tools/testing/selftests/x86/fsgsbase.c

   1 /*
   2  * fsgsbase.c, an fsgsbase test
   3  * Copyright (c) 2014-2016 Andy Lutomirski
   4  * GPL v2
   5  */
   6
   7 #define _GNU_SOURCE
   8 #include <stdio.h>
   9 #include <stdlib.h>
  10 #include <stdbool.h>
  11 #include <string.h>
  12 #include <sys/syscall.h>
  13 #include <unistd.h>
  14 #include <err.h>
  15 #include <sys/user.h>
  16 #include <asm/prctl.h>
  17 #include <sys/prctl.h>
  18 #include <signal.h>
  19 #include <limits.h>
  20 #include <sys/ucontext.h>
  21 #include <sched.h>
  22 #include <linux/futex.h>
  23 #include <pthread.h>
  24 #include <asm/ldt.h>
  25 #include <sys/mman.h>
  26
  27 #ifndef __x86_64__
  28 # error This test is 64-bit only
  29 #endif
  30
  31 static volatile sig_atomic_t want_segv;
  32 static volatile unsigned long segv_addr;
  33
  34 static int nerrs;
  35
  36 static void sethandler(int sig, void (*handler)(int, siginfo_t *, void *),
  37                        int flags)
  38 {
  39         struct sigaction sa;
  40         memset(&sa, 0, sizeof(sa));
  41         sa.sa_sigaction = handler;
  42         sa.sa_flags = SA_SIGINFO | flags;
  43         sigemptyset(&sa.sa_mask);
  44         if (sigaction(sig, &sa, 0))
  45                 err(1, "sigaction");
  46 }
  47
  48 static void clearhandler(int sig)
  49 {
  50         struct sigaction sa;
  51         memset(&sa, 0, sizeof(sa));
  52         sa.sa_handler = SIG_DFL;
  53         sigemptyset(&sa.sa_mask);
  54         if (sigaction(sig, &sa, 0))
  55                 err(1, "sigaction");
  56 }
  57
  58 static void sigsegv(int sig, siginfo_t *si, void *ctx_void)
  59 {
  60         ucontext_t *ctx = (ucontext_t*)ctx_void;
  61
  62         if (!want_segv) {
  63                 clearhandler(SIGSEGV);
  64                 return;  /* Crash cleanly. */
  65         }
  66
  67         want_segv = false;
  68         segv_addr = (unsigned long)si->si_addr;
  69
  70         ctx->uc_mcontext.gregs[REG_RIP] += 4;   /* Skip the faulting mov */
  71
  72 }
  73
  74 enum which_base { FS, GS };
  75
  76 static unsigned long read_base(enum which_base which)
  77 {
  78         unsigned long offset;
  79         /*
  80          * Unless we have FSGSBASE, there's no direct way to do this from
  81          * user mode.  We can get at it indirectly using signals, though.
  82          */
  83
  84         want_segv = true;
  85
  86         offset = 0;
  87         if (which == FS) {
  88                 /* Use a constant-length instruction here. */
  89                 asm volatile ("mov %%fs:(%%rcx), %%rax" : : "c" (offset) : "rax");
  90         } else {
  91                 asm volatile ("mov %%gs:(%%rcx), %%rax" : : "c" (offset) : "rax");
  92         }
  93         if (!want_segv)
  94                 return segv_addr + offset;
  95
  96         /*
  97          * If that didn't segfault, try the other end of the address space.
  98          * Unless we get really unlucky and run into the vsyscall page, this
  99          * is guaranteed to segfault.
