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Diffstat (limited to 'tools/testing/selftests/x86/sigreturn.c')
| -rw-r--r-- | tools/testing/selftests/x86/sigreturn.c | 876 |
1 files changed, 876 insertions, 0 deletions
diff --git a/tools/testing/selftests/x86/sigreturn.c b/tools/testing/selftests/x86/sigreturn.c new file mode 100644 index 000000000..57c4f67f1 --- /dev/null +++ b/tools/testing/selftests/x86/sigreturn.c @@ -0,0 +1,876 @@ +// SPDX-License-Identifier: GPL-2.0-only +/* + * sigreturn.c - tests for x86 sigreturn(2) and exit-to-userspace + * Copyright (c) 2014-2015 Andrew Lutomirski + * + * This is a series of tests that exercises the sigreturn(2) syscall and + * the IRET / SYSRET paths in the kernel. + * + * For now, this focuses on the effects of unusual CS and SS values, + * and it has a bunch of tests to make sure that ESP/RSP is restored + * properly. + * + * The basic idea behind these tests is to raise(SIGUSR1) to create a + * sigcontext frame, plug in the values to be tested, and then return, + * which implicitly invokes sigreturn(2) and programs the user context + * as desired. + * + * For tests for which we expect sigreturn and the subsequent return to + * user mode to succeed, we return to a short trampoline that generates + * SIGTRAP so that the meat of the tests can be ordinary C code in a + * SIGTRAP handler. + * + * The inner workings of each test is documented below. + * + * Do not run on outdated, unpatched kernels at risk of nasty crashes. + */ + +#define _GNU_SOURCE + +#include <sys/time.h> +#include <time.h> +#include <stdlib.h> +#include <sys/syscall.h> +#include <unistd.h> +#include <stdio.h> +#include <string.h> +#include <inttypes.h> +#include <sys/mman.h> +#include <sys/signal.h> +#include <sys/ucontext.h> +#include <asm/ldt.h> +#include <err.h> +#include <setjmp.h> +#include <stddef.h> +#include <stdbool.h> +#include <sys/ptrace.h> +#include <sys/user.h> + +/* Pull in AR_xyz defines. */ +typedef unsigned int u32; +typedef unsigned short u16; +#include "../../../../arch/x86/include/asm/desc_defs.h" + +/* + * Copied from asm/ucontext.h, as asm/ucontext.h conflicts badly with the glibc + * headers. + */ +#ifdef __x86_64__ +/* + * UC_SIGCONTEXT_SS will be set when delivering 64-bit or x32 signals on + * kernels that save SS in the sigcontext. All kernels that set + * UC_SIGCONTEXT_SS will correctly restore at least the low 32 bits of esp + * regardless of SS (i.e. they implement espfix). + * + * Kernels that set UC_SIGCONTEXT_SS will also set UC_STRICT_RESTORE_SS + * when delivering a signal that came from 64-bit code. + * + * Sigreturn restores SS as follows: + * + * if (saved SS is valid || UC_STRICT_RESTORE_SS is set || + * saved CS is not 64-bit) + * new SS = saved SS (will fail IRET and signal if invalid) + * else + * new SS = a flat 32-bit data segment + */ +#define UC_SIGCONTEXT_SS 0x2 +#define UC_STRICT_RESTORE_SS 0x4 +#endif + +/* + * In principle, this test can run on Linux emulation layers (e.g. + * Illumos "LX branded zones"). Solaris-based kernels reserve LDT + * entries 0-5 for their own internal purposes, so start our LDT + * allocations above that reservation. (The tests don't pass on LX + * branded zones, but at least this lets them run.) + */ +#define LDT_OFFSET 6 + +/* An aligned stack accessible through some of our segments. */ +static unsigned char stack16[65536] __attribute__((aligned(4096))); + +/* + * An aligned int3 instruction used as a