| Linux Socket Filtering aka Berkeley Packet Filter (BPF) |
| ======================================================= |
| |
| Introduction |
| ------------ |
| |
| Linux Socket Filtering (LSF) is derived from the Berkeley Packet Filter. |
| Though there are some distinct differences between the BSD and Linux |
| Kernel filtering, but when we speak of BPF or LSF in Linux context, we |
| mean the very same mechanism of filtering in the Linux kernel. |
| |
| BPF allows a user-space program to attach a filter onto any socket and |
| allow or disallow certain types of data to come through the socket. LSF |
| follows exactly the same filter code structure as BSD's BPF, so referring |
| to the BSD bpf.4 manpage is very helpful in creating filters. |
| |
| On Linux, BPF is much simpler than on BSD. One does not have to worry |
| about devices or anything like that. You simply create your filter code, |
| send it to the kernel via the SO_ATTACH_FILTER option and if your filter |
| code passes the kernel check on it, you then immediately begin filtering |
| data on that socket. |
| |
| You can also detach filters from your socket via the SO_DETACH_FILTER |
| option. This will probably not be used much since when you close a socket |
| that has a filter on it the filter is automagically removed. The other |
| less common case may be adding a different filter on the same socket where |
| you had another filter that is still running: the kernel takes care of |
| removing the old one and placing your new one in its place, assuming your |
| filter has passed the checks, otherwise if it fails the old filter will |
| remain on that socket. |
| |
| SO_LOCK_FILTER option allows to lock the filter attached to a socket. Once |
| set, a filter cannot be removed or changed. This allows one process to |
| setup a socket, attach a filter, lock it then drop privileges and be |
| assured that the filter will be kept until the socket is closed. |
| |
| The biggest user of this construct might be libpcap. Issuing a high-level |
| filter command like `tcpdump -i em1 port 22` passes through the libpcap |
| internal compiler that generates a structure that can eventually be loaded |
| via SO_ATTACH_FILTER to the kernel. `tcpdump -i em1 port 22 -ddd` |
| displays what is being placed into this structure. |
| |
| Although we were only speaking about sockets here, BPF in Linux is used |
| in many more places. There's xt_bpf for netfilter, cls_bpf in the kernel |
| qdisc layer, SECCOMP-BPF (SECure COMPuting [1]), and lots of other places |
| such as team driver, PTP code, etc where BPF is being used. |
| |
| [1] Documentation/userspace-api/seccomp_filter.rst |
| |
| Original BPF paper: |
| |
| Steven McCanne and Van Jacobson. 1993. The BSD packet filter: a new |
| architecture for user-level packet capture. In Proceedings of the |
| USENIX Winter 1993 Conference Proceedings on USENIX Winter 1993 |
| Conference Proceedings (USENIX'93). USENIX Association, Berkeley, |
| CA, USA, 2-2. [http://www.tcpdump.org/papers/bpf-usenix93.pdf] |
| |
| Structure |
| --------- |
| |
| User space applications include <linux/filter.h> which contains the |
| following relevant structures: |
| |
| struct sock_filter { /* Filter block */ |
| __u16 code; /* Actual filter code */ |
| __u8 jt; /* Jump true */ |
| __u8 jf; /* Jump false */ |
| __u32 k; /* Generic multiuse field */ |
| }; |
| |
| Such a structure is assembled as an array of 4-tuples, that contains |
| a code, jt, jf and k value. jt and jf are jump offsets and k a generic |
| value to be used for a provided code. |
| |
| struct sock_fprog { /* Required for SO_ATTACH_FILTER. */ |
| unsigned short len; /* Number of filter blocks */ |
| struct sock_filter __user *filter; |
| }; |
| |
| For socket filtering, a pointer to this structure (as shown in |
| follow-up example) is being passed to the kernel through setsockopt(2). |
| |
| Example |
| ------- |
| |
| #include <sys/socket.h> |
| #include <sys/types.h> |
| #include <arpa/inet.h> |
| #include <linux/if_ether.h> |
| /* ... */ |
| |
| /* From the example above: tcpdump -i em1 port 22 -dd */ |
| struct sock_filter code[] = { |
| { 0x28, 0, 0, 0x0000000c }, |
| { 0x15, 0, 8, 0x000086dd }, |
| { 0x30, 0, 0, 0x00000014 }, |
| { 0x15, 2, 0, 0x00000084 }, |
| { 0x15, 1, 0, 0x00000006 }, |
| { 0x15, 0, 17, 0x00000011 }, |
| { 0x28, 0, 0, 0x00000036 }, |
| { 0x15, 14, 0, 0x00000016 }, |
| { 0x28, 0, 0, 0x00000038 }, |
| { 0x15, 12, 13, 0x00000016 }, |
| { 0x15, 0, 12, 0x00000800 }, |
| { 0x30, 0, 0, 0x00000017 }, |
| { 0x15, 2, 0, 0x00000084 }, |
| { 0x15, 1, 0, 0x00000006 }, |
| { 0x15, 0, 8, 0x00000011 }, |
| { 0x28, 0, 0, 0x00000014 }, |
| { 0x45, 6, 0, 0x00001fff }, |
| { 0xb1, 0, 0, 0x0000000e }, |
| { 0x48, 0, 0, 0x0000000e }, |
| { 0x15, 2, 0, 0x00000016 }, |
| { 0x48, 0, 0, 0x00000010 }, |
| { 0x15, 0, 1, 0x00000016 }, |
| { 0x06, 0, 0, 0x0000ffff }, |
| { 0x06, 0, 0, 0x00000000 }, |
| }; |
| |
| struct sock_fprog bpf = { |
| .len = ARRAY_SIZE(code), |
| .filter = code, |
| }; |
| |
| sock = socket(PF_PACKET, SOCK_RAW, htons(ETH_P_ALL)); |
| if (sock < 0) |
| /* ... bail out ... */ |
| |
| ret = setsockopt(sock, SOL_SOCKET, SO_ATTACH_FILTER, &bpf, sizeof(bpf)); |
| if (ret < 0) |
| /* ... bail out ... */ |
| |
| /* ... */ |
| close(sock); |
| |
| The above example code attaches a socket filter for a PF_PACKET socket |
| in order to let all IPv4/IPv6 packets with port 22 pass. The rest will |
| be dropped for this socket. |
| |
| The setsockopt(2) call to SO_DETACH_FILTER doesn't need any arguments |
| and SO_LOCK_FILTER for preventing the filter to be detached, takes an |
| integer value with 0 or 1. |
| |
| Note that socket filters are not restricted to PF_PACKET sockets only, |
| but can also be used on other socket families. |
| |
| Summary of system calls: |
| |
| * setsockopt(sockfd, SOL_SOCKET, SO_ATTACH_FILTER, &val, sizeof(val)); |
| * setsockopt(sockfd, SOL_SOCKET, SO_DETACH_FILTER, &val, sizeof(val)); |
| * setsockopt(sockfd, SOL_SOCKET, SO_LOCK_FILTER, &val, sizeof(val)); |
| |
| Normally, most use cases for socket filtering on packet sockets will be |
| covered by libpcap in high-level syntax, so as an application developer |
| you should stick to that. libpcap wraps its own layer around all that. |
| |
| Unless i) using/linking to libpcap is not an option, ii) the required BPF |
| filters use Linux extensions that are not supported by libpcap's compiler, |
| iii) a filter might be more complex and not cleanly implementable with |
| libpcap's compiler, or iv) particular filter codes should be optimized |
| differently than libpcap's internal compiler does; then in such cases |
| writing such a filter "by hand" can be of an alternative. For example, |
| xt_bpf and cls_bpf users might have requirements that could result in |
| more complex filter code, or one that cannot be expressed with libpcap |
| (e.g. different return codes for various code paths). Moreover, BPF JIT |
| implementors may wish to manually write test cases and thus need low-level |
| access to BPF code as well. |
| |
| BPF engine and instruction set |
| ------------------------------ |
| |
| Under tools/bpf/ there's a small helper tool called bpf_asm which can |
| be used to write low-level filters for example scenarios mentioned in the |
| previous section. Asm-like syntax mentioned here has been implemented in |
| bpf_asm and will be used for further explanations (instead of dealing with |