 100          */
 101
 102         offset = (ULONG_MAX >> 1) + 1;
 103         if (which == FS) {
 104                 asm volatile ("mov %%fs:(%%rcx), %%rax"
 105                               : : "c" (offset) : "rax");
 106         } else {
 107                 asm volatile ("mov %%gs:(%%rcx), %%rax"
 108                               : : "c" (offset) : "rax");
 109         }
 110         if (!want_segv)
 111                 return segv_addr + offset;
 112
 113         abort();
 114 }
 115
 116 static void check_gs_value(unsigned long value)
 117 {
 118         unsigned long base;
 119         unsigned short sel;
 120
 121         printf("[RUN]\tARCH_SET_GS to 0x%lx\n", value);
 122         if (syscall(SYS_arch_prctl, ARCH_SET_GS, value) != 0)
 123                 err(1, "ARCH_SET_GS");
 124
 125         asm volatile ("mov %%gs, %0" : "=rm" (sel));
 126         base = read_base(GS);
 127         if (base == value) {
 128                 printf("[OK]\tGSBASE was set as expected (selector 0x%hx)\n",
 129                        sel);
 130         } else {
 131                 nerrs++;
 132                 printf("[FAIL]\tGSBASE was not as expected: got 0x%lx (selector 0x%hx)\n",
 133                        base, sel);
 134         }
 135
 136         if (syscall(SYS_arch_prctl, ARCH_GET_GS, &base) != 0)
 137                 err(1, "ARCH_GET_GS");
 138         if (base == value) {
 139                 printf("[OK]\tARCH_GET_GS worked as expected (selector 0x%hx)\n",
 140                        sel);
 141         } else {
 142                 nerrs++;
 143                 printf("[FAIL]\tARCH_GET_GS was not as expected: got 0x%lx (selector 0x%hx)\n",
 144                        base, sel);
 145         }
 146 }
 147
 148 static void mov_0_gs(unsigned long initial_base, bool schedule)
 149 {
 150         unsigned long base, arch_base;
 151
 152         printf("[RUN]\tARCH_SET_GS to 0x%lx then mov 0 to %%gs%s\n", initial_base, schedule ? " and schedule " : "");
 153         if (syscall(SYS_arch_prctl, ARCH_SET_GS, initial_base) != 0)
 154                 err(1, "ARCH_SET_GS");
 155
 156         if (schedule)
 157                 usleep(10);
 158
 159         asm volatile ("mov %0, %%gs" : : "rm" (0));
 160         base = read_base(GS);
 161         if (syscall(SYS_arch_prctl, ARCH_GET_GS, &arch_base) != 0)
 162                 err(1, "ARCH_GET_GS");
 163         if (base == arch_base) {
 164                 printf("[OK]\tGSBASE is 0x%lx\n", base);
 165         } else {
 166                 nerrs++;
 167                 printf("[FAIL]\tGSBASE changed to 0x%lx but kernel reports 0x%lx\n", base, arch_base);
 168         }
 169 }
 170
 171 static volatile unsigned long remote_base;
 172 static volatile bool remote_hard_zero;
 173 static volatile unsigned int ftx;
 174
 175 /*
 176  * ARCH_SET_FS/GS(0) may or may not program a selector of zero.  HARD_ZERO
 177  * means to force the selector to zero to improve test coverage.
 178  */
 179 #define HARD_ZERO 0xa1fa5f343cb85fa4
 180
 181 static void do_remote_base()
 182 {
 183         unsigned long to_set = remote_base;
 184         bool hard_zero = false;
 185         if (to_set == HARD_ZERO) {
 186                 to_set = 0;
 187                 hard_zero = true;
 188         }
 189
 190         if (syscall(SYS_arch_prctl, ARCH_SET_GS, to_set) != 0)
 191                 err(1, "ARCH_SET_GS");
 192
 193         if (hard_zero)
 194                 asm volatile ("mov %0, %%gs" : : "rm" ((unsigned short)0));
 195
 196         unsigned short sel;
 197         asm volatile ("mov %%gs, %0" : "=rm" (sel));
 198         printf("\tother thread: ARCH_SET_GS(0x%lx)%s -- sel is 0x%hx\n",
 199                to_set, hard_zero ? " and clear gs" : "", sel);
 200 }
 201
 202 void do_unexpected_base(void)
 203 {
 204         /*
 205          * The goal here is to try to arrange for GS == 0, GSBASE !=
 206          * 0, and for the the kernel the think that GSBASE == 0.
 207          *
 208          * To make the test as reliable as possible, this uses
 209          * explicit descriptorss.  (This is not the only way.  This
 210          * could use ARCH_SET_GS with a low, nonzero base, but the
 211          * relevant side effect of ARCH_SET_GS could change.)