trampoline. Some of the tests + * want to fish out their ss values, so this trampoline copies ss to eax + * before the int3. + */ +asm (".pushsection .text\n\t" + ".type int3, @function\n\t" + ".align 4096\n\t" + "int3:\n\t" + "mov %ss,%ecx\n\t" + "int3\n\t" + ".size int3, . - int3\n\t" + ".align 4096, 0xcc\n\t" + ".popsection"); +extern char int3[4096]; + +/* + * At startup, we prepapre: + * + * - ldt_nonexistent_sel: An LDT entry that doesn't exist (all-zero + * descriptor or out of bounds). + * - code16_sel: A 16-bit LDT code segment pointing to int3. + * - data16_sel: A 16-bit LDT data segment pointing to stack16. + * - npcode32_sel: A 32-bit not-present LDT code segment pointing to int3. + * - npdata32_sel: A 32-bit not-present LDT data segment pointing to stack16. + * - gdt_data16_idx: A 16-bit GDT data segment pointing to stack16. + * - gdt_npdata32_idx: A 32-bit not-present GDT data segment pointing to + * stack16. + * + * For no particularly good reason, xyz_sel is a selector value with the + * RPL and LDT bits filled in, whereas xyz_idx is just an index into the + * descriptor table. These variables will be zero if their respective + * segments could not be allocated. + */ +static unsigned short ldt_nonexistent_sel; +static unsigned short code16_sel, data16_sel, npcode32_sel, npdata32_sel; + +static unsigned short gdt_data16_idx, gdt_npdata32_idx; + +static unsigned short GDT3(int idx) +{ + return (idx << 3) | 3; +} + +static unsigned short LDT3(int idx) +{ + return (idx << 3) | 7; +} + +/* Our sigaltstack scratch space. */ +static char altstack_data[SIGSTKSZ]; + +static void sethandler(int sig, void (*handler)(int, siginfo_t *, void *), + int flags) +{ + struct sigaction sa; + memset(&sa, 0, sizeof(sa)); + sa.sa_sigaction = handler; + sa.sa_flags = SA_SIGINFO | flags; + sigemptyset(&sa.sa_mask); + if (sigaction(sig, &sa, 0)) + err(1, "sigaction"); +} + +static void clearhandler(int sig) +{ + struct sigaction sa; + memset(&sa, 0, sizeof(sa)); + sa.sa_handler = SIG_DFL; + sigemptyset(&sa.sa_mask); + if (sigaction(sig, &sa, 0)) + err(1, "sigaction"); +} + +static void add_ldt(const struct user_desc *desc, unsigned short *var, + const char *name) +{ + if (syscall(SYS_modify_ldt, 1, desc, sizeof(*desc)) == 0) { + *var = LDT3(desc->entry_number); + } else { + printf("[NOTE]\tFailed to create %s segment\n", name); + *var = 0; + } +} + +static void setup_ldt(void) +{ + if ((unsigned long)stack16 > (1ULL << 32) - sizeof(stack16)) + errx(1, "stack16 is too high\n"); + if ((unsigned long)int3 > (1ULL << 32) - sizeof(int3)) + errx(1, "int3 is too high\n"); + + ldt_nonexistent_sel = LDT3(LDT_OFFSET + 2); + + const struct user_desc code16_desc = { + .entry_number = LDT_OFFSET + 0, + .base_addr = (unsigned long)int3, + .limit = 4095, + .seg_32bit = 0, + .contents = 2, /* Code, not conforming */ + .read_exec_only = 0, + .limit_in_pages = 0, + .seg_not_present = 0, + .useable = 0 + }; + add_ldt(&code16_desc, &code16_sel, "code16"); + + const struct user_desc data16_desc = { + .entry_number = LDT_OFFSET + 1, + .base_addr = (unsigned long)stack16, + .limit = 0xffff, + .seg_32bit = 0, + .contents = 0, /* Data, grow-up */ + .read_exec_only = 0, + .limit_in_pages = 0, + .seg_not_present = 0, + .useable = 0 + }; + add_ldt(&data16_desc, &data16_sel, "data16"); + + const struct user_desc npcode32_desc = { + .entry_number = LDT_OFFSET + 3, + .base_addr = (unsigned long)int3, + .limit = 4095, + .seg_32bit = 1, + .contents = 2, /* Code, not conforming */ + .read_exec_only = 0, + .limit_in_pages = 0, + .seg_not_present = 1, + .useable = 0 + }; + add_ldt(&npcode32_desc, &npcode32_sel, "npcode32"); + + const struct user_desc npdata32_desc = { + .entry_number = LDT_OFFSET + 4, + .base_addr = (unsigned long)stack16, + .limit = 0xffff, + .seg_32bit = 1, + .contents = 0, /* Data, grow-up */ + .read_exec_only = 0, + .limit_in_pages = 0, + .seg_not_present = 1, + .useable = 0 + }; + add_ldt(&npdata32_desc, &npdata32_sel, "npdata32"); + + struct user_desc gdt_data16_desc = { + .entry_number = -1, + .base_addr = (unsigned long)stack16, + .limit = 0xffff, + .seg_32bit = 0, + .contents = 0, /* Data, grow-up */ + .read_exec_only = 0, + .limit_in_pages = 0, + .seg_not_present = 0, + .useable = 0 + }; + + if (syscall(SYS_set_thread_area, &gdt_data16_desc) == 0) { + /* + * This probably indicates vulnerability to CVE-2014-8133. + * Merely getting here isn't definitive, though, and we'll + * diagnose the problem for real later on. + */ + printf("[WARN]\tset_thread_area allocated data16 at index %d\n", + gdt_data16_desc.entry_number); + gdt_data16_idx = gdt_data16_desc.entry_number; + } else { + printf("[OK]\tset_thread_area refused 16-bit data\n"); + } + + struct user_desc gdt_npdata32_desc = { + .entry_number = -1, + .base_addr = (unsigned long)stack16, + .limit = 0xffff, + .seg_32bit = 1, + .contents = 0, /* Data, grow-up */ + .read_exec_only = 0, + .limit_in_pages = 0, + .seg_not_present = 1, + .useable = 0 + }; + + if (syscall(SYS_set_thread_area, &gdt_npdata32_desc) == 0) { + /* + * As a hardening measure, newer kernels don't allow this. + */ + printf("[WARN]\tset_thread_area allocated npdata32 at index %d\n", + gdt_npdata32_desc.entry_number); + gdt_npdata32_idx = gdt_npdata32_desc.entry_number; + } else { + printf("[OK]\tset_thread_area refused 16-bit data\n"); + } +} + +/* State used by our signal handlers. */ +static gregset_t initial_regs, requested_regs, resulting_regs; + +/* Instructions for the SIGUSR1 handler. */ +static volatile unsigned short sig_cs, sig_ss; +static volatile sig_atomic_t sig_trapped, sig_err, sig_trapno; +#ifdef __x86_64__ +static volatile sig_atomic_t sig_corrupt_final_ss; +#endif + +/* Abstractions for some 32-bit vs 64-bit differences. */ +#ifdef __x86_64__ +# define REG_IP REG_RIP +# define REG_SP REG_RSP +# define REG_CX REG_RCX + +struct selectors { + unsigned short cs, gs, fs, ss; +}; + +static unsigned short *ssptr(ucontext_t *ctx) +{ + struct selectors *sels = (void *)&ctx->uc_mcontext.gregs[REG_CSGSFS]; + return &sels->ss; +} + +static unsigned short *csptr(ucontext_t *ctx) +{ + struct selectors *sels = (void *)&ctx->uc_mcontext.gregs[REG_CSGSFS]; + return &sels->cs; +} +#else +# define REG_IP REG_EIP +# define REG_SP REG_ESP +# define REG_CX REG_ECX + +static greg_t *ssptr(ucontext_t *ctx) +{ + return &ctx->uc_mcontext.gregs[REG_SS]; +} + +static greg_t *csptr(ucontext_t *ctx) +{ + return &ctx->uc_mcontext.gregs[REG_CS]; +} +#endif + +/* + * Checks a given selector for its code bitness or returns -1 if it's not + * a usable code segment selector. + */ +int cs_bitness(unsigned short cs) +{ + uint32_t valid = 0, ar; + asm ("lar %[cs], %[ar]\n\t" + "jnz 1f\n\t" + "mov $1, %[valid]\n\t" + "1:" + : [ar] "=r" (ar), [valid] "+rm" (valid) + : [cs] "r" (cs)); + + if (!valid) + return -1; + + bool db = (ar & (1 << 22)); + bool l = (ar & (1 << 21)); + + if (!