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| 1 | +// SPDX-License-Identifier: GPL-2.0-only |
| 2 | +/* |
| 3 | + * fsgsbase_restore.c, test ptrace vs fsgsbase |
| 4 | + * Copyright (c) 2020 Andy Lutomirski |
| 5 | + * |
| 6 | + * This test case simulates a tracer redirecting tracee execution to |
| 7 | + * a function and then restoring tracee state using PTRACE_GETREGS and |
| 8 | + * PTRACE_SETREGS. This is similar to what gdb does when doing |
| 9 | + * 'p func()'. The catch is that this test has the called function |
| 10 | + * modify a segment register. This makes sure that ptrace correctly |
| 11 | + * restores segment state when using PTRACE_SETREGS. |
| 12 | + * |
| 13 | + * This is not part of fsgsbase.c, because that test is 64-bit only. |
| 14 | + */ |
| 15 | + |
| 16 | +#define _GNU_SOURCE |
| 17 | +#include <stdio.h> |
| 18 | +#include <stdlib.h> |
| 19 | +#include <stdbool.h> |
| 20 | +#include <string.h> |
| 21 | +#include <sys/syscall.h> |
| 22 | +#include <unistd.h> |
| 23 | +#include <err.h> |
| 24 | +#include <sys/user.h> |
| 25 | +#include <asm/prctl.h> |
| 26 | +#include <sys/prctl.h> |
| 27 | +#include <asm/ldt.h> |
| 28 | +#include <sys/mman.h> |
| 29 | +#include <stddef.h> |
| 30 | +#include <sys/ptrace.h> |
| 31 | +#include <sys/wait.h> |
| 32 | +#include <stdint.h> |
| 33 | + |
| 34 | +#define EXPECTED_VALUE 0x1337f00d |
| 35 | + |
| 36 | +#ifdef __x86_64__ |
| 37 | +# define SEG "%gs" |
| 38 | +#else |
| 39 | +# define SEG "%fs" |
| 40 | +#endif |
| 41 | + |
| 42 | +static unsigned int dereference_seg_base(void) |
| 43 | +{ |
| 44 | + int ret; |
| 45 | + asm volatile ("mov %" SEG ":(0), %0" : "=rm" (ret)); |
| 46 | + return ret; |
| 47 | +} |
| 48 | + |
| 49 | +static void init_seg(void) |
| 50 | +{ |
| 51 | + unsigned int *target = mmap( |
| 52 | + NULL, sizeof(unsigned int), |
| 53 | + PROT_READ | PROT_WRITE, |
| 54 | + MAP_PRIVATE | MAP_ANONYMOUS | MAP_32BIT, -1, 0); |
| 55 | + if (target == MAP_FAILED) |
| 56 | + err(1, "mmap"); |
| 57 | + |
| 58 | + *target = EXPECTED_VALUE; |
| 59 | + |
| 60 | + printf("\tsegment base address = 0x%lx\n", (unsigned long)target); |
| 61 | + |
| 62 | + struct user_desc desc = { |
| 63 | + .entry_number = 0, |
| 64 | + .base_addr = (unsigned int)(uintptr_t)target, |
| 65 | + .limit = sizeof(unsigned int) - 1, |
| 66 | + .seg_32bit = 1, |
| 67 | + .contents = 0, /* Data, grow-up */ |
| 68 | + .read_exec_only = 0, |
| 69 | + .limit_in_pages = 0, |
| 70 | + .seg_not_present = 0, |
| 71 | + .useable = 0 |
| 72 | + }; |
| 73 | + if (syscall(SYS_modify_ldt, 1, &desc, sizeof(desc)) == 0) { |
| 74 | + printf("\tusing LDT slot 0\n"); |
| 75 | + asm volatile ("mov %0, %" SEG :: "rm" ((unsigned short)0x7)); |
| 76 | + } else { |
| 77 | + /* No modify_ldt for us (configured out, perhaps) */ |
| 78 | + |
| 79 | + struct user_desc *low_desc = mmap( |
| 80 | + NULL, sizeof(desc), |
| 81 | + PROT_READ | PROT_WRITE, |