| less readable opcodes directly, principles are the same). The syntax is |
| closely modelled after Steven McCanne's and Van Jacobson's BPF paper. |
| |
| The BPF architecture consists of the following basic elements: |
| |
| Element Description |
| |
| A 32 bit wide accumulator |
| X 32 bit wide X register |
| M[] 16 x 32 bit wide misc registers aka "scratch memory |
| store", addressable from 0 to 15 |
| |
| A program, that is translated by bpf_asm into "opcodes" is an array that |
| consists of the following elements (as already mentioned): |
| |
| op:16, jt:8, jf:8, k:32 |
| |
| The element op is a 16 bit wide opcode that has a particular instruction |
| encoded. jt and jf are two 8 bit wide jump targets, one for condition |
| "jump if true", the other one "jump if false". Eventually, element k |
| contains a miscellaneous argument that can be interpreted in different |
| ways depending on the given instruction in op. |
| |
| The instruction set consists of load, store, branch, alu, miscellaneous |
| and return instructions that are also represented in bpf_asm syntax. This |
| table lists all bpf_asm instructions available resp. what their underlying |
| opcodes as defined in linux/filter.h stand for: |
| |
| Instruction Addressing mode Description |
| |
| ld 1, 2, 3, 4, 10 Load word into A |
| ldi 4 Load word into A |
| ldh 1, 2 Load half-word into A |
| ldb 1, 2 Load byte into A |
| ldx 3, 4, 5, 10 Load word into X |
| ldxi 4 Load word into X |
| ldxb 5 Load byte into X |
| |
| st 3 Store A into M[] |
| stx 3 Store X into M[] |
| |
| jmp 6 Jump to label |
| ja 6 Jump to label |
| jeq 7, 8 Jump on A == k |
| jneq 8 Jump on A != k |
| jne 8 Jump on A != k |
| jlt 8 Jump on A < k |
| jle 8 Jump on A <= k |
| jgt 7, 8 Jump on A > k |
| jge 7, 8 Jump on A >= k |
| jset 7, 8 Jump on A & k |
| |
| add 0, 4 A + <x> |
| sub 0, 4 A - <x> |
| mul 0, 4 A * <x> |
| div 0, 4 A / <x> |
| mod 0, 4 A % <x> |
| neg !A |
| and 0, 4 A & <x> |
| or 0, 4 A | <x> |
| xor 0, 4 A ^ <x> |
| lsh 0, 4 A << <x> |
| rsh 0, 4 A >> <x> |
| |
| tax Copy A into X |
| txa Copy X into A |
| |
| ret 4, 9 Return |
| |
| The next table shows addressing formats from the 2nd column: |
| |
| Addressing mode Syntax Description |
| |
| 0 x/%x Register X |
| 1 [k] BHW at byte offset k in the packet |
| 2 [x + k] BHW at the offset X + k in the packet |
| 3 M[k] Word at offset k in M[] |
| 4 #k Literal value stored in k |
| 5 4*([k]&0xf) Lower nibble * 4 at byte offset k in the packet |
| 6 L Jump label L |
| 7 #k,Lt,Lf Jump to Lt if true, otherwise jump to Lf |
| 8 #k,Lt Jump to Lt if predicate is true |
| 9 a/%a Accumulator A |
| 10 extension BPF extension |
| |
| The Linux kernel also has a couple of BPF extensions that are used along |
| with the class of load instructions by "overloading" the k argument with |
| a negative offset + a particular extension offset. The result of such BPF |
| extensions are loaded into A. |
| |
| Possible BPF extensions are shown in the following table: |
| |
| Extension Description |
| |
| len skb->len |
| proto skb->protocol |
| type skb->pkt_type |
| poff Payload start offset |
| ifidx skb->dev->ifindex |
| nla Netlink attribute of type X with offset A |
| nlan Nested Netlink attribute of type X with offset A |
| mark skb->mark |
| queue skb->queue_mapping |
| hatype skb->dev->type |
| rxhash skb->hash |
| cpu raw_smp_processor_id() |
| vlan_tci skb_vlan_tag_get(skb) |
| vlan_avail skb_vlan_tag_present(skb) |
| vlan_tpid skb->vlan_proto |
| rand prandom_u32() |
| |
| These extensions can also be prefixed with '#'. |
| Examples for low-level BPF: |
| |
| ** ARP packets: |
| |
| ldh [12] |
| jne #0x806, drop |
| ret #-1 |
| drop: ret #0 |
| |
| ** IPv4 TCP packets: |
| |
| ldh [12] |
| jne #0x800, drop |
| ldb [23] |
| jneq #6, drop |
| ret #-1 |
| drop: ret #0 |
| |
| ** (Accelerated) VLAN w/ id 10: |
| |
| ld vlan_tci |
| jneq #10, drop |
| ret #-1 |
| drop: ret #0 |
| |
| ** icmp random packet sampling, 1 in 4 |
| ldh [12] |
| jne #0x800, drop |
| ldb [23] |
| jneq #1, drop |
| # get a random uint32 number |
| ld rand |
| mod #4 |
| jneq #1, drop |
| ret #-1 |
| drop: ret #0 |
| |
| ** SECCOMP filter example: |
| |
| ld [4] /* offsetof(struct seccomp_data, arch) */ |
| jne #0xc000003e, bad /* AUDIT_ARCH_X86_64 */ |
| ld [0] /* offsetof(struct seccomp_data, nr) */ |
| jeq #15, good /* __NR_rt_sigreturn */ |
| jeq #231, good /* __NR_exit_group */ |
| jeq #60, good /* __NR_exit */ |
| jeq #0, good /* __NR_read */ |
| jeq #1, good /* __NR_write */ |
| jeq #5, good /* __NR_fstat */ |
| jeq #9, good /* __NR_mmap */ |
| jeq #14, good /* __NR_rt_sigprocmask */ |
| jeq #13, good /* __NR_rt_sigaction */ |
| jeq #35, good /* __NR_nanosleep */ |
| bad: ret #0 /* SECCOMP_RET_KILL_THREAD */ |
| good: ret #0x7fff0000 /* SECCOMP_RET_ALLOW */ |
| |
| The above example code can be placed into a file (here called "foo"), and |
| then be passed to the bpf_asm tool for generating opcodes, output that xt_bpf |
| and cls_bpf understands and can directly be loaded with. Example with above |
| ARP code: |
| |
| $ ./bpf_asm foo |
| 4,40 0 0 12,21 0 1 2054,6 0 0 4294967295,6 0 0 0, |
| |
| In copy and paste C-like output: |
| |
| $ ./bpf_asm -c foo |
| { 0x28, 0, 0, 0x0000000c }, |
| { 0x15, 0, 1, 0x00000806 }, |
| { 0x06, 0, 0, 0xffffffff }, |
| { 0x06, 0, 0, 0000000000 }, |
| |
| In particular, as usage with xt_bpf or cls_bpf can result in more complex BPF |
| filters that might not be obvious at first, it's good to test filters before |
| attaching to a live system. For that purpose, there's a small tool called |
| bpf_dbg under tools/bpf/ in the kernel source directory. This debugger allows |
| for testing BPF filters against given pcap files, single stepping through the |
| BPF code on the pcap's packets and to do BPF machine register dumps. |
| |
| Starting bpf_dbg is trivial and just requires issuing: |
| |
| # ./bpf_dbg |
| |
| In case input and output do not equal stdin/stdout, bpf_dbg takes an |
| alternative stdin source as a first argument, and an alternative stdout |
| sink as a second one, e.g. `./bpf_dbg test_in.txt test_out.txt`. |
| |
| Other than that, a particular libreadline configuration can be set via |
| file "~/.bpf_dbg_init" and the command history is stored in the file |
| "~/.bpf_dbg_history". |
| |
| Interaction in bpf_dbg happens through a shell that also has auto-completion |
| support (follow-up example commands starting with '>' denote bpf_dbg shell). |
| The usual workflow would be to ... |
| |
| > load bpf 6,40 0 0 12,21 0 3 2048,48 0 0 23,21 0 1 1,6 0 0 65535,6 0 0 0 |
| Loads a BPF filter from standard output of bpf_asm, or transformed via |
| e.g. `tcpdump -iem1 -ddd port 22 | tr '\n' ','`. Note that for JIT |
| debugging (next section), this command creates a temporary socket and |
| loads the BPF code into the kernel. Thus, this will also be useful for |
| JIT developers. |
| |
| > load pcap foo.pcap |
| Loads standard tcpdump pcap file. |
| |
| > run [<n>] |
| bpf passes:1 fails:9 |
| Runs through all packets from a pcap to account how many passes and fails |
| the filter will generate. A limit of packets to traverse can be given. |
| |
| > disassemble |
| l0: ldh [12] |
| l1: jeq #0x800, l2, l5 |
| l2: ldb [23] |
| l3: jeq #0x1, l4, l5 |
| l4: ret #0xffff |
| l5: ret #0 |
| Prints out BPF code disassembly. |
| |
| > dump |
| /* { op, jt, jf, k }, */ |
| { 0x28, 0, 0, 0x0000000c }, |
| { 0x15, 0, 3, 0x00000800 }, |
