| 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/prctl/seccomp_filter.txt |
| |
| 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/net/ 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 k == A |
| jneq 8 Jump on k != A |
| jne 8 Jump on k != A |
| jlt 8 Jump on k < A |
| jle 8 Jump on k <= A |
| jgt 7, 8 Jump on k > A |
| jge 7, 8 Jump on k >= A |
| jset 7, 8 Jump on k & A |
| |
| 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 0, 4 !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->rxhash |
| cpu raw_smp_processor_id() |
| vlan_tci vlan_tx_tag_get(skb) |
| vlan_pr vlan_tx_tag_present(skb) |
| |
| 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 |
| |
| ** 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 */ |
| 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/net/ 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 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/net/, 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. |
| |
| 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 <dborkman@redhat.com> |
