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 
threadproc(void * ctx)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 
main()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 }
105