 212          */
 213
 214         /* Step 1: tell the kernel that we have GSBASE == 0. */
 215         if (syscall(SYS_arch_prctl, ARCH_SET_GS, 0) != 0)
 216                 err(1, "ARCH_SET_GS");
 217
 218         /* Step 2: change GSBASE without telling the kernel. */
 219         struct user_desc desc = {
 220                 .entry_number    = 0,
 221                 .base_addr       = 0xBAADF00D,
 222                 .limit           = 0xfffff,
 223                 .seg_32bit       = 1,
 224                 .contents        = 0, /* Data, grow-up */
 225                 .read_exec_only  = 0,
 226                 .limit_in_pages  = 1,
 227                 .seg_not_present = 0,
 228                 .useable         = 0
 229         };
 230         if (syscall(SYS_modify_ldt, 1, &desc, sizeof(desc)) == 0) {
 231                 printf("\tother thread: using LDT slot 0\n");
 232                 asm volatile ("mov %0, %%gs" : : "rm" ((unsigned short)0x7));
 233         } else {
 234                 /* No modify_ldt for us (configured out, perhaps) */
 235
 236                 struct user_desc *low_desc = mmap(
 237                         NULL, sizeof(desc),
 238                         PROT_READ | PROT_WRITE,
 239                         MAP_PRIVATE | MAP_ANONYMOUS | MAP_32BIT, -1, 0);
 240                 memcpy(low_desc, &desc, sizeof(desc));
 241
 242                 low_desc->entry_number = -1;
 243
 244                 /* 32-bit set_thread_area */
 245                 long ret;
 246                 asm volatile ("int $0x80"
 247                               : "=a" (ret) : "a" (243), "b" (low_desc)
 248                               : "r8", "r9", "r10", "r11");
 249                 memcpy(&desc, low_desc, sizeof(desc));
 250                 munmap(low_desc, sizeof(desc));
 251
 252                 if (ret != 0) {
 253                         printf("[NOTE]\tcould not create a segment -- test won't do anything\n");
 254                         return;
 255                 }
 256                 printf("\tother thread: using GDT slot %d\n", desc.entry_number);
 257                 asm volatile ("mov %0, %%gs" : : "rm" ((unsigned short)((desc.entry_number << 3) | 0x3)));
 258         }
 259
 260         /*
 261          * Step 3: set the selector back to zero.  On AMD chips, this will
 262          * preserve GSBASE.
 263          */
 264
 265         asm volatile ("mov %0, %%gs" : : "rm" ((unsigned short)0));
 266 }
 267
 268 static void *threadproc(void *ctx)
 269 {
 270         while (1) {
 271                 while (ftx == 0)
 272                         syscall(SYS_futex, &ftx, FUTEX_WAIT, 0, NULL, NULL, 0);
 273                 if (ftx == 3)
 274                         return NULL;
 275
 276                 if (ftx == 1)
 277                         do_remote_base();
 278                 else if (ftx == 2)
 279                         do_unexpected_base();
 280                 else
 281                         errx(1, "helper thread got bad command");
 282
 283                 ftx = 0;
 284                 syscall(SYS_futex, &ftx, FUTEX_WAKE, 0, NULL, NULL, 0);
 285         }
 286 }
 287
 288 static void set_gs_and_switch_to(unsigned long local,
 289                                  unsigned short force_sel,
 290                                  unsigned long remote)
 291 {
 292         unsigned long base;
 293         unsigned short sel_pre_sched, sel_post_sched;
 294
 295         bool hard_zero = false;
 296         if (local == HARD_ZERO) {
 297                 hard_zero = true;
 298                 local = 0;
 299         }
 300
 301         printf("[RUN]\tARCH_SET_GS(0x%lx)%s, then schedule to 0x%lx\n",
 302                local, hard_zero ? " and clear gs" : "", remote);
 303         if (force_sel)
 304                 printf("\tBefore schedule, set selector to 0x%hx\n", force_sel);
 305         if (syscall(SYS_arch_prctl, ARCH_SET_GS, local) != 0)
 306                 err(1, "ARCH_SET_GS");
 307         if (hard_zero)
 308                 asm volatile ("mov %0, %%gs" : : "rm" ((unsigned short)0));
 309
 310         if (read_base(GS) != local) {
 311                 nerrs++;
 312                 printf("[FAIL]\tGSBASE wasn't set as expected\n");
 313         }
 314
 315         if (force_sel) {
 316                 asm volatile ("mov %0, %%gs" : : "rm" (force_sel));
 317                 sel_pre_sched = force_sel;
 318                 local = read_base(GS);
 319
 320                 /*
 321                  * Signal delivery seems to mess up weird selectors.  Put it
 322                  * back.