(ar & (1<<11))) + return -1; /* Not code. */ + + if (l && !db) + return 64; + else if (!l && db) + return 32; + else if (!l && !db) + return 16; + else + return -1; /* Unknown bitness. */ +} + +/* + * Checks a given selector for its code bitness or returns -1 if it's not + * a usable code segment selector. + */ +bool is_valid_ss(unsigned short cs) +{ + uint32_t valid = 0, ar; + asm ("lar %[cs], %[ar]\n\t" + "jnz 1f\n\t" + "mov $1, %[valid]\n\t" + "1:" + : [ar] "=r" (ar), [valid] "+rm" (valid) + : [cs] "r" (cs)); + + if (!valid) + return false; + + if ((ar & AR_TYPE_MASK) != AR_TYPE_RWDATA && + (ar & AR_TYPE_MASK) != AR_TYPE_RWDATA_EXPDOWN) + return false; + + return (ar & AR_P); +} + +/* Number of errors in the current test case. */ +static volatile sig_atomic_t nerrs; + +static void validate_signal_ss(int sig, ucontext_t *ctx) +{ +#ifdef __x86_64__ + bool was_64bit = (cs_bitness(*csptr(ctx)) == 64); + + if (!(ctx->uc_flags & UC_SIGCONTEXT_SS)) { + printf("[FAIL]\tUC_SIGCONTEXT_SS was not set\n"); + nerrs++; + + /* + * This happens on Linux 4.1. The rest will fail, too, so + * return now to reduce the noise. + */ + return; + } + + /* UC_STRICT_RESTORE_SS is set iff we came from 64-bit mode. */ + if (!!(ctx->uc_flags & UC_STRICT_RESTORE_SS) != was_64bit) { + printf("[FAIL]\tUC_STRICT_RESTORE_SS was wrong in signal %d\n", + sig); + nerrs++; + } + + if (is_valid_ss(*ssptr(ctx))) { + /* + * DOSEMU was written before 64-bit sigcontext had SS, and + * it tries to figure out the signal source SS by looking at + * the physical register. Make sure that keeps working. + */ + unsigned short hw_ss; + asm ("mov %%ss, %0" : "=rm" (hw_ss)); + if (hw_ss != *ssptr(ctx)) { + printf("[FAIL]\tHW SS didn't match saved SS\n"); + nerrs++; + } + } +#endif +} + +/* + * SIGUSR1 handler. Sets CS and SS as requested and points IP to the + * int3 trampoline. Sets SP to a large known value so that we can see + * whether the value round-trips back to user mode correctly. + */ +static void sigusr1(int sig, siginfo_t *info, void *ctx_void) +{ + ucontext_t *ctx = (ucontext_t*)ctx_void; + + validate_signal_ss(sig, ctx); + + memcpy(&initial_regs, &ctx->uc_mcontext.gregs, sizeof(gregset_t)); + + *csptr(ctx) = sig_cs; + *ssptr(ctx) = sig_ss; + + ctx->uc_mcontext.gregs[REG_IP] = + sig_cs == code16_sel ? 0 : (unsigned long)&int3; + ctx->uc_mcontext.gregs[REG_SP] = (unsigned long)0x8badf00d5aadc0deULL; + ctx->uc_mcontext.gregs[REG_CX] = 0; + +#ifdef __i386__ + /* + * Make sure the kernel doesn't inadvertently use DS or ES-relative + * accesses in a region where user DS or ES is loaded. + * + * Skip this for 64-bit builds because long mode doesn't care about + * DS and ES and skipping it increases test coverage a little bit, + * since 64-bit kernels can still run the 32-bit build. + */ + ctx->uc_mcontext.gregs[REG_DS] = 0; + ctx->uc_mcontext.gregs[REG_ES] = 0; +#endif + + memcpy(&requested_regs, &ctx->uc_mcontext.gregs, sizeof(gregset_t)); + requested_regs[REG_CX] = *ssptr(ctx); /* The asm code does this. */ + + return; +} + +/* + * Called after a successful sigreturn (via int3) or from a failed + * sigreturn (directly by kernel). Restores our state so that the + * original raise(SIGUSR1) returns. + */ +static void sigtrap(int sig, siginfo_t *info, void *ctx_void) +{ + ucontext_t *ctx = (ucontext_t*)ctx_void; + + validate_signal_ss(sig, ctx); + + sig_err = ctx->uc_mcontext.gregs[REG_ERR]; + sig_trapno = ctx->uc_mcontext.gregs[REG_TRAPNO]; + + unsigned short ss; + asm ("mov %%ss,%0" : "=r" (ss)); + + greg_t asm_ss = ctx->uc_mcontext.gregs[REG_CX]; + if (asm_ss != sig_ss && sig == SIGTRAP) { + /* Sanity check failure. */ + printf("[FAIL]\tSIGTRAP: ss = %hx, frame ss = %hx, ax = %llx\n", + ss, *ssptr(ctx), (unsigned long long)asm_ss); + nerrs++; + } + + memcpy(&resulting_regs, &ctx->uc_mcontext.gregs, sizeof(gregset_t)); + memcpy(&ctx->uc_mcontext.gregs, &initial_regs, sizeof(gregset_t)); + +#ifdef __x86_64__ + if (sig_corrupt_final_ss) { + if (ctx->uc_flags & UC_STRICT_RESTORE_SS) { + printf("[FAIL]\tUC_STRICT_RESTORE_SS was set inappropriately\n"); + nerrs++; + } else { + /* + * DOSEMU transitions from 32-bit to 64-bit mode by + * adjusting sigcontext, and it requires that this work + * even if the saved SS is bogus. + */ + printf("\tCorrupting SS on return to 64-bit mode\n"); + *ssptr(ctx) = 0; + } + } +#endif + + sig_trapped = sig; +} + +#ifdef __x86_64__ +/* Tests recovery if !UC_STRICT_RESTORE_SS */ +static void sigusr2(int sig, siginfo_t *info, void *ctx_void) +{ + ucontext_t *ctx = (ucontext_t*)ctx_void; + + if (!(ctx->uc_flags & UC_STRICT_RESTORE_SS)) { + printf("[FAIL]\traise(2) didn't set UC_STRICT_RESTORE_SS\n"); + nerrs++; + return; /* We can't do the rest. */ + } + + ctx->uc_flags &= ~UC_STRICT_RESTORE_SS; + *ssptr(ctx) = 0; + + /* Return. The kernel should recover without sending another signal. */ +} + +static int test_nonstrict_ss(void) +{ + clearhandler(SIGUSR1); + clearhandler(SIGTRAP); + clearhandler(SIGSEGV); + clearhandler(SIGILL); + sethandler(SIGUSR2, sigusr2, 0); + + nerrs = 0; + + printf("[RUN]\tClear UC_STRICT_RESTORE_SS and corrupt SS\n"); + raise(SIGUSR2); + if (!nerrs) + printf("[OK]\tIt worked\n"); + + return nerrs; +} +#endif + +/* Finds a usable code segment of the requested bitness. */ +int find_cs(int bitness) +{ + unsigned short my_cs; + + asm ("mov %%cs,%0" : "=r" (my_cs)); + + if (cs_bitness(my_cs) == bitness) + return my_cs; + if (cs_bitness(my_cs + (2 << 3)) == bitness) + return my_cs + (2 << 3); + if (my_cs > (2<<3) && cs_bitness(my_cs - (2 << 3)) == bitness) + return my_cs - (2 << 3); + if (cs_bitness(code16_sel) == bitness) + return code16_sel; + + printf("[WARN]\tCould not find %d-bit CS\n", bitness); + return -1; +} + +static int test_valid_sigreturn(int cs_bits, bool use_16bit_ss, int force_ss) +{ + int cs = find_cs(cs_bits); + if (cs == -1) { + printf("[SKIP]\tCode segment unavailable for %d-bit CS, %d-bit SS\n", + cs_bits, use_16bit_ss ? 16 : 32); + return 0; + } + + if (force_ss != -1) { + sig_ss = force_ss; + } else { + if (use_16bit_ss) { + if (!data16_sel) { + printf("[SKIP]\tData segment unavailable for %d-bit CS, 16-bit SS\n", + cs_bits); + return 0; + } + sig_ss = data16_sel; + } else { + asm volatile ("mov %%ss,%0" : "=r" (sig_ss)); + } + } + + sig_cs = cs; + + printf("[RUN]\tValid sigreturn: %d-bit CS (%hx), %d-bit SS (%hx%s)\n", + cs_bits, sig_cs, use_16bit_ss ? 16 : 32, sig_ss, + (sig_ss & 4) ? "" : ", GDT"); + + raise(SIGUSR1); + + nerrs = 0; + + /* + * Check that each register had an acceptable value when the + * int3 trampoline was invoked. + */ + for (int i = 0; i < NGREG; i++) { + greg_t req = requested_regs[i], res = resulting_regs[i]; + + if (i == REG_TRAPNO || i == REG_IP) + continue; /* don't care */ + + if (i == REG_SP) { + /* + * If we were using a 16-bit stack segment, then + * the kernel is a bit stuck: IRET only restores + * the low 16 bits of ESP/RSP if SS is 16-bit. + * The kernel uses a hack to restore bits 