tools/testing/selftests/x86/fsgsbase_restore.c

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