| { 0x30, 0, 0, 0x00000017 }, |
| { 0x15, 0, 1, 0x00000001 }, |
| { 0x06, 0, 0, 0x0000ffff }, |
| { 0x06, 0, 0, 0000000000 }, |
| Prints out C-style BPF code dump. |
| |
| > breakpoint 0 |
| breakpoint at: l0: ldh [12] |
| > breakpoint 1 |
| breakpoint at: l1: jeq #0x800, l2, l5 |
| ... |
| Sets breakpoints at particular BPF instructions. Issuing a `run` command |
| will walk through the pcap file continuing from the current packet and |
| break when a breakpoint is being hit (another `run` will continue from |
| the currently active breakpoint executing next instructions): |
| |
| > run |
| -- register dump -- |
| pc: [0] <-- program counter |
| code: [40] jt[0] jf[0] k[12] <-- plain BPF code of current instruction |
| curr: l0: ldh [12] <-- disassembly of current instruction |
| A: [00000000][0] <-- content of A (hex, decimal) |
| X: [00000000][0] <-- content of X (hex, decimal) |
| M[0,15]: [00000000][0] <-- folded content of M (hex, decimal) |
| -- packet dump -- <-- Current packet from pcap (hex) |
| len: 42 |
| 0: 00 19 cb 55 55 a4 00 14 a4 43 78 69 08 06 00 01 |
| 16: 08 00 06 04 00 01 00 14 a4 43 78 69 0a 3b 01 26 |
| 32: 00 00 00 00 00 00 0a 3b 01 01 |
| (breakpoint) |
| > |
| |
| > breakpoint |
| breakpoints: 0 1 |
| Prints currently set breakpoints. |
| |
| > step [-<n>, +<n>] |
| Performs single stepping through the BPF program from the current pc |
| offset. Thus, on each step invocation, above register dump is issued. |
| This can go forwards and backwards in time, a plain `step` will break |
| on the next BPF instruction, thus +1. (No `run` needs to be issued here.) |
| |
| > select <n> |
| Selects a given packet from the pcap file to continue from. Thus, on |
| the next `run` or `step`, the BPF program is being evaluated against |
| the user pre-selected packet. Numbering starts just as in Wireshark |
| with index 1. |
| |
| > quit |
| # |
| Exits bpf_dbg. |
| |
| JIT compiler |
| ------------ |
| |
| The Linux kernel has a built-in BPF JIT compiler for x86_64, SPARC, PowerPC, |
| ARM, ARM64, MIPS and s390 and can be enabled through CONFIG_BPF_JIT. The JIT |
| compiler is transparently invoked for each attached filter from user space |
| or for internal kernel users if it has been previously enabled by root: |
| |
| echo 1 > /proc/sys/net/core/bpf_jit_enable |
| |
| For JIT developers, doing audits etc, each compile run can output the generated |
| opcode image into the kernel log via: |
| |
| echo 2 > /proc/sys/net/core/bpf_jit_enable |
| |
| Example output from dmesg: |
| |
| [ 3389.935842] flen=6 proglen=70 pass=3 image=ffffffffa0069c8f |
| [ 3389.935847] JIT code: 00000000: 55 48 89 e5 48 83 ec 60 48 89 5d f8 44 8b 4f 68 |
| [ 3389.935849] JIT code: 00000010: 44 2b 4f 6c 4c 8b 87 d8 00 00 00 be 0c 00 00 00 |
| [ 3389.935850] JIT code: 00000020: e8 1d 94 ff e0 3d 00 08 00 00 75 16 be 17 00 00 |
| [ 3389.935851] JIT code: 00000030: 00 e8 28 94 ff e0 83 f8 01 75 07 b8 ff ff 00 00 |
| [ 3389.935852] JIT code: 00000040: eb 02 31 c0 c9 c3 |
| |
| In the kernel source tree under tools/bpf/, there's bpf_jit_disasm for |
| generating disassembly out of the kernel log's hexdump: |
| |
| # ./bpf_jit_disasm |
| 70 bytes emitted from JIT compiler (pass:3, flen:6) |
| ffffffffa0069c8f + <x>: |
| 0: push %rbp |
| 1: mov %rsp,%rbp |
| 4: sub $0x60,%rsp |
| 8: mov %rbx,-0x8(%rbp) |
| c: mov 0x68(%rdi),%r9d |
| 10: sub 0x6c(%rdi),%r9d |
| 14: mov 0xd8(%rdi),%r8 |
| 1b: mov $0xc,%esi |
| 20: callq 0xffffffffe0ff9442 |
| 25: cmp $0x800,%eax |
| 2a: jne 0x0000000000000042 |
| 2c: mov $0x17,%esi |
| 31: callq 0xffffffffe0ff945e |
| 36: cmp $0x1,%eax |
| 39: jne 0x0000000000000042 |
| 3b: mov $0xffff,%eax |
| 40: jmp 0x0000000000000044 |
| 42: xor %eax,%eax |
| 44: leaveq |
| 45: retq |
| |
| Issuing option `-o` will "annotate" opcodes to resulting assembler |
| instructions, which can be very useful for JIT developers: |
| |
| # ./bpf_jit_disasm -o |
| 70 bytes emitted from JIT compiler (pass:3, flen:6) |
| ffffffffa0069c8f + <x>: |
| 0: push %rbp |
| 55 |
| 1: mov %rsp,%rbp |
| 48 89 e5 |
| 4: sub $0x60,%rsp |
| 48 83 ec 60 |
| 8: mov %rbx,-0x8(%rbp) |
| 48 89 5d f8 |
| c: mov 0x68(%rdi),%r9d |
| 44 8b 4f 68 |
| 10: sub 0x6c(%rdi),%r9d |
| 44 2b 4f 6c |
| 14: mov 0xd8(%rdi),%r8 |
| 4c 8b 87 d8 00 00 00 |
| 1b: mov $0xc,%esi |
| be 0c 00 00 00 |
| 20: callq 0xffffffffe0ff9442 |
| e8 1d 94 ff e0 |
| 25: cmp $0x800,%eax |
| 3d 00 08 00 00 |
| 2a: jne 0x0000000000000042 |
| 75 16 |
| 2c: mov $0x17,%esi |
| be 17 00 00 00 |
| 31: callq 0xffffffffe0ff945e |
| e8 28 94 ff e0 |
| 36: cmp $0x1,%eax |
| 83 f8 01 |
| 39: jne 0x0000000000000042 |
| 75 07 |
| 3b: mov $0xffff,%eax |
| b8 ff ff 00 00 |
| 40: jmp 0x0000000000000044 |
| eb 02 |
| 42: xor %eax,%eax |
| 31 c0 |
| 44: leaveq |
| c9 |
| 45: retq |
| c3 |
| |
| For BPF JIT developers, bpf_jit_disasm, bpf_asm and bpf_dbg provides a useful |
| toolchain for developing and testing the kernel's JIT compiler. |
| |
| BPF kernel internals |
| -------------------- |
| Internally, for the kernel interpreter, a different instruction set |
| format with similar underlying principles from BPF described in previous |
| paragraphs is being used. However, the instruction set format is modelled |
| closer to the underlying architecture to mimic native instruction sets, so |
| that a better performance can be achieved (more details later). This new |
| ISA is called 'eBPF' or 'internal BPF' interchangeably. (Note: eBPF which |
| originates from [e]xtended BPF is not the same as BPF extensions! While |
| eBPF is an ISA, BPF extensions date back to classic BPF's 'overloading' |
| of BPF_LD | BPF_{B,H,W} | BPF_ABS instruction.) |
| |
| It is designed to be JITed with one to one mapping, which can also open up |
| the possibility for GCC/LLVM compilers to generate optimized eBPF code through |
| an eBPF backend that performs almost as fast as natively compiled code. |
| |
| The new instruction set was originally designed with the possible goal in |
| mind to write programs in "restricted C" and compile into eBPF with a optional |
| GCC/LLVM backend, so that it can just-in-time map to modern 64-bit CPUs with |
| minimal performance overhead over two steps, that is, C -> eBPF -> native code. |
| |
| Currently, the new format is being used for running user BPF programs, which |
| includes seccomp BPF, classic socket filters, cls_bpf traffic classifier, |
| team driver's classifier for its load-balancing mode, netfilter's xt_bpf |
| extension, PTP dissector/classifier, and much more. They are all internally |
| converted by the kernel into the new instruction set representation and run |
| in the eBPF interpreter. For in-kernel handlers, this all works transparently |
| by using bpf_prog_create() for setting up the filter, resp. |
| bpf_prog_destroy() for destroying it. The macro |
| BPF_PROG_RUN(filter, ctx) transparently invokes eBPF interpreter or JITed |
| code to run the filter. 'filter' is a pointer to struct bpf_prog that we |
| got from bpf_prog_create(), and 'ctx' the given context (e.g. |
| skb pointer). All constraints and restrictions from bpf_check_classic() apply |
| before a conversion to the new layout is being done behind the scenes! |
| |
| Currently, the classic BPF format is being used for JITing on most 32-bit |
| architectures, whereas x86-64, aarch64, s390x, powerpc64, sparc64, arm32 perform |
| JIT compilation from eBPF instruction set. |
| |
| Some core changes of the new internal format: |