31:16, + * but that hack doesn't help with bits 63:32. + * On Intel CPUs, bits 63:32 end up zeroed, and, on + * AMD CPUs, they leak the high bits of the kernel + * espfix64 stack pointer. There's very little that + * the kernel can do about it. + * + * Similarly, if we are returning to a 32-bit context, + * the CPU will often lose the high 32 bits of RSP. + */ + + if (res == req) + continue; + + if (cs_bits != 64 && ((res ^ req) & 0xFFFFFFFF) == 0) { + printf("[NOTE]\tSP: %llx -> %llx\n", + (unsigned long long)req, + (unsigned long long)res); + continue; + } + + printf("[FAIL]\tSP mismatch: requested 0x%llx; got 0x%llx\n", + (unsigned long long)requested_regs[i], + (unsigned long long)resulting_regs[i]); + nerrs++; + continue; + } + + bool ignore_reg = false; +#if __i386__ + if (i == REG_UESP) + ignore_reg = true; +#else + if (i == REG_CSGSFS) { + struct selectors *req_sels = + (void *)&requested_regs[REG_CSGSFS]; + struct selectors *res_sels = + (void *)&resulting_regs[REG_CSGSFS]; + if (req_sels->cs != res_sels->cs) { + printf("[FAIL]\tCS mismatch: requested 0x%hx; got 0x%hx\n", + req_sels->cs, res_sels->cs); + nerrs++; + } + + if (req_sels->ss != res_sels->ss) { + printf("[FAIL]\tSS mismatch: requested 0x%hx; got 0x%hx\n", + req_sels->ss, res_sels->ss); + nerrs++; + } + + continue; + } +#endif + + /* Sanity check on the kernel */ + if (i == REG_CX && req != res) { + printf("[FAIL]\tCX (saved SP) mismatch: requested 0x%llx; got 0x%llx\n", + (unsigned long long)req, + (unsigned long long)res); + nerrs++; + continue; + } + + if (req != res && !ignore_reg) { + printf("[FAIL]\tReg %d mismatch: requested 0x%llx; got 0x%llx\n", + i, (unsigned long long)req, + (unsigned long long)res); + nerrs++; + } + } + + if (nerrs == 0) + printf("[OK]\tall registers okay\n"); + + return nerrs; +} + +static int test_bad_iret(int cs_bits, unsigned short ss, int force_cs) +{ + int cs = force_cs == -1 ? find_cs(cs_bits) : force_cs; + if (cs == -1) + return 0; + + sig_cs = cs; + sig_ss = ss; + + printf("[RUN]\t%d-bit CS (%hx), bogus SS (%hx)\n", + cs_bits, sig_cs, sig_ss); + + sig_trapped = 0; + raise(SIGUSR1); + if (sig_trapped) { + char errdesc[32] = ""; + if (sig_err) { + const char *src = (sig_err & 1) ? " EXT" : ""; + const char *table; + if ((sig_err & 0x6) == 0x0) + table = "GDT"; + else if ((sig_err & 0x6) == 0x4) + table = "LDT"; + else if ((sig_err & 0x6) == 0x2) + table = "IDT"; + else + table = "???"; + + sprintf(errdesc, "%s%s index %d, ", + table, src, sig_err >> 3); + } + + char trapname[32]; + if (sig_trapno == 13) + strcpy(trapname, "GP"); + else if (sig_trapno == 11) + strcpy(trapname, "NP"); + else if (sig_trapno == 12) + strcpy(trapname, "SS"); + else if (sig_trapno == 32) + strcpy(trapname, "IRET"); /* X86_TRAP_IRET */ + else + sprintf(trapname, "%d", sig_trapno); + + printf("[OK]\tGot #%s(0x%lx) (i.e. %s%s)\n", + trapname, (unsigned long)sig_err, + errdesc, strsignal(sig_trapped)); + return 0; + } else { + /* + * This also implicitly tests UC_STRICT_RESTORE_SS: + * We check that these signals set UC_STRICT_RESTORE_SS and, + * if UC_STRICT_RESTORE_SS doesn't cause strict behavior, + * then we won't get SIGSEGV. + */ + printf("[FAIL]\tDid not get SIGSEGV\n"); + return 1; + } +} + +int main() +{ + int total_nerrs = 0; + unsigned short my_cs, my_ss; + + asm volatile ("mov %%cs,%0" : "=r" (my_cs)); + asm volatile ("mov %%ss,%0" : "=r" (my_ss)); + setup_ldt(); + + stack_t stack = { + .ss_sp = altstack_data, + .ss_size = SIGSTKSZ, + }; + if (sigaltstack(&stack, NULL) != 0) + err(1, "sigaltstack"); + + sethandler(SIGUSR1, sigusr1, 0); + sethandler(SIGTRAP, sigtrap, SA_ONSTACK); + + /* Easy cases: return to a 32-bit SS in each possible CS bitness. */ + total_nerrs += test_valid_sigreturn(64, false, -1); + total_nerrs += test_valid_sigreturn(32, false, -1); + total_nerrs += test_valid_sigreturn(16, false, -1); + + /* + * Test easy espfix cases: return to a 16-bit LDT SS in each possible + * CS bitness. NB: with a long mode CS, the SS bitness is irrelevant. + * + * This catches the original missing-espfix-on-64-bit-kernels issue + * as well as CVE-2014-8134. + */ + total_nerrs += test_valid_sigreturn(64, true, -1); + total_nerrs += test_valid_sigreturn(32, true, -1); + total_nerrs += test_valid_sigreturn(16, true, -1); + + if (gdt_data16_idx) { + /* + * For performance reasons, Linux skips espfix if SS points + * to the GDT. If we were able to allocate a 16-bit SS in + * the GDT, see if it leaks parts of the kernel stack pointer. + * + * This tests for CVE-2014-8133. + */ + total_nerrs += test_valid_sigreturn(64, true, + GDT3(gdt_data16_idx)); + total_nerrs += test_valid_sigreturn(32, true, + GDT3(gdt_data16_idx)); + total_nerrs += test_valid_sigreturn(16, true, + GDT3(gdt_data16_idx)); + } + +#ifdef __x86_64__ + /* Nasty ABI case: check SS corruption handling. */ + sig_corrupt_final_ss = 1; + total_nerrs += test_valid_sigreturn(32, false, -1); + total_nerrs += test_valid_sigreturn(32, true, -1); + sig_corrupt_final_ss = 0; +#endif + + /* + * We're done testing valid sigreturn cases. Now we test states + * for which sigreturn itself will succeed but the subsequent + * entry to user mode will fail. + * + * Depending on the failure mode and the kernel bitness, these + * entry failures can generate SIGSEGV, SIGBUS, or SIGILL. + */ + clearhandler(SIGTRAP); + sethandler(SIGSEGV, sigtrap, SA_ONSTACK); + sethandler(SIGBUS, sigtrap, SA_ONSTACK); + sethandler(SIGILL, sigtrap, SA_ONSTACK); /* 32-bit kernels do this */ + + /* Easy failures: invalid SS, resulting in #GP(0) */ + test_bad_iret(64, ldt_nonexistent_sel, -1); + test_bad_iret(32, ldt_nonexistent_sel, -1); + test_bad_iret(16, ldt_nonexistent_sel, -1); + + /* These fail because SS isn't a data segment, resulting in #GP(SS) */ + test_bad_iret(64, my_cs, -1); + test_bad_iret(32, my_cs, -1); + test_bad_iret(16, my_cs, -1); + + /* Try to return to a not-present code segment, triggering #NP(SS). */ + test_bad_iret(32, my_ss, npcode32_sel); + + /* + * Try to return to a not-present but otherwise valid data segment. + * This will cause IRET to fail with #SS on the espfix stack. This + * exercises CVE-2014-9322. + * + * Note that, if espfix is enabled, 64-bit Linux will lose track + * of the actual cause of failure and report #GP(0) instead. + * This would be very difficult for Linux to avoid, because + * espfix64 causes IRET failures to be promoted to #DF, so the + * original exception frame is never pushed onto the stack. + */ + test_bad_iret(32, npdata32_sel, -1); + + /* + * Try to return to a not-present but otherwise valid data + * segment without invoking espfix. Newer kernels don't allow + * this to happen in the first place. On older kernels, though, + * this can trigger CVE-2014-9322. + */ + if (gdt_npdata32_idx) + test_bad_iret(32, GDT3(gdt_npdata32_idx), -1); + +#ifdef __x86_64__ + total_nerrs += test_nonstrict_ss(); +#endif + + return total_nerrs ? 1 : 0; +} |