blob: ce42d5a6400904b85295f315b2443b86514b000b [file] [log] [blame]
* sysret_ss_attrs.c - test that syscalls return valid hidden SS attributes
* Copyright (c) 2015 Andrew Lutomirski
* This program is free software; you can redistribute it and/or modify
* it under the terms and conditions of the GNU General Public License,
* version 2, as published by the Free Software Foundation.
* This program is distributed in the hope it will be useful, but
* WITHOUT ANY WARRANTY; without even the implied warranty of
* General Public License for more details.
* On AMD CPUs, SYSRET can return with a valid SS descriptor with with
* the hidden attributes set to an unusable state. Make sure the kernel
* doesn't let this happen.
#define _GNU_SOURCE
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>
#include <sys/mman.h>
#include <err.h>
#include <stddef.h>
#include <stdbool.h>
#include <pthread.h>
static void *threadproc(void *ctx)
* Do our best to cause sleeps on this CPU to exit the kernel and
* re-enter with SS = 0.
while (true)
return NULL;
#ifdef __x86_64__
extern unsigned long call32_from_64(void *stack, void (*function)(void));
asm (".pushsection .text\n\t"
"pushl $0\n\t"
"popl %eax\n\t"
extern void test_ss(void);
int main()
* Start a busy-looping thread on the same CPU we're on.
* For simplicity, just stick everything to CPU 0. This will
* fail in some containers, but that's probably okay.
cpu_set_t cpuset;
CPU_SET(0, &cpuset);
if (sched_setaffinity(0, sizeof(cpuset), &cpuset) != 0)
printf("[WARN]\tsched_setaffinity failed\n");
pthread_t thread;
if (pthread_create(&thread, 0, threadproc, 0) != 0)
err(1, "pthread_create");
#ifdef __x86_64__
unsigned char *stack32 = mmap(NULL, 4096, PROT_READ | PROT_WRITE,
-1, 0);
if (stack32 == MAP_FAILED)
err(1, "mmap");
printf("[RUN]\tSyscalls followed by SS validation\n");
for (int i = 0; i < 1000; i++) {
* Go to sleep and return using sysret (if we're 64-bit
* or we're 32-bit on AMD on a 64-bit kernel). On AMD CPUs,
* SYSRET doesn't fix up the cached SS descriptor, so the
* kernel needs some kind of workaround to make sure that we
* end the system call with a valid stack segment. This
* can be a confusing failure because the SS *selector*
* is the same regardless.
#ifdef __x86_64__
* On 32-bit, just doing a syscall through glibc is enough
* to cause a crash if our cached SS descriptor is invalid.
* On 64-bit, it's not, so try extra hard.
call32_from_64(stack32 + 4088, test_ss);
printf("[OK]\tWe survived\n");
#ifdef __x86_64__
munmap(stack32, 4096);
return 0;