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Diffstat (limited to 'arch/x86/kernel/umip.c')
| -rw-r--r-- | arch/x86/kernel/umip.c | 411 |
1 files changed, 411 insertions, 0 deletions
diff --git a/arch/x86/kernel/umip.c b/arch/x86/kernel/umip.c new file mode 100644 index 000000000..5a4b21389 --- /dev/null +++ b/arch/x86/kernel/umip.c @@ -0,0 +1,411 @@ +/* + * umip.c Emulation for instruction protected by the User-Mode Instruction + * Prevention feature + * + * Copyright (c) 2017, Intel Corporation. + * Ricardo Neri <ricardo.neri-calderon@linux.intel.com> + */ + +#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_GDT_IDT_BASE_SIZE_64BIT 8 +#define UMIP_GDT_IDT_BASE_SIZE_32BIT 4 +#define UMIP_GDT_IDT_LIMIT_SIZE 2 + +#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] = { + [UMIP_INST_SGDT] = "SGDT", + [UMIP_INST_SIDT] = "SIDT", + [UMIP_INST_SMSW] = "SMSW", + [UMIP_INST_SLDT] = "SLDT", + [UMIP_INST_STR] = "STR", +}; + +#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)) + return; + + va_start(args, fmt); + vaf.fmt = fmt; + vaf.va = &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); + va_end(args); +} + +/** + * 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. */ + insn_get_modrm(insn); + + 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: + return UMIP_INST_SGDT; + case 1: + return UMIP_INST_SIDT; + case 4: + return UMIP_INST_SMSW; + default: + return -EINVAL; + } + } else if (insn->opcode.bytes[1] == 0x0) { + if (X86_MODRM_REG(insn->modrm.value) == 0) + return UMIP_INST_SLDT; + else if (X86_MODRM_REG(insn->modrm.value) == 1) + return UMIP_INST_STR; + else + 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 + + * UMIP_GDT_IDT_LIMIT_SIZE bytes. + * + * 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; + else + 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; + else + *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) { +#ifdef CONFIG_MODIFY_LDT_SYSCALL + down_read(¤t->mm->context.ldt_usr_sem); + if (current->mm->context.ldt) + dummy_value = GDT_ENTRY_LDT * 8; + else + dummy_value = 0; + up_read(¤t->mm->context.ldt_usr_sem); +#else + dummy_value = 0; +#endif + } + + /* + * 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; + else + *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))) + return; + + umip_pr_err(regs, "segfault in emulation. error%x\n", + X86_PF_USER | X86_PF_WRITE); +} + +/** + * 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_insns[umip_inst]); + + 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, + user_64bit_mode(regs))) + 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; +} |