blob: 5a4b21389b1d98b4e4fd65149ec1fd0b8fd485a7 [file] [log] [blame]
* umip.c Emulation for instruction protected by the User-Mode Instruction
* Prevention feature
* Copyright (c) 2017, Intel Corporation.
* Ricardo Neri <>
#include <linux/uaccess.h>
#include <asm/umip.h>
#include <asm/traps.h>
#include <asm/insn.h>
#include <asm/insn-eval.h>
#include <linux/ratelimit.h>
#undef pr_fmt
#define pr_fmt(fmt) "umip: " fmt
/** DOC: Emulation for User-Mode Instruction Prevention (UMIP)
* User-Mode Instruction Prevention is a security feature present in recent
* x86 processors that, when enabled, prevents a group of instructions (SGDT,
* SIDT, SLDT, SMSW and STR) from being run in user mode by issuing a general
* protection fault if the instruction is executed with CPL > 0.
* Rather than relaying to the user space the general protection fault caused by
* the UMIP-protected instructions (in the form of a SIGSEGV signal), it can be
* trapped and emulate the result of such instructions to provide dummy values.
* This allows to both conserve the current kernel behavior and not reveal the
* system resources that UMIP intends to protect (i.e., the locations of the
* global descriptor and interrupt descriptor tables, the segment selectors of
* the local descriptor table, the value of the task state register and the
* contents of the CR0 register).
* This emulation is needed because certain applications (e.g., WineHQ and
* DOSEMU2) rely on this subset of instructions to function.
* The instructions protected by UMIP can be split in two groups. Those which
* return a kernel memory address (SGDT and SIDT) and those which return a
* value (SLDT, STR and SMSW).
* For the instructions that return a kernel memory address, applications
* such as WineHQ rely on the result being located in the kernel memory space,
* not the actual location of the table. The result is emulated as a hard-coded
* value that, lies close to the top of the kernel memory. The limit for the GDT
* and the IDT are set to zero.
* The instruction SMSW is emulated to return the value that the register CR0
* has at boot time as set in the head_32.
* SLDT and STR are emulated to return the values that the kernel programmatically
* assigns:
* - SLDT returns (GDT_ENTRY_LDT * 8) if an LDT has been set, 0 if not.
* - STR returns (GDT_ENTRY_TSS * 8).
* Emulation is provided for both 32-bit and 64-bit processes.
* Care is taken to appropriately emulate the results when segmentation is
* used. That is, rather than relying on USER_DS and USER_CS, the function
* insn_get_addr_ref() inspects the segment descriptor pointed by the
* registers in pt_regs. This ensures that we correctly obtain the segment
* base address and the address and operand sizes even if the user space
* application uses a local descriptor table.
#define UMIP_DUMMY_GDT_BASE 0xfffffffffffe0000ULL
#define UMIP_DUMMY_IDT_BASE 0xffffffffffff0000ULL
* The SGDT and SIDT instructions store the contents of the global descriptor
* table and interrupt table registers, respectively. The destination is a
* memory operand of X+2 bytes. X bytes are used to store the base address of
* the table and 2 bytes are used to store the limit. In 32-bit processes X
* has a value of 4, in 64-bit processes X has a value of 8.
#define UMIP_INST_SGDT 0 /* 0F 01 /0 */
#define UMIP_INST_SIDT 1 /* 0F 01 /1 */
#define UMIP_INST_SMSW 2 /* 0F 01 /4 */
#define UMIP_INST_SLDT 3 /* 0F 00 /0 */
#define UMIP_INST_STR 4 /* 0F 00 /1 */
static const char * const umip_insns[5] = {
#define umip_pr_err(regs, fmt, ...) \
umip_printk(regs, KERN_ERR, fmt, ##__VA_ARGS__)
#define umip_pr_debug(regs, fmt, ...) \
umip_printk(regs, KERN_DEBUG, fmt, ##__VA_ARGS__)
* umip_printk() - Print a rate-limited message
* @regs: Register set with the context in which the warning is printed
* @log_level: Kernel log level to print the message
* @fmt: The text string to print
* Print the text contained in @fmt. The print rate is limited to bursts of 5
* messages every two minutes. The purpose of this customized version of
* printk() is to print messages when user space processes use any of the
* UMIP-protected instructions. Thus, the printed text is prepended with the
* task name and process ID number of the current task as well as the
* instruction and stack pointers in @regs as seen when entering kernel mode.
* Returns:
* None.
static __printf(3, 4)
void umip_printk(const struct pt_regs *regs, const char *log_level,
const char *fmt, ...)
