tools/testing/selftests/x86/sysret_ss_attrs.c

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 /*
   3  * sysret_ss_attrs.c - test that syscalls return valid hidden SS attributes
   4  * Copyright (c) 2015 Andrew Lutomirski
   5  *
   6  * On AMD CPUs, SYSRET can return with a valid SS descriptor with with
   7  * the hidden attributes set to an unusable state.  Make sure the kernel
   8  * doesn't let this happen.
   9  */
  10
  11 #define _GNU_SOURCE
  12
  13 #include <stdlib.h>
  14 #include <unistd.h>
  15 #include <stdio.h>
  16 #include <string.h>
  17 #include <sys/mman.h>
  18 #include <err.h>
  19 #include <stddef.h>
  20 #include <stdbool.h>
  21 #include <pthread.h>
  22
  23 static void *threadproc(void *ctx)
  24 {
  25         /*
  26          * Do our best to cause sleeps on this CPU to exit the kernel and
  27          * re-enter with SS = 0.
  28          */
  29         while (true)
  30                 ;
  31
  32         return NULL;
  33 }
  34
  35 #ifdef __x86_64__
  36 extern unsigned long call32_from_64(void *stack, void (*function)(void));
  37
  38 asm (".pushsection .text\n\t"
  39      ".code32\n\t"
  40      "test_ss:\n\t"
  41      "pushl $0\n\t"
  42      "popl %eax\n\t"
  43      "ret\n\t"
  44      ".code64");
  45 extern void test_ss(void);
  46 #endif
  47
  48 int main()
  49 {
  50         /*
  51          * Start a busy-looping thread on the same CPU we're on.
  52          * For simplicity, just stick everything to CPU 0.  This will
  53          * fail in some containers, but that's probably okay.
  54          */
  55         cpu_set_t cpuset;
  56         CPU_ZERO(&cpuset);
  57         CPU_SET(0, &cpuset);
  58         if (sched_setaffinity(0, sizeof(cpuset), &cpuset) != 0)
  59                 printf("[WARN]\tsched_setaffinity failed\n");
  60
  61         pthread_t thread;
  62         if (pthread_create(&thread, 0, threadproc, 0) != 0)
  63                 err(1, "pthread_create");
  64
  65 #ifdef __x86_64__
  66         unsigned char *stack32 = mmap(NULL, 4096, PROT_READ | PROT_WRITE,
  67                                       MAP_32BIT | MAP_ANONYMOUS | MAP_PRIVATE,
  68                                       -1, 0);
  69         if (stack32 == MAP_FAILED)
  70                 err(1, "mmap");
  71 #endif
  72
  73         printf("[RUN]\tSyscalls followed by SS validation\n");
  74
  75         for (int i = 0; i < 1000; i++) {
  76                 /*
  77                  * Go to sleep and return using sysret (if we're 64-bit
  78                  * or we're 32-bit on AMD on a 64-bit kernel).  On AMD CPUs,
  79                  * SYSRET doesn't fix up the cached SS descriptor, so the
  80                  * kernel needs some kind of workaround to make sure that we
  81                  * end the system call with a valid stack segment.  This
  82                  * can be a confusing failure because the SS *selector*
  83                  * is the same regardless.
  84                  */
  85                 usleep(2);
  86
  87 #ifdef __x86_64__
  88                 /*
  89                  * On 32-bit, just doing a syscall through glibc is enough
  90                  * to cause a crash if our cached SS descriptor is invalid.
  91                  * On 64-bit, it's not, so try extra hard.
  92                  */
  93                 call32_from_64(stack32 + 4088, test_ss);
  94 #endif
  95         }
  96
  97         printf("[OK]\tWe survived\n");
  98
  99 #ifdef __x86_64__
 100         munmap(stack32, 4096);
 101 #endif
 102
 103         return 0;
 104 }