| 82 | + MAP_PRIVATE | MAP_ANONYMOUS | MAP_32BIT, -1, 0); |
| 83 | + memcpy(low_desc, &desc, sizeof(desc)); |
| 84 | + |
| 85 | + low_desc->entry_number = -1; |
| 86 | + |
| 87 | + /* 32-bit set_thread_area */ |
| 88 | + long ret; |
| 89 | + asm volatile ("int $0x80" |
| 90 | + : "=a" (ret), "+m" (*low_desc) |
| 91 | + : "a" (243), "b" (low_desc) |
| 92 | +#ifdef __x86_64__ |
| 93 | + : "r8", "r9", "r10", "r11" |
| 94 | +#endif |
| 95 | + ); |
| 96 | + memcpy(&desc, low_desc, sizeof(desc)); |
| 97 | + munmap(low_desc, sizeof(desc)); |
| 98 | + |
| 99 | + if (ret != 0) { |
| 100 | + printf("[NOTE]\tcould not create a segment -- can't test anything\n"); |
| 101 | + exit(0); |
| 102 | + } |
| 103 | + printf("\tusing GDT slot %d\n", desc.entry_number); |
| 104 | + |
| 105 | + unsigned short sel = (unsigned short)((desc.entry_number << 3) | 0x3); |
| 106 | + asm volatile ("mov %0, %" SEG :: "rm" (sel)); |
| 107 | + } |
| 108 | +} |
| 109 | + |
| 110 | +static void tracee_zap_segment(void) |
| 111 | +{ |
| 112 | + /* |
| 113 | + * The tracer will redirect execution here. This is meant to |
| 114 | + * work like gdb's 'p func()' feature. The tricky bit is that |
| 115 | + * we modify a segment register in order to make sure that ptrace |
| 116 | + * can correctly restore segment registers. |
| 117 | + */ |
| 118 | + printf("\tTracee: in tracee_zap_segment()\n"); |
| 119 | + |
| 120 | + /* |
| 121 | + * Write a nonzero selector with base zero to the segment register. |
| 122 | + * Using a null selector would defeat the test on AMD pre-Zen2 |
| 123 | + * CPUs, as such CPUs don't clear the base when loading a null |
| 124 | + * selector. |
| 125 | + */ |
| 126 | + unsigned short sel; |
| 127 | + asm volatile ("mov %%ss, %0\n\t" |
| 128 | + "mov %0, %" SEG |
| 129 | + : "=rm" (sel)); |
| 130 | + |
| 131 | + pid_t pid = getpid(), tid = syscall(SYS_gettid); |
| 132 | + |
| 133 | + printf("\tTracee is going back to sleep\n"); |
| 134 | + syscall(SYS_tgkill, pid, tid, SIGSTOP); |
| 135 | + |
| 136 | + /* Should not get here. */ |
| 137 | + while (true) { |
| 138 | + printf("[FAIL]\tTracee hit unreachable code\n"); |
| 139 | + pause(); |
| 140 | + } |
| 141 | +} |
| 142 | + |
| 143 | +int main() |
| 144 | +{ |
| 145 | + printf("\tSetting up a segment\n"); |
| 146 | + init_seg(); |
| 147 | + |
| 148 | + unsigned int val = dereference_seg_base(); |
| 149 | + if (val != EXPECTED_VALUE) { |
| 150 | + printf("[FAIL]\tseg[0] == %x; should be %x\n", val, EXPECTED_VALUE); |
| 151 | + return 1; |
| 152 | + } |
| 153 | + printf("[OK]\tThe segment points to the right place.\n"); |
| 154 | + |
| 155 | + pid_t chld = fork(); |
| 156 | + if (chld < 0) |
| 157 | + err(1, "fork"); |
| 158 | + |
| 159 | + if (chld == 0) { |
| 160 | + prctl(PR_SET_PDEATHSIG, SIGKILL, 0, 0, 0, 0); |
| 161 | + |
| 162 | + if (ptrace(PTRACE_TRACEME, 0, 0, 0) != 0) |
| 163 | + err(1, "PTRACE_TRACEME"); |
| 164 | + |
| 165 | + pid_t pid = getpid(), tid = syscall(SYS_gettid); |