| |
| - Number of registers increase from 2 to 10: |
| |
| The old format had two registers A and X, and a hidden frame pointer. The |
| new layout extends this to be 10 internal registers and a read-only frame |
| pointer. Since 64-bit CPUs are passing arguments to functions via registers |
| the number of args from eBPF program to in-kernel function is restricted |
| to 5 and one register is used to accept return value from an in-kernel |
| function. Natively, x86_64 passes first 6 arguments in registers, aarch64/ |
| sparcv9/mips64 have 7 - 8 registers for arguments; x86_64 has 6 callee saved |
| registers, and aarch64/sparcv9/mips64 have 11 or more callee saved registers. |
| |
| Therefore, eBPF calling convention is defined as: |
| |
| * R0 - return value from in-kernel function, and exit value for eBPF program |
| * R1 - R5 - arguments from eBPF program to in-kernel function |
| * R6 - R9 - callee saved registers that in-kernel function will preserve |
| * R10 - read-only frame pointer to access stack |
| |
| Thus, all eBPF registers map one to one to HW registers on x86_64, aarch64, |
| etc, and eBPF calling convention maps directly to ABIs used by the kernel on |
| 64-bit architectures. |
| |
| On 32-bit architectures JIT may map programs that use only 32-bit arithmetic |
| and may let more complex programs to be interpreted. |
| |
| R0 - R5 are scratch registers and eBPF program needs spill/fill them if |
| necessary across calls. Note that there is only one eBPF program (== one |
| eBPF main routine) and it cannot call other eBPF functions, it can only |
| call predefined in-kernel functions, though. |
| |
| - Register width increases from 32-bit to 64-bit: |
| |
| Still, the semantics of the original 32-bit ALU operations are preserved |
| via 32-bit subregisters. All eBPF registers are 64-bit with 32-bit lower |
| subregisters that zero-extend into 64-bit if they are being written to. |
| That behavior maps directly to x86_64 and arm64 subregister definition, but |
| makes other JITs more difficult. |
| |
| 32-bit architectures run 64-bit internal BPF programs via interpreter. |
| Their JITs may convert BPF programs that only use 32-bit subregisters into |
| native instruction set and let the rest being interpreted. |
| |
| Operation is 64-bit, because on 64-bit architectures, pointers are also |
| 64-bit wide, and we want to pass 64-bit values in/out of kernel functions, |
| so 32-bit eBPF registers would otherwise require to define register-pair |
| ABI, thus, there won't be able to use a direct eBPF register to HW register |
| mapping and JIT would need to do combine/split/move operations for every |
| register in and out of the function, which is complex, bug prone and slow. |
| Another reason is the use of atomic 64-bit counters. |
| |
| - Conditional jt/jf targets replaced with jt/fall-through: |
| |
| While the original design has constructs such as "if (cond) jump_true; |
| else jump_false;", they are being replaced into alternative constructs like |
| "if (cond) jump_true; /* else fall-through */". |
| |
| - Introduces bpf_call insn and register passing convention for zero overhead |
| calls from/to other kernel functions: |
| |
| Before an in-kernel function call, the internal BPF program needs to |
| place function arguments into R1 to R5 registers to satisfy calling |
| convention, then the interpreter will take them from registers and pass |
| to in-kernel function. If R1 - R5 registers are mapped to CPU registers |
| that are used for argument passing on given architecture, the JIT compiler |
| doesn't need to emit extra moves. Function arguments will be in the correct |
| registers and BPF_CALL instruction will be JITed as single 'call' HW |
| instruction. This calling convention was picked to cover common call |
| situations without performance penalty. |
| |
| After an in-kernel function call, R1 - R5 are reset to unreadable and R0 has |
| a return value of the function. Since R6 - R9 are callee saved, their state |
| is preserved across the call. |
| |
| For example, consider three C functions: |
| |
| u64 f1() { return (*_f2)(1); } |
| u64 f2(u64 a) { return f3(a + 1, a); } |
| u64 f3(u64 a, u64 b) { return a - b; } |
| |
| GCC can compile f1, f3 into x86_64: |
| |
| f1: |
| movl $1, %edi |
| movq _f2(%rip), %rax |
| jmp *%rax |
| f3: |
| movq %rdi, %rax |
| subq %rsi, %rax |
| ret |
| |
| Function f2 in eBPF may look like: |
| |
| f2: |
| bpf_mov R2, R1 |
| bpf_add R1, 1 |
| bpf_call f3 |
| bpf_exit |
| |
| If f2 is JITed and the pointer stored to '_f2'. The calls f1 -> f2 -> f3 and |
| returns will be seamless. Without JIT, __bpf_prog_run() interpreter needs to |
| be used to call into f2. |
| |
| For practical reasons all eBPF programs have only one argument 'ctx' which is |
| already placed into R1 (e.g. on __bpf_prog_run() startup) and the programs |
| can call kernel functions with up to 5 arguments. Calls with 6 or more arguments |
| are currently not supported, but these restrictions can be lifted if necessary |
| in the future. |
| |
| On 64-bit architectures all register map to HW registers one to one. For |
| example, x86_64 JIT compiler can map them as ... |
| |
| R0 - rax |
| R1 - rdi |
| R2 - rsi |
| R3 - rdx |
| R4 - rcx |
| R5 - r8 |
| R6 - rbx |
| R7 - r13 |
| R8 - r14 |
| R9 - r15 |
| R10 - rbp |
| |
| ... since x86_64 ABI mandates rdi, rsi, rdx, rcx, r8, r9 for argument passing |
| and rbx, r12 - r15 are callee saved. |
| |
| Then the following internal BPF pseudo-program: |
| |
| bpf_mov R6, R1 /* save ctx */ |
| bpf_mov R2, 2 |
| bpf_mov R3, 3 |
| bpf_mov R4, 4 |
| bpf_mov R5, 5 |
| bpf_call foo |
| bpf_mov R7, R0 /* save foo() return value */ |
| bpf_mov R1, R6 /* restore ctx for next call */ |
| bpf_mov R2, 6 |
| bpf_mov R3, 7 |
| bpf_mov R4, 8 |
| bpf_mov R5, 9 |
| bpf_call bar |
| bpf_add R0, R7 |
| bpf_exit |
| |
| After JIT to x86_64 may look like: |
| |
| push %rbp |
| mov %rsp,%rbp |
| sub $0x228,%rsp |
| mov %rbx,-0x228(%rbp) |
| mov %r13,-0x220(%rbp) |
| mov %rdi,%rbx |
| mov $0x2,%esi |
| mov $0x3,%edx |
| mov $0x4,%ecx |
| mov $0x5,%r8d |
| callq foo |
| mov %rax,%r13 |
| mov %rbx,%rdi |
| mov $0x2,%esi |
| mov $0x3,%edx |
| mov $0x4,%ecx |
| mov $0x5,%r8d |
| callq bar |
| add %r13,%rax |
| mov -0x228(%rbp),%rbx |
| mov -0x220(%rbp),%r13 |
| leaveq |
| retq |
| |
| Which is in this example equivalent in C to: |
| |
| u64 bpf_filter(u64 ctx) |
| { |
| return foo(ctx, 2, 3, 4, 5) + bar(ctx, 6, 7, 8, 9); |
| } |
| |
| In-kernel functions foo() and bar() with prototype: u64 (*)(u64 arg1, u64 |
| arg2, u64 arg3, u64 arg4, u64 arg5); will receive arguments in proper |
| registers and place their return value into '%rax' which is R0 in eBPF. |
| Prologue and epilogue are emitted by JIT and are implicit in the |
| interpreter. R0-R5 are scratch registers, so eBPF program needs to preserve |
| them across the calls as defined by calling convention. |
| |
| For example the following program is invalid: |
| |
| bpf_mov R1, 1 |
| bpf_call foo |
| bpf_mov R0, R1 |
| bpf_exit |
| |
| After the call the registers R1-R5 contain junk values and cannot be read. |
| An in-kernel eBPF verifier is used to validate internal BPF programs. |
| |
| Also in the new design, eBPF is limited to 4096 insns, which means that any |
| program will terminate quickly and will only call a fixed number of kernel |
| functions. Original BPF and the new format are two operand instructions, |
