cachepc-linux

Fork of AMDESE/linux with modifications for CachePC side-channel attack
git clone https://git.sinitax.com/sinitax/cachepc-linux
Log | Files | Refs | README | LICENSE | sfeed.txt

sysret_ss_attrs.c (2514B)

      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}