tjh.dev
Linux kernel mirror (for testing)
git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
kernel
os
linux
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}