 323                  */
 324                 asm volatile ("mov %0, %%gs" : : "rm" (force_sel));
 325         } else {
 326                 asm volatile ("mov %%gs, %0" : "=rm" (sel_pre_sched));
 327         }
 328
 329         remote_base = remote;
 330         ftx = 1;
 331         syscall(SYS_futex, &ftx, FUTEX_WAKE, 0, NULL, NULL, 0);
 332         while (ftx != 0)
 333                 syscall(SYS_futex, &ftx, FUTEX_WAIT, 1, NULL, NULL, 0);
 334
 335         asm volatile ("mov %%gs, %0" : "=rm" (sel_post_sched));
 336         base = read_base(GS);
 337         if (base == local && sel_pre_sched == sel_post_sched) {
 338                 printf("[OK]\tGS/BASE remained 0x%hx/0x%lx\n",
 339                        sel_pre_sched, local);
 340         } else {
 341                 nerrs++;
 342                 printf("[FAIL]\tGS/BASE changed from 0x%hx/0x%lx to 0x%hx/0x%lx\n",
 343                        sel_pre_sched, local, sel_post_sched, base);
 344         }
 345 }
 346
 347 static void test_unexpected_base(void)
 348 {
 349         unsigned long base;
 350
 351         printf("[RUN]\tARCH_SET_GS(0), clear gs, then manipulate GSBASE in a different thread\n");
 352         if (syscall(SYS_arch_prctl, ARCH_SET_GS, 0) != 0)
 353                 err(1, "ARCH_SET_GS");
 354         asm volatile ("mov %0, %%gs" : : "rm" ((unsigned short)0));
 355
 356         ftx = 2;
 357         syscall(SYS_futex, &ftx, FUTEX_WAKE, 0, NULL, NULL, 0);
 358         while (ftx != 0)
 359                 syscall(SYS_futex, &ftx, FUTEX_WAIT, 1, NULL, NULL, 0);
 360
 361         base = read_base(GS);
 362         if (base == 0) {
 363                 printf("[OK]\tGSBASE remained 0\n");
 364         } else {
 365                 nerrs++;
 366                 printf("[FAIL]\tGSBASE changed to 0x%lx\n", base);
 367         }
 368 }
 369
 370 int main()
 371 {
 372         pthread_t thread;
 373
 374         sethandler(SIGSEGV, sigsegv, 0);
 375
 376         check_gs_value(0);
 377         check_gs_value(1);
 378         check_gs_value(0x200000000);
 379         check_gs_value(0);
 380         check_gs_value(0x200000000);
 381         check_gs_value(1);
 382
 383         for (int sched = 0; sched < 2; sched++) {
 384                 mov_0_gs(0, !!sched);
 385                 mov_0_gs(1, !!sched);
 386                 mov_0_gs(0x200000000, !!sched);
 387         }
 388
 389         /* Set up for multithreading. */
 390
 391         cpu_set_t cpuset;
 392         CPU_ZERO(&cpuset);
 393         CPU_SET(0, &cpuset);
 394         if (sched_setaffinity(0, sizeof(cpuset), &cpuset) != 0)
 395                 err(1, "sched_setaffinity to CPU 0");   /* should never fail */
 396
 397         if (pthread_create(&thread, 0, threadproc, 0) != 0)
 398                 err(1, "pthread_create");
 399
 400         static unsigned long bases_with_hard_zero[] = {
 401                 0, HARD_ZERO, 1, 0x200000000,
 402         };
 403
 404         for (int local = 0; local < 4; local++) {
 405                 for (int remote = 0; remote < 4; remote++) {
 406                         for (unsigned short s = 0; s < 5; s++) {
 407                                 unsigned short sel = s;
 408                                 if (s == 4)
 409                                         asm ("mov %%ss, %0" : "=rm" (sel));
 410                                 set_gs_and_switch_to(
 411                                         bases_with_hard_zero[local],
 412                                         sel,
 413                                         bases_with_hard_zero[remote]);
 414                         }
 415                 }
 416         }
 417
 418         test_unexpected_base();
 419
 420         ftx = 3;  /* Kill the thread. */
 421         syscall(SYS_futex, &ftx, FUTEX_WAKE, 0, NULL, NULL, 0);
 422
 423         if (pthread_join(thread, NULL) != 0)
 424                 err(1, "pthread_join");
 425
 426         return nerrs == 0 ? 0 : 1;
 427 }