| which helps to do one-to-one mapping between eBPF insn and x86 insn during JIT. |
| |
| The input context pointer for invoking the interpreter function is generic, |
| its content is defined by a specific use case. For seccomp register R1 points |
| to seccomp_data, for converted BPF filters R1 points to a skb. |
| |
| A program, that is translated internally consists of the following elements: |
| |
| op:16, jt:8, jf:8, k:32 ==> op:8, dst_reg:4, src_reg:4, off:16, imm:32 |
| |
| So far 87 internal BPF instructions were implemented. 8-bit 'op' opcode field |
| has room for new instructions. Some of them may use 16/24/32 byte encoding. New |
| instructions must be multiple of 8 bytes to preserve backward compatibility. |
| |
| Internal BPF is a general purpose RISC instruction set. Not every register and |
| every instruction are used during translation from original BPF to new format. |
| For example, socket filters are not using 'exclusive add' instruction, but |
| tracing filters may do to maintain counters of events, for example. Register R9 |
| is not used by socket filters either, but more complex filters may be running |
| out of registers and would have to resort to spill/fill to stack. |
| |
| Internal BPF can used as generic assembler for last step performance |
| optimizations, socket filters and seccomp are using it as assembler. Tracing |
| filters may use it as assembler to generate code from kernel. In kernel usage |
| may not be bounded by security considerations, since generated internal BPF code |
| may be optimizing internal code path and not being exposed to the user space. |
| Safety of internal BPF can come from a verifier (TBD). In such use cases as |
| described, it may be used as safe instruction set. |
| |
| Just like the original BPF, the new format runs within a controlled environment, |
| is deterministic and the kernel can easily prove that. The safety of the program |
| can be determined in two steps: first step does depth-first-search to disallow |
| loops and other CFG validation; second step starts from the first insn and |
| descends all possible paths. It simulates execution of every insn and observes |
| the state change of registers and stack. |
| |
| eBPF opcode encoding |
| -------------------- |
| |
| eBPF is reusing most of the opcode encoding from classic to simplify conversion |
| of classic BPF to eBPF. For arithmetic and jump instructions the 8-bit 'code' |
| field is divided into three parts: |
| |
| +----------------+--------+--------------------+ |
| | 4 bits | 1 bit | 3 bits | |
| | operation code | source | instruction class | |
| +----------------+--------+--------------------+ |
| (MSB) (LSB) |
| |
| Three LSB bits store instruction class which is one of: |
| |
| Classic BPF classes: eBPF classes: |
| |
| BPF_LD 0x00 BPF_LD 0x00 |
| BPF_LDX 0x01 BPF_LDX 0x01 |
| BPF_ST 0x02 BPF_ST 0x02 |
| BPF_STX 0x03 BPF_STX 0x03 |
| BPF_ALU 0x04 BPF_ALU 0x04 |
| BPF_JMP 0x05 BPF_JMP 0x05 |
| BPF_RET 0x06 [ class 6 unused, for future if needed ] |
| BPF_MISC 0x07 BPF_ALU64 0x07 |
| |
| When BPF_CLASS(code) == BPF_ALU or BPF_JMP, 4th bit encodes source operand ... |
| |
| BPF_K 0x00 |
| BPF_X 0x08 |
| |
| * in classic BPF, this means: |
| |
| BPF_SRC(code) == BPF_X - use register X as source operand |
| BPF_SRC(code) == BPF_K - use 32-bit immediate as source operand |
| |
| * in eBPF, this means: |
| |
| BPF_SRC(code) == BPF_X - use 'src_reg' register as source operand |
| BPF_SRC(code) == BPF_K - use 32-bit immediate as source operand |
| |
| ... and four MSB bits store operation code. |
| |
| If BPF_CLASS(code) == BPF_ALU or BPF_ALU64 [ in eBPF ], BPF_OP(code) is one of: |
| |
| BPF_ADD 0x00 |
| BPF_SUB 0x10 |
| BPF_MUL 0x20 |
| BPF_DIV 0x30 |
| BPF_OR 0x40 |
| BPF_AND 0x50 |
| BPF_LSH 0x60 |
| BPF_RSH 0x70 |
| BPF_NEG 0x80 |
| BPF_MOD 0x90 |
| BPF_XOR 0xa0 |
| BPF_MOV 0xb0 /* eBPF only: mov reg to reg */ |
| BPF_ARSH 0xc0 /* eBPF only: sign extending shift right */ |
| BPF_END 0xd0 /* eBPF only: endianness conversion */ |
| |
| If BPF_CLASS(code) == BPF_JMP, BPF_OP(code) is one of: |
| |
| BPF_JA 0x00 |
| BPF_JEQ 0x10 |
| BPF_JGT 0x20 |
| BPF_JGE 0x30 |
| BPF_JSET 0x40 |
| BPF_JNE 0x50 /* eBPF only: jump != */ |
| BPF_JSGT 0x60 /* eBPF only: signed '>' */ |
| BPF_JSGE 0x70 /* eBPF only: signed '>=' */ |
| BPF_CALL 0x80 /* eBPF only: function call */ |
| BPF_EXIT 0x90 /* eBPF only: function return */ |
| BPF_JLT 0xa0 /* eBPF only: unsigned '<' */ |
| BPF_JLE 0xb0 /* eBPF only: unsigned '<=' */ |
| BPF_JSLT 0xc0 /* eBPF only: signed '<' */ |
| BPF_JSLE 0xd0 /* eBPF only: signed '<=' */ |
| |
| So BPF_ADD | BPF_X | BPF_ALU means 32-bit addition in both classic BPF |
| and eBPF. There are only two registers in classic BPF, so it means A += X. |
| In eBPF it means dst_reg = (u32) dst_reg + (u32) src_reg; similarly, |
| BPF_XOR | BPF_K | BPF_ALU means A ^= imm32 in classic BPF and analogous |
| src_reg = (u32) src_reg ^ (u32) imm32 in eBPF. |
| |
| Classic BPF is using BPF_MISC class to represent A = X and X = A moves. |
| eBPF is using BPF_MOV | BPF_X | BPF_ALU code instead. Since there are no |
| BPF_MISC operations in eBPF, the class 7 is used as BPF_ALU64 to mean |
| exactly the same operations as BPF_ALU, but with 64-bit wide operands |
| instead. So BPF_ADD | BPF_X | BPF_ALU64 means 64-bit addition, i.e.: |
| dst_reg = dst_reg + src_reg |
| |
| Classic BPF wastes the whole BPF_RET class to represent a single 'ret' |
| operation. Classic BPF_RET | BPF_K means copy imm32 into return register |
| and perform function exit. eBPF is modeled to match CPU, so BPF_JMP | BPF_EXIT |
| in eBPF means function exit only. The eBPF program needs to store return |
| value into register R0 before doing a BPF_EXIT. Class 6 in eBPF is currently |
| unused and reserved for future use. |
| |
| For load and store instructions the 8-bit 'code' field is divided as: |
| |
| +--------+--------+-------------------+ |
| | 3 bits | 2 bits | 3 bits | |
| | mode | size | instruction class | |
| +--------+--------+-------------------+ |
| (MSB) (LSB) |
| |
| Size modifier is one of ... |
| |
| BPF_W 0x00 /* word */ |
| BPF_H 0x08 /* half word */ |
| BPF_B 0x10 /* byte */ |
| BPF_DW 0x18 /* eBPF only, double word */ |
| |
| ... which encodes size of load/store operation: |
| |
| B - 1 byte |
| H - 2 byte |
| W - 4 byte |
| DW - 8 byte (eBPF only) |
| |
| Mode modifier is one of: |
| |
| BPF_IMM 0x00 /* used for 32-bit mov in classic BPF and 64-bit in eBPF */ |
| BPF_ABS 0x20 |
| BPF_IND 0x40 |
| BPF_MEM 0x60 |
| BPF_LEN 0x80 /* classic BPF only, reserved in eBPF */ |
| BPF_MSH 0xa0 /* classic BPF only, reserved in eBPF */ |
| BPF_XADD 0xc0 /* eBPF only, exclusive add */ |
| |
| eBPF has two non-generic instructions: (BPF_ABS | <size> | BPF_LD) and |
| (BPF_IND | <size> | BPF_LD) which are used to access packet data. |
| |
| They had to be carried over from classic to have strong performance of |
| socket filters running in eBPF interpreter. These instructions can only |
| be used when interpreter context is a pointer to 'struct sk_buff' and |
| have seven implicit operands. Register R6 is an implicit input that must |
| contain pointer to sk_buff. Register R0 is an implicit output which contains |
| the data fetched from the packet. Registers R1-R5 are scratch registers |
| and must not be used to store the data across BPF_ABS | BPF_LD or |
| BPF_IND | BPF_LD instructions. |
| |
