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
 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}