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/*
* 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
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* 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"
".code32\n\t"
"test_ss:\n\t"
"pushl $0\n\t"
"popl %eax\n\t"
"ret\n\t"
".code64");
extern void test_ss(void);
#endif
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_ZERO(&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,
MAP_32BIT | MAP_ANONYMOUS | MAP_PRIVATE,
-1, 0);
if (stack32 == MAP_FAILED)
err(1, "mmap");
#endif
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.
*/
usleep(2);
#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);
#endif
}
printf("[OK]\tWe survived\n");
#ifdef __x86_64__
munmap(stack32, 4096);
#endif
return 0;
}
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