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

   1/*
   2 * sysret_ss_attrs.c - test that syscalls return valid hidden SS attributes
   3 * Copyright (c) 2015 Andrew Lutomirski
   4 *
   5 * This program is free software; you can redistribute it and/or modify
   6 * it under the terms and conditions of the GNU General Public License,
   7 * version 2, as published by the Free Software Foundation.
   8 *
   9 * This program is distributed in the hope it will be useful, but
  10 * WITHOUT ANY WARRANTY; without even the implied warranty of
  11 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12 * General Public License for more details.
  13 *
  14 * On AMD CPUs, SYSRET can return with a valid SS descriptor with with
  15 * the hidden attributes set to an unusable state.  Make sure the kernel
  16 * doesn't let this happen.
  17 */
  18
  19#define _GNU_SOURCE
  20
  21#include <stdlib.h>
  22#include <unistd.h>
  23#include <stdio.h>
  24#include <string.h>
  25#include <sys/mman.h>
  26#include <err.h>
  27#include <stddef.h>
  28#include <stdbool.h>
  29#include <pthread.h>
  30
  31static void *threadproc(void *ctx)
  32{
  33        /*
  34         * Do our best to cause sleeps on this CPU to exit the kernel and
  35         * re-enter with SS = 0.
  36         */
  37        while (true)
  38                ;
  39
  40        return NULL;
  41}
  42
  43#ifdef __x86_64__
  44extern unsigned long call32_from_64(void *stack, void (*function)(void));
  45
  46asm (".pushsection .text\n\t"
  47     ".code32\n\t"
  48     "test_ss:\n\t"
  49     "pushl $0\n\t"
  50     "popl %eax\n\t"
  51     "ret\n\t"
  52     ".code64");
  53extern void test_ss(void);
  54#endif
  55
  56int main()
  57{
  58        /*
  59         * Start a busy-looping thread on the same CPU we're on.
  60         * For simplicity, just stick everything to CPU 0.  This will
  61         * fail in some containers, but that's probably okay.
  62         */
  63        cpu_set_t cpuset;
  64        CPU_ZERO(&cpuset);
  65        CPU_SET(0, &cpuset);
  66        if (sched_setaffinity(0, sizeof(cpuset), &cpuset) != 0)
  67                printf("[WARN]\tsched_setaffinity failed\n");
  68
  69        pthread_t thread;
  70        if (pthread_create(&thread, 0, threadproc, 0) != 0)
  71                err(1, "pthread_create");
  72
  73#ifdef __x86_64__
  74        unsigned char *stack32 = mmap(NULL, 4096, PROT_READ | PROT_WRITE,
  75                                      MAP_32BIT | MAP_ANONYMOUS | MAP_PRIVATE,
  76                                      -1, 0);
  77        if (stack32 == MAP_FAILED)
  78                err(1, "mmap");
  79#endif
  80
  81        printf("[RUN]\tSyscalls followed by SS validation\n");
  82
  83        for (int i = 0; i < 1000; i++) {
  84                /*
  85                 * Go to sleep and return using sysret (if we're 64-bit
  86                 * or we're 32-bit on AMD on a 64-bit kernel).  On AMD CPUs,
  87                 * SYSRET doesn't fix up the cached SS descriptor, so the
  88                 * kernel needs some kind of workaround to make sure that we
  89                 * end the system call with a valid stack segment.  This
  90                 * can be a confusing failure because the SS *selector*
  91                 * is the same regardless.
  92                 */
  93                usleep(2);
  94
  95#ifdef __x86_64__
  96                /*
  97                 * On 32-bit, just doing a syscall through glibc is enough
  98                 * to cause a crash if our cached SS descriptor is invalid.
  99                 * On 64-bit, it's not, so try extra hard.
 100                 */
 101                call32_from_64(stack32 + 4088, test_ss);
 102#endif
 103        }
 104
 105        printf("[OK]\tWe survived\n");
 106
 107#ifdef __x86_64__
 108        munmap(stack32, 4096);
 109#endif
 110
 111        return 0;
 112}
 113