| These instructions have implicit program exit condition as well. When |
| eBPF program is trying to access the data beyond the packet boundary, |
| the interpreter will abort the execution of the program. JIT compilers |
| therefore must preserve this property. src_reg and imm32 fields are |
| explicit inputs to these instructions. |
| |
| For example: |
| |
| BPF_IND | BPF_W | BPF_LD means: |
| |
| R0 = ntohl(*(u32 *) (((struct sk_buff *) R6)->data + src_reg + imm32)) |
| and R1 - R5 were scratched. |
| |
| Unlike classic BPF instruction set, eBPF has generic load/store operations: |
| |
| BPF_MEM | <size> | BPF_STX: *(size *) (dst_reg + off) = src_reg |
| BPF_MEM | <size> | BPF_ST: *(size *) (dst_reg + off) = imm32 |
| BPF_MEM | <size> | BPF_LDX: dst_reg = *(size *) (src_reg + off) |
| BPF_XADD | BPF_W | BPF_STX: lock xadd *(u32 *)(dst_reg + off16) += src_reg |
| BPF_XADD | BPF_DW | BPF_STX: lock xadd *(u64 *)(dst_reg + off16) += src_reg |
| |
| Where size is one of: BPF_B or BPF_H or BPF_W or BPF_DW. Note that 1 and |
| 2 byte atomic increments are not supported. |
| |
| eBPF has one 16-byte instruction: BPF_LD | BPF_DW | BPF_IMM which consists |
| of two consecutive 'struct bpf_insn' 8-byte blocks and interpreted as single |
| instruction that loads 64-bit immediate value into a dst_reg. |
| Classic BPF has similar instruction: BPF_LD | BPF_W | BPF_IMM which loads |
| 32-bit immediate value into a register. |
| |
| eBPF verifier |
| ------------- |
| The safety of the eBPF program is determined in two steps. |
| |
| First step does DAG check to disallow loops and other CFG validation. |
| In particular it will detect programs that have unreachable instructions. |
| (though classic BPF checker allows them) |
| |
| Second step starts from the first insn and descends all possible paths. |
| It simulates execution of every insn and observes the state change of |
| registers and stack. |
| |
| At the start of the program the register R1 contains a pointer to context |
| and has type PTR_TO_CTX. |
| If verifier sees an insn that does R2=R1, then R2 has now type |
| PTR_TO_CTX as well and can be used on the right hand side of expression. |
| If R1=PTR_TO_CTX and insn is R2=R1+R1, then R2=SCALAR_VALUE, |
| since addition of two valid pointers makes invalid pointer. |
| (In 'secure' mode verifier will reject any type of pointer arithmetic to make |
| sure that kernel addresses don't leak to unprivileged users) |
| |
| If register was never written to, it's not readable: |
| bpf_mov R0 = R2 |
| bpf_exit |
| will be rejected, since R2 is unreadable at the start of the program. |
| |
| After kernel function call, R1-R5 are reset to unreadable and |
| R0 has a return type of the function. |
| |
| Since R6-R9 are callee saved, their state is preserved across the call. |
| bpf_mov R6 = 1 |
| bpf_call foo |
| bpf_mov R0 = R6 |
| bpf_exit |
| is a correct program. If there was R1 instead of R6, it would have |
| been rejected. |
| |
| load/store instructions are allowed only with registers of valid types, which |
| are PTR_TO_CTX, PTR_TO_MAP, PTR_TO_STACK. They are bounds and alignment checked. |
| For example: |
| bpf_mov R1 = 1 |
| bpf_mov R2 = 2 |
| bpf_xadd *(u32 *)(R1 + 3) += R2 |
| bpf_exit |
| will be rejected, since R1 doesn't have a valid pointer type at the time of |
| execution of instruction bpf_xadd. |
| |
| At the start R1 type is PTR_TO_CTX (a pointer to generic 'struct bpf_context') |
| A callback is used to customize verifier to restrict eBPF program access to only |
| certain fields within ctx structure with specified size and alignment. |
| |
| For example, the following insn: |
| bpf_ld R0 = *(u32 *)(R6 + 8) |
| intends to load a word from address R6 + 8 and store it into R0 |
| If R6=PTR_TO_CTX, via is_valid_access() callback the verifier will know |
| that offset 8 of size 4 bytes can be accessed for reading, otherwise |
| the verifier will reject the program. |
| If R6=PTR_TO_STACK, then access should be aligned and be within |
| stack bounds, which are [-MAX_BPF_STACK, 0). In this example offset is 8, |
| so it will fail verification, since it's out of bounds. |
| |
| The verifier will allow eBPF program to read data from stack only after |
| it wrote into it. |
| Classic BPF verifier does similar check with M[0-15] memory slots. |
| For example: |
| bpf_ld R0 = *(u32 *)(R10 - 4) |
| bpf_exit |
| is invalid program. |
| Though R10 is correct read-only register and has type PTR_TO_STACK |
| and R10 - 4 is within stack bounds, there were no stores into that location. |
| |
| Pointer register spill/fill is tracked as well, since four (R6-R9) |
| callee saved registers may not be enough for some programs. |
| |
| Allowed function calls are customized with bpf_verifier_ops->get_func_proto() |
| The eBPF verifier will check that registers match argument constraints. |
| After the call register R0 will be set to return type of the function. |
| |
| Function calls is a main mechanism to extend functionality of eBPF programs. |
| Socket filters may let programs to call one set of functions, whereas tracing |
| filters may allow completely different set. |
| |
| If a function made accessible to eBPF program, it needs to be thought through |
| from safety point of view. The verifier will guarantee that the function is |
| called with valid arguments. |
| |
| seccomp vs socket filters have different security restrictions for classic BPF. |
| Seccomp solves this by two stage verifier: classic BPF verifier is followed |
| by seccomp verifier. In case of eBPF one configurable verifier is shared for |
| all use cases. |
| |
| See details of eBPF verifier in kernel/bpf/verifier.c |
| |
| Register value tracking |
| ----------------------- |
| In order to determine the safety of an eBPF program, the verifier must track |
| the range of possible values in each register and also in each stack slot. |
| This is done with 'struct bpf_reg_state', defined in include/linux/ |
| bpf_verifier.h, which unifies tracking of scalar and pointer values. Each |
| register state has a type, which is either NOT_INIT (the register has not been |
| written to), SCALAR_VALUE (some value which is not usable as a pointer), or a |
| pointer type. The types of pointers describe their base, as follows: |
| PTR_TO_CTX Pointer to bpf_context. |
| CONST_PTR_TO_MAP Pointer to struct bpf_map. "Const" because arithmetic |
| on these pointers is forbidden. |
| PTR_TO_MAP_VALUE Pointer to the value stored in a map element. |
| PTR_TO_MAP_VALUE_OR_NULL |
| Either a pointer to a map value, or NULL; map accesses |
| (see section 'eBPF maps', below) return this type, |
| which becomes a PTR_TO_MAP_VALUE when checked != NULL. |
| Arithmetic on these pointers is forbidden. |
| PTR_TO_STACK Frame pointer. |
| PTR_TO_PACKET skb->data. |
| PTR_TO_PACKET_END skb->data + headlen; arithmetic forbidden. |
| However, a pointer may be offset from this base (as a result of pointer |
| arithmetic), and this is tracked in two parts: the 'fixed offset' and 'variable |
| offset'. The former is used when an exactly-known value (e.g. an immediate |
| operand) is added to a pointer, while the latter is used for values which are |
| not exactly known. The variable offset is also used in SCALAR_VALUEs, to track |
| the range of possible values in the register. |
| The verifier's knowledge about the variable offset consists of: |
| * minimum and maximum values as unsigned |
| * minimum and maximum values as signed |
