LXR linux/tools/testing/selftests/x86/sysret_ss

   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
  23static 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__
  36extern unsigned long call32_from_64(void *stack, void (*function)(void));
  37
  38asm (".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");
  45extern void test_ss(void);
  46#endif
  47
  48int 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}
 105