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
23 static 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__
36 extern unsigned long call32_from_64(void *stack, void (*function)(void));
37
38 asm (".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");
45 extern void test_ss(void);
46 #endif
47
48 int 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 }