| * knowledge of the values of individual bits, in the form of a 'tnum': a u64 |
| 'mask' and a u64 'value'. 1s in the mask represent bits whose value is unknown; |
| 1s in the value represent bits known to be 1. Bits known to be 0 have 0 in both |
| mask and value; no bit should ever be 1 in both. For example, if a byte is read |
| into a register from memory, the register's top 56 bits are known zero, while |
| the low 8 are unknown - which is represented as the tnum (0x0; 0xff). If we |
| then OR this with 0x40, we get (0x40; 0xbf), then if we add 1 we get (0x0; |
| 0x1ff), because of potential carries. |
| Besides arithmetic, the register state can also be updated by conditional |
| branches. For instance, if a SCALAR_VALUE is compared > 8, in the 'true' branch |
| it will have a umin_value (unsigned minimum value) of 9, whereas in the 'false' |
| branch it will have a umax_value of 8. A signed compare (with BPF_JSGT or |
| BPF_JSGE) would instead update the signed minimum/maximum values. Information |
| from the signed and unsigned bounds can be combined; for instance if a value is |
| first tested < 8 and then tested s> 4, the verifier will conclude that the value |
| is also > 4 and s< 8, since the bounds prevent crossing the sign boundary. |
| PTR_TO_PACKETs with a variable offset part have an 'id', which is common to all |
| pointers sharing that same variable offset. This is important for packet range |
| checks: after adding some variable to a packet pointer, if you then copy it to |
| another register and (say) add a constant 4, both registers will share the same |
| 'id' but one will have a fixed offset of +4. Then if it is bounds-checked and |
| found to be less than a PTR_TO_PACKET_END, the other register is now known to |
| have a safe range of at least 4 bytes. See 'Direct packet access', below, for |
| more on PTR_TO_PACKET ranges. |
| The 'id' field is also used on PTR_TO_MAP_VALUE_OR_NULL, common to all copies of |
| the pointer returned from a map lookup. This means that when one copy is |
| checked and found to be non-NULL, all copies can become PTR_TO_MAP_VALUEs. |
| As well as range-checking, the tracked information is also used for enforcing |
| alignment of pointer accesses. For instance, on most systems the packet pointer |
| is 2 bytes after a 4-byte alignment. If a program adds 14 bytes to that to jump |
| over the Ethernet header, then reads IHL and addes (IHL * 4), the resulting |
| pointer will have a variable offset known to be 4n+2 for some n, so adding the 2 |
| bytes (NET_IP_ALIGN) gives a 4-byte alignment and so word-sized accesses through |
| that pointer are safe. |
| |
| Direct packet access |
| -------------------- |
| In cls_bpf and act_bpf programs the verifier allows direct access to the packet |
| data via skb->data and skb->data_end pointers. |
| Ex: |
| 1: r4 = *(u32 *)(r1 +80) /* load skb->data_end */ |
| 2: r3 = *(u32 *)(r1 +76) /* load skb->data */ |
| 3: r5 = r3 |
| 4: r5 += 14 |
| 5: if r5 > r4 goto pc+16 |
| R1=ctx R3=pkt(id=0,off=0,r=14) R4=pkt_end R5=pkt(id=0,off=14,r=14) R10=fp |
| 6: r0 = *(u16 *)(r3 +12) /* access 12 and 13 bytes of the packet */ |
| |
| this 2byte load from the packet is safe to do, since the program author |
| did check 'if (skb->data + 14 > skb->data_end) goto err' at insn #5 which |
| means that in the fall-through case the register R3 (which points to skb->data) |
| has at least 14 directly accessible bytes. The verifier marks it |
| as R3=pkt(id=0,off=0,r=14). |
| id=0 means that no additional variables were added to the register. |
| off=0 means that no additional constants were added. |
| r=14 is the range of safe access which means that bytes [R3, R3 + 14) are ok. |
| Note that R5 is marked as R5=pkt(id=0,off=14,r=14). It also points |
| to the packet data, but constant 14 was added to the register, so |
| it now points to 'skb->data + 14' and accessible range is [R5, R5 + 14 - 14) |
| which is zero bytes. |
| |
| More complex packet access may look like: |
| R0=inv1 R1=ctx R3=pkt(id=0,off=0,r=14) R4=pkt_end R5=pkt(id=0,off=14,r=14) R10=fp |
| 6: r0 = *(u8 *)(r3 +7) /* load 7th byte from the packet */ |
| 7: r4 = *(u8 *)(r3 +12) |
| 8: r4 *= 14 |
| 9: r3 = *(u32 *)(r1 +76) /* load skb->data */ |
| 10: r3 += r4 |
| 11: r2 = r1 |
| 12: r2 <<= 48 |
| 13: r2 >>= 48 |
| 14: r3 += r2 |
| 15: r2 = r3 |
| 16: r2 += 8 |
| 17: r1 = *(u32 *)(r1 +80) /* load skb->data_end */ |
| 18: if r2 > r1 goto pc+2 |
| R0=inv(id=0,umax_value=255,var_off=(0x0; 0xff)) R1=pkt_end R2=pkt(id=2,off=8,r=8) R3=pkt(id=2,off=0,r=8) R4=inv(id=0,umax_value=3570,var_off=(0x0; 0xfffe)) R5=pkt(id=0,off=14,r=14) R10=fp |
| 19: r1 = *(u8 *)(r3 +4) |
| The state of the register R3 is R3=pkt(id=2,off=0,r=8) |
| id=2 means that two 'r3 += rX' instructions were seen, so r3 points to some |
| offset within a packet and since the program author did |
| 'if (r3 + 8 > r1) goto err' at insn #18, the safe range is [R3, R3 + 8). |
| The verifier only allows 'add'/'sub' operations on packet registers. Any other |
| operation will set the register state to 'SCALAR_VALUE' and it won't be |
| available for direct packet access. |
| Operation 'r3 += rX' may overflow and become less than original skb->data, |
| therefore the verifier has to prevent that. So when it sees 'r3 += rX' |
| instruction and rX is more than 16-bit value, any subsequent bounds-check of r3 |
| against skb->data_end will not give us 'range' information, so attempts to read |
| through the pointer will give "invalid access to packet" error. |
| Ex. after insn 'r4 = *(u8 *)(r3 +12)' (insn #7 above) the state of r4 is |
| R4=inv(id=0,umax_value=255,var_off=(0x0; 0xff)) which means that upper 56 bits |
| of the register are guaranteed to be zero, and nothing is known about the lower |
| 8 bits. After insn 'r4 *= 14' the state becomes |
| R4=inv(id=0,umax_value=3570,var_off=(0x0; 0xfffe)), since multiplying an 8-bit |
| value by constant 14 will keep upper 52 bits as zero, also the least significant |
| bit will be zero as 14 is even. Similarly 'r2 >>= 48' will make |
| R2=inv(id=0,umax_value=65535,var_off=(0x0; 0xffff)), since the shift is not sign |
| extending. This logic is implemented in adjust_reg_min_max_vals() function, |
| which calls adjust_ptr_min_max_vals() for adding pointer to scalar (or vice |
| versa) and adjust_scalar_min_max_vals() for operations on two scalars. |
| |
| The end result is that bpf program author can access packet directly |
| using normal C code as: |
| void *data = (void *)(long)skb->data; |
| void *data_end = (void *)(long)skb->data_end; |
| struct eth_hdr *eth = data; |
| struct iphdr *iph = data + sizeof(*eth); |
| struct udphdr *udp = data + sizeof(*eth) + sizeof(*iph); |
| |
| if (data + sizeof(*eth) + sizeof(*iph) + sizeof(*udp) > data_end) |
| return 0; |
| if (eth->h_proto != htons(ETH_P_IP)) |
| return 0; |
| if (iph->protocol != IPPROTO_UDP || iph->ihl != 5) |
| return 0; |
| if (udp->dest == 53 || udp->source == 9) |
| ...; |
| which makes such programs easier to write comparing to LD_ABS insn |
| and significantly faster. |
| |
| eBPF maps |
| --------- |
| 'maps' is a generic storage of different types for sharing data between kernel |
| and userspace. |
| |
| The maps are accessed from user space via BPF syscall, which has commands: |
| - create a map with given type and attributes |
| map_fd = bpf(BPF_MAP_CREATE, union bpf_attr *attr, u32 size) |
| using attr->map_type, attr->key_size, attr->value_size, attr->max_entries |