/* Bursts of 5 messages every two minutes */
static DEFINE_RATELIMIT_STATE(ratelimit, 2 * 60 * HZ, 5);
struct task_struct *tsk = current;
struct va_format vaf;
va_list args;
if (!__ratelimit(&ratelimit))
va_start(args, fmt);
vaf.fmt = fmt; = &args;
printk("%s" pr_fmt("%s[%d] ip:%lx sp:%lx: %pV"), log_level, tsk->comm,
task_pid_nr(tsk), regs->ip, regs->sp, &vaf);
* identify_insn() - Identify a UMIP-protected instruction
* @insn: Instruction structure with opcode and ModRM byte.
* From the opcode and ModRM.reg in @insn identify, if any, a UMIP-protected
* instruction that can be emulated.
* Returns:
* On success, a constant identifying a specific UMIP-protected instruction that
* can be emulated.
* -EINVAL on error or when not an UMIP-protected instruction that can be
* emulated.
static int identify_insn(struct insn *insn)
/* By getting modrm we also get the opcode. */
if (!insn->modrm.nbytes)
return -EINVAL;
/* All the instructions of interest start with 0x0f. */
if (insn->opcode.bytes[0] != 0xf)
return -EINVAL;
if (insn->opcode.bytes[1] == 0x1) {
switch (X86_MODRM_REG(insn->modrm.value)) {
case 0:
case 1:
case 4:
return -EINVAL;
} else if (insn->opcode.bytes[1] == 0x0) {
if (X86_MODRM_REG(insn->modrm.value) == 0)
else if (X86_MODRM_REG(insn->modrm.value) == 1)
return -EINVAL;
} else {
return -EINVAL;
* emulate_umip_insn() - Emulate UMIP instructions and return dummy values
* @insn: Instruction structure with operands
* @umip_inst: A constant indicating the instruction to emulate
* @data: Buffer into which the dummy result is stored
* @data_size: Size of the emulated result
* @x86_64: true if process is 64-bit, false otherwise
* Emulate an instruction protected by UMIP and provide a dummy result. The
* result of the emulation is saved in @data. The size of the results depends
* on both the instruction and type of operand (register vs memory address).
* The size of the result is updated in @data_size. Caller is responsible
* of providing a @data buffer of at least UMIP_GDT_IDT_BASE_SIZE +
* Returns:
* 0 on success, -EINVAL on error while emulating.
static int emulate_umip_insn(struct insn *insn, int umip_inst,
unsigned char *data, int *data_size, bool x86_64)
if (!data || !data_size || !insn)
return -EINVAL;
* These two instructions return the base address and limit of the
* global and interrupt descriptor table, respectively. According to the
* Intel Software Development manual, the base address can be 24-bit,
* 32-bit or 64-bit. Limit is always 16-bit. If the operand size is
* 16-bit, the returned value of the base address is supposed to be a
* zero-extended 24-byte number. However, it seems that a 32-byte number
* is always returned irrespective of the operand size.
if (umip_inst == UMIP_INST_SGDT || umip_inst == UMIP_INST_SIDT) {
u64 dummy_base_addr;
u16 dummy_limit = 0;
/* SGDT and SIDT do not use registers operands. */
if (X86_MODRM_MOD(insn->modrm.value) == 3)
return -EINVAL;
if (umip_inst == UMIP_INST_SGDT)
dummy_base_addr = UMIP_DUMMY_GDT_BASE;
dummy_base_addr = UMIP_DUMMY_IDT_BASE;
* 64-bit processes use the entire dummy base address.
* 32-bit processes use the lower 32 bits of the base address.
* dummy_base_addr is always 64 bits, but we memcpy the correct
* number of bytes from it to the destination.
if (x86_64)
*data_size = UMIP_GDT_IDT_BASE_SIZE_64BIT;
*data_size = UMIP_GDT_IDT_BASE_SIZE_32BIT;
memcpy(data + 2, &dummy_base_addr, *data_size);
*data_size += UMIP_GDT_IDT_LIMIT_SIZE;
memcpy(data, &dummy_limit, UMIP_GDT_IDT_LIMIT_SIZE);
} else if (umip_inst == UMIP_INST_SMSW || umip_inst == UMIP_INST_SLDT ||
umip_inst == UMIP_INST_STR) {
unsigned long dummy_value;
if (umip_inst == UMIP_INST_SMSW) {
dummy_value = CR0_STATE;
} else if (umip_inst == UMIP_INST_STR) {
dummy_value = GDT_ENTRY_TSS * 8;
} else if (umip_inst == UMIP_INST_SLDT) {
if (current->mm->context.ldt)
dummy_value = GDT_ENTRY_LDT * 8;
dummy_value = 0;
dummy_value = 0;
* For these 3 instructions, the number
* of bytes to be copied in the result buffer is determined
* by whether the operand is a register or a memory location.