| 166 | + |
| 167 | + printf("\tTracee will take a nap until signaled\n"); |
| 168 | + syscall(SYS_tgkill, pid, tid, SIGSTOP); |
| 169 | + |
| 170 | + printf("\tTracee was resumed. Will re-check segment.\n"); |
| 171 | + |
| 172 | + val = dereference_seg_base(); |
| 173 | + if (val != EXPECTED_VALUE) { |
| 174 | + printf("[FAIL]\tseg[0] == %x; should be %x\n", val, EXPECTED_VALUE); |
| 175 | + exit(1); |
| 176 | + } |
| 177 | + |
| 178 | + printf("[OK]\tThe segment points to the right place.\n"); |
| 179 | + exit(0); |
| 180 | + } |
| 181 | + |
| 182 | + int status; |
| 183 | + |
| 184 | + /* Wait for SIGSTOP. */ |
| 185 | + if (waitpid(chld, &status, 0) != chld || !WIFSTOPPED(status)) |
| 186 | + err(1, "waitpid"); |
| 187 | + |
| 188 | + struct user_regs_struct regs; |
| 189 | + |
| 190 | + if (ptrace(PTRACE_GETREGS, chld, NULL, ®s) != 0) |
| 191 | + err(1, "PTRACE_GETREGS"); |
| 192 | + |
| 193 | +#ifdef __x86_64__ |
| 194 | + printf("\tChild GS=0x%lx, GSBASE=0x%lx\n", (unsigned long)regs.gs, (unsigned long)regs.gs_base); |
| 195 | +#else |
| 196 | + printf("\tChild FS=0x%lx\n", (unsigned long)regs.xfs); |
| 197 | +#endif |
| 198 | + |
| 199 | + struct user_regs_struct regs2 = regs; |
| 200 | +#ifdef __x86_64__ |
| 201 | + regs2.rip = (unsigned long)tracee_zap_segment; |
| 202 | + regs2.rsp -= 128; /* Don't clobber the redzone. */ |
| 203 | +#else |
| 204 | + regs2.eip = (unsigned long)tracee_zap_segment; |
| 205 | +#endif |
| 206 | + |
| 207 | + printf("\tTracer: redirecting tracee to tracee_zap_segment()\n"); |
| 208 | + if (ptrace(PTRACE_SETREGS, chld, NULL, ®s2) != 0) |
| 209 | + err(1, "PTRACE_GETREGS"); |
| 210 | + if (ptrace(PTRACE_CONT, chld, NULL, NULL) != 0) |
| 211 | + err(1, "PTRACE_GETREGS"); |
| 212 | + |
| 213 | + /* Wait for SIGSTOP. */ |
| 214 | + if (waitpid(chld, &status, 0) != chld || !WIFSTOPPED(status)) |
| 215 | + err(1, "waitpid"); |
| 216 | + |
| 217 | + printf("\tTracer: restoring tracee state\n"); |
| 218 | + if (ptrace(PTRACE_SETREGS, chld, NULL, ®s) != 0) |
| 219 | + err(1, "PTRACE_GETREGS"); |
| 220 | + if (ptrace(PTRACE_DETACH, chld, NULL, NULL) != 0) |
| 221 | + err(1, "PTRACE_GETREGS"); |
| 222 | + |
| 223 | + /* Wait for SIGSTOP. */ |
| 224 | + if (waitpid(chld, &status, 0) != chld) |
| 225 | + err(1, "waitpid"); |
| 226 | + |
| 227 | + if (WIFSIGNALED(status)) { |
| 228 | + printf("[FAIL]\tTracee crashed\n"); |
| 229 | + return 1; |
| 230 | + } |
| 231 | + |
| 232 | + if (!WIFEXITED(status)) { |
| 233 | + printf("[FAIL]\tTracee stopped for an unexpected reason: %d\n", status); |
| 234 | + return 1; |
| 235 | + } |
| 236 | + |
| 237 | + int exitcode = WEXITSTATUS(status); |
| 238 | + if (exitcode != 0) { |
| 239 | + printf("[FAIL]\tTracee reported failure\n"); |
| 240 | + return 1; |
| 241 | + } |
| 242 | + |
| 243 | + printf("[OK]\tAll is well.\n"); |
| 244 | + return 0; |
| 245 | +} |
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