| returns process-local file descriptor or negative error |
| |
| - lookup key in a given map |
| err = bpf(BPF_MAP_LOOKUP_ELEM, union bpf_attr *attr, u32 size) |
| using attr->map_fd, attr->key, attr->value |
| returns zero and stores found elem into value or negative error |
| |
| - create or update key/value pair in a given map |
| err = bpf(BPF_MAP_UPDATE_ELEM, union bpf_attr *attr, u32 size) |
| using attr->map_fd, attr->key, attr->value |
| returns zero or negative error |
| |
| - find and delete element by key in a given map |
| err = bpf(BPF_MAP_DELETE_ELEM, union bpf_attr *attr, u32 size) |
| using attr->map_fd, attr->key |
| |
| - to delete map: close(fd) |
| Exiting process will delete maps automatically |
| |
| userspace programs use this syscall to create/access maps that eBPF programs |
| are concurrently updating. |
| |
| maps can have different types: hash, array, bloom filter, radix-tree, etc. |
| |
| The map is defined by: |
| . type |
| . max number of elements |
| . key size in bytes |
| . value size in bytes |
| |
| Pruning |
| ------- |
| The verifier does not actually walk all possible paths through the program. For |
| each new branch to analyse, the verifier looks at all the states it's previously |
| been in when at this instruction. If any of them contain the current state as a |
| subset, the branch is 'pruned' - that is, the fact that the previous state was |
| accepted implies the current state would be as well. For instance, if in the |
| previous state, r1 held a packet-pointer, and in the current state, r1 holds a |
| packet-pointer with a range as long or longer and at least as strict an |
| alignment, then r1 is safe. Similarly, if r2 was NOT_INIT before then it can't |
| have been used by any path from that point, so any value in r2 (including |
| another NOT_INIT) is safe. The implementation is in the function regsafe(). |
| Pruning considers not only the registers but also the stack (and any spilled |
| registers it may hold). They must all be safe for the branch to be pruned. |
| This is implemented in states_equal(). |
| |
| Understanding eBPF verifier messages |
| ------------------------------------ |
| |
| The following are few examples of invalid eBPF programs and verifier error |
| messages as seen in the log: |
| |
| Program with unreachable instructions: |
| static struct bpf_insn prog[] = { |
| BPF_EXIT_INSN(), |
| BPF_EXIT_INSN(), |
| }; |
| Error: |
| unreachable insn 1 |
| |
| Program that reads uninitialized register: |
| BPF_MOV64_REG(BPF_REG_0, BPF_REG_2), |
| BPF_EXIT_INSN(), |
| Error: |
| 0: (bf) r0 = r2 |
| R2 !read_ok |
| |
| Program that doesn't initialize R0 before exiting: |
| BPF_MOV64_REG(BPF_REG_2, BPF_REG_1), |
| BPF_EXIT_INSN(), |
| Error: |
| 0: (bf) r2 = r1 |
| 1: (95) exit |
| R0 !read_ok |
| |
| Program that accesses stack out of bounds: |
| BPF_ST_MEM(BPF_DW, BPF_REG_10, 8, 0), |
| BPF_EXIT_INSN(), |
| Error: |
| 0: (7a) *(u64 *)(r10 +8) = 0 |
| invalid stack off=8 size=8 |
| |
| Program that doesn't initialize stack before passing its address into function: |
| BPF_MOV64_REG(BPF_REG_2, BPF_REG_10), |
| BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8), |
| BPF_LD_MAP_FD(BPF_REG_1, 0), |
| BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem), |
| BPF_EXIT_INSN(), |
| Error: |
| 0: (bf) r2 = r10 |
| 1: (07) r2 += -8 |
| 2: (b7) r1 = 0x0 |
| 3: (85) call 1 |
| invalid indirect read from stack off -8+0 size 8 |
| |
| Program that uses invalid map_fd=0 while calling to map_lookup_elem() function: |
| BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0), |
| BPF_MOV64_REG(BPF_REG_2, BPF_REG_10), |
| BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8), |
| BPF_LD_MAP_FD(BPF_REG_1, 0), |
| BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem), |
| BPF_EXIT_INSN(), |
| Error: |
| 0: (7a) *(u64 *)(r10 -8) = 0 |
| 1: (bf) r2 = r10 |
| 2: (07) r2 += -8 |
| 3: (b7) r1 = 0x0 |
| 4: (85) call 1 |
| fd 0 is not pointing to valid bpf_map |
| |
| Program that doesn't check return value of map_lookup_elem() before accessing |
| map element: |
| BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0), |
| BPF_MOV64_REG(BPF_REG_2, BPF_REG_10), |
| BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8), |
| BPF_LD_MAP_FD(BPF_REG_1, 0), |
| BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem), |
| BPF_ST_MEM(BPF_DW, BPF_REG_0, 0, 0), |
| BPF_EXIT_INSN(), |
| Error: |
| 0: (7a) *(u64 *)(r10 -8) = 0 |
| 1: (bf) r2 = r10 |
| 2: (07) r2 += -8 |
| 3: (b7) r1 = 0x0 |
| 4: (85) call 1 |
| 5: (7a) *(u64 *)(r0 +0) = 0 |
| R0 invalid mem access 'map_value_or_null' |
| |
| Program that correctly checks map_lookup_elem() returned value for NULL, but |
| accesses the memory with incorrect alignment: |
| BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0), |
| BPF_MOV64_REG(BPF_REG_2, BPF_REG_10), |
| BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8), |
| BPF_LD_MAP_FD(BPF_REG_1, 0), |
| BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem), |
| BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 1), |
| BPF_ST_MEM(BPF_DW, BPF_REG_0, 4, 0), |
| BPF_EXIT_INSN(), |
| Error: |
| 0: (7a) *(u64 *)(r10 -8) = 0 |
| 1: (bf) r2 = r10 |
| 2: (07) r2 += -8 |
| 3: (b7) r1 = 1 |
| 4: (85) call 1 |
| 5: (15) if r0 == 0x0 goto pc+1 |
| R0=map_ptr R10=fp |
| 6: (7a) *(u64 *)(r0 +4) = 0 |
| misaligned access off 4 size 8 |
| |
| Program that correctly checks map_lookup_elem() returned value for NULL and |
| accesses memory with correct alignment in one side of 'if' branch, but fails |
| to do so in the other side of 'if' branch: |
| BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0), |
| BPF_MOV64_REG(BPF_REG_2, BPF_REG_10), |
| BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8), |
| BPF_LD_MAP_FD(BPF_REG_1, 0), |
| BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem), |
| BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 2), |
| BPF_ST_MEM(BPF_DW, BPF_REG_0, 0, 0), |
| BPF_EXIT_INSN(), |
| BPF_ST_MEM(BPF_DW, BPF_REG_0, 0, 1), |
| BPF_EXIT_INSN(), |
| Error: |
| 0: (7a) *(u64 *)(r10 -8) = 0 |
| 1: (bf) r2 = r10 |
| 2: (07) r2 += -8 |
| 3: (b7) r1 = 1 |
| 4: (85) call 1 |
| 5: (15) if r0 == 0x0 goto pc+2 |
| R0=map_ptr R10=fp |
| 6: (7a) *(u64 *)(r0 +0) = 0 |
| 7: (95) exit |
| |
| from 5 to 8: R0=imm0 R10=fp |
| 8: (7a) *(u64 *)(r0 +0) = 1 |
| R0 invalid mem access 'imm' |
| |
| Testing |
| ------- |
| |
| Next to the BPF toolchain, the kernel also ships a test module that contains |
| various test cases for classic and internal BPF that can be executed against |
| the BPF interpreter and JIT compiler. It can be found in lib/test_bpf.c and |
| enabled via Kconfig: |
| |
| CONFIG_TEST_BPF=m |
| |
| After the module has been built and installed, the test suite can be executed |
| via insmod or modprobe against 'test_bpf' module. Results of the test cases |
| including timings in nsec can be found in the kernel log (dmesg). |
| |
| Misc |
| ---- |
| |
| Also trinity, the Linux syscall fuzzer, has built-in support for BPF and |
| SECCOMP-BPF kernel fuzzing. |
| |
| Written by |
| ---------- |
| |
| The document was written in the hope that it is found useful and in order |
| to give potential BPF hackers or security auditors a better overview of |
| the underlying architecture. |
| |
| Jay Schulist <jschlst@samba.org> |
| Daniel Borkmann <daniel@iogearbox.net> |
| Alexei Starovoitov <ast@kernel.org> |