* If operand is a register, return as many bytes as the operand
* size. If operand is memory, return only the two least
* significant bytes.
if (X86_MODRM_MOD(insn->modrm.value) == 3)
*data_size = insn->opnd_bytes;
*data_size = 2;
memcpy(data, &dummy_value, *data_size);
} else {
return -EINVAL;
return 0;
* force_sig_info_umip_fault() - Force a SIGSEGV with SEGV_MAPERR
* @addr: Address that caused the signal
* @regs: Register set containing the instruction pointer
* Force a SIGSEGV signal with SEGV_MAPERR as the error code. This function is
* intended to be used to provide a segmentation fault when the result of the
* UMIP emulation could not be copied to the user space memory.
* Returns: none
static void force_sig_info_umip_fault(void __user *addr, struct pt_regs *regs)
struct task_struct *tsk = current;
tsk->thread.cr2 = (unsigned long)addr;
tsk->thread.error_code = X86_PF_USER | X86_PF_WRITE;
tsk->thread.trap_nr = X86_TRAP_PF;
force_sig_fault(SIGSEGV, SEGV_MAPERR, addr);
if (!(show_unhandled_signals && unhandled_signal(tsk, SIGSEGV)))
umip_pr_err(regs, "segfault in emulation. error%x\n",
* fixup_umip_exception() - Fixup a general protection fault caused by UMIP
* @regs: Registers as saved when entering the #GP handler
* The instructions SGDT, SIDT, STR, SMSW and SLDT cause a general protection
* fault if executed with CPL > 0 (i.e., from user space). This function fixes
* the exception up and provides dummy results for SGDT, SIDT and SMSW; STR
* and SLDT are not fixed up.
* If operands are memory addresses, results are copied to user-space memory as
* indicated by the instruction pointed by eIP using the registers indicated in
* the instruction operands. If operands are registers, results are copied into
* the context that was saved when entering kernel mode.
* Returns:
* True if emulation was successful; false if not.
bool fixup_umip_exception(struct pt_regs *regs)
int nr_copied, reg_offset, dummy_data_size, umip_inst;
/* 10 bytes is the maximum size of the result of UMIP instructions */
unsigned char dummy_data[10] = { 0 };
unsigned char buf[MAX_INSN_SIZE];
unsigned long *reg_addr;
void __user *uaddr;
struct insn insn;
if (!regs)
return false;
* Give up on emulation if fetching the instruction failed. Should a
* page fault or a #GP be issued?
nr_copied = insn_fetch_from_user(regs, buf);
if (nr_copied <= 0)
return false;
if (!insn_decode_from_regs(&insn, regs, buf, nr_copied))
return false;
umip_inst = identify_insn(&insn);
if (umip_inst < 0)
return false;
umip_pr_debug(regs, "%s instruction cannot be used by applications.\n",
umip_pr_debug(regs, "For now, expensive software emulation returns the result.\n");
if (emulate_umip_insn(&insn, umip_inst, dummy_data, &dummy_data_size,
return false;
* If operand is a register, write result to the copy of the register
* value that was pushed to the stack when entering into kernel mode.
* Upon exit, the value we write will be restored to the actual hardware
* register.
if (X86_MODRM_MOD(insn.modrm.value) == 3) {
reg_offset = insn_get_modrm_rm_off(&insn, regs);
* Negative values are usually errors. In memory addressing,
* the exception is -EDOM. Since we expect a register operand,
* all negative values are errors.
if (reg_offset < 0)
return false;
reg_addr = (unsigned long *)((unsigned long)regs + reg_offset);
memcpy(reg_addr, dummy_data, dummy_data_size);
} else {
uaddr = insn_get_addr_ref(&insn, regs);
if ((unsigned long)uaddr == -1L)
return false;
nr_copied = copy_to_user(uaddr, dummy_data, dummy_data_size);
if (nr_copied > 0) {
* If copy fails, send a signal and tell caller that
* fault was fixed up.
force_sig_info_umip_fault(uaddr, regs);
return true;
/* increase IP to let the program keep going */
regs->ip += insn.length;
return true;