1 // SPDX-License-Identifier: GPL-2.0-only
2 /*
3 * fsgsbase_restore.c, test ptrace vs fsgsbase
4 * Copyright (c) 2020 Andy Lutomirski
5 *
6 * This test case simulates a tracer redirecting tracee execution to
7 * a function and then restoring tracee state using PTRACE_GETREGS and
8 * PTRACE_SETREGS. This is similar to what gdb does when doing
9 * 'p func()'. The catch is that this test has the called function
10 * modify a segment register. This makes sure that ptrace correctly
11 * restores segment state when using PTRACE_SETREGS.
12 *
13 * This is not part of fsgsbase.c, because that test is 64-bit only.
14 */
15
16 #define _GNU_SOURCE
17 #include <stdio.h>
18 #include <stdlib.h>
19 #include <stdbool.h>
20 #include <string.h>
21 #include <sys/syscall.h>
22 #include <unistd.h>
23 #include <err.h>
24 #include <sys/user.h>
25 #include <asm/prctl.h>
26 #include <sys/prctl.h>
27 #include <asm/ldt.h>
28 #include <sys/mman.h>
29 #include <stddef.h>
30 #include <sys/ptrace.h>
31 #include <sys/wait.h>
32 #include <stdint.h>
33
34 #define EXPECTED_VALUE 0x1337f00d
35
36 #ifdef __x86_64__
37 # define SEG "%gs"
38 #else
39 # define SEG "%fs"
40 #endif
41
dereference_seg_base(void)42 static unsigned int dereference_seg_base(void)
43 {
44 int ret;
45 asm volatile ("mov %" SEG ":(0), %0" : "=rm" (ret));
46 return ret;
47 }
48
init_seg(void)49 static void init_seg(void)
50 {
51 unsigned int *target = mmap(
52 NULL, sizeof(unsigned int),
53 PROT_READ | PROT_WRITE,
54 MAP_PRIVATE | MAP_ANONYMOUS | MAP_32BIT, -1, 0);
55 if (target == MAP_FAILED)
56 err(1, "mmap");
57
58 *target = EXPECTED_VALUE;
59
60 printf("\tsegment base address = 0x%lx\n", (unsigned long)target);
61
62 struct user_desc desc = {
63 .entry_number = 0,
64 .base_addr = (unsigned int)(uintptr_t)target,
65 .limit = sizeof(unsigned int) - 1,
66 .seg_32bit = 1,
67 .contents = 0, /* Data, grow-up */
68 .read_exec_only = 0,
69 .limit_in_pages = 0,
70 .seg_not_present = 0,
71 .useable = 0
72 };
73 if (syscall(SYS_modify_ldt, 1, &desc, sizeof(desc)) == 0) {
74 printf("\tusing LDT slot 0\n");
75 asm volatile ("mov %0, %" SEG :: "rm" ((unsigned short)0x7));
76 } else {
77 /* No modify_ldt for us (configured out, perhaps) */
78
79 struct user_desc *low_desc = mmap(
80 NULL, sizeof(desc),
81 PROT_READ | PROT_WRITE,
82 MAP_PRIVATE | MAP_ANONYMOUS | MAP_32BIT, -1, 0);
83 memcpy(low_desc, &desc, sizeof(desc));
84
85 low_desc->entry_number = -1;
86
87 /* 32-bit set_thread_area */
88 long ret;
89 asm volatile ("int $0x80"
90 : "=a" (ret), "+m" (*low_desc)
91 : "a" (243), "b" (low_desc)
92 #ifdef __x86_64__
93 : "r8", "r9", "r10", "r11"
94 #endif
95 );
96 memcpy(&desc, low_desc, sizeof(desc));
97 munmap(low_desc, sizeof(desc));
98
99 if (ret != 0) {
100 printf("[NOTE]\tcould not create a segment -- can't test anything\n");
101 exit(0);
102 }
103 printf("\tusing GDT slot %d\n", desc.entry_number);
104
105 unsigned short sel = (unsigned short)((desc.entry_number << 3) | 0x3);
106 asm volatile ("mov %0, %" SEG :: "rm" (sel));
107 }
108 }
109
tracee_zap_segment(void)110 static void tracee_zap_segment(void)
111 {
112 /*
113 * The tracer will redirect execution here. This is meant to
114 * work like gdb's 'p func()' feature. The tricky bit is that
115 * we modify a segment register in order to make sure that ptrace
116 * can correctly restore segment registers.
117 */
118 printf("\tTracee: in tracee_zap_segment()\n");
119
120 /*
121 * Write a nonzero selector with base zero to the segment register.
122 * Using a null selector would defeat the test on AMD pre-Zen2
123 * CPUs, as such CPUs don't clear the base when loading a null
124 * selector.
125 */
126 unsigned short sel;
127 asm volatile ("mov %%ss, %0\n\t"
128 "mov %0, %" SEG
129 : "=rm" (sel));
130
131 pid_t pid = getpid(), tid = syscall(SYS_gettid);
132
133 printf("\tTracee is going back to sleep\n");
134 syscall(SYS_tgkill, pid, tid, SIGSTOP);
135
136 /* Should not get here. */
137 while (true) {
138 printf("[FAIL]\tTracee hit unreachable code\n");
139 pause();
140 }
141 }
142
main()143 int main()
144 {
145 printf("\tSetting up a segment\n");
146 init_seg();
147
148 unsigned int val = dereference_seg_base();
149 if (val != EXPECTED_VALUE) {
150 printf("[FAIL]\tseg[0] == %x; should be %x\n", val, EXPECTED_VALUE);
151 return 1;
152 }
153 printf("[OK]\tThe segment points to the right place.\n");
154
155 pid_t chld = fork();
156 if (chld < 0)
157 err(1, "fork");
158
159 if (chld == 0) {
160 prctl(PR_SET_PDEATHSIG, SIGKILL, 0, 0, 0, 0);
161
162 if (ptrace(PTRACE_TRACEME, 0, 0, 0) != 0)
163 err(1, "PTRACE_TRACEME");
164
165 pid_t pid = getpid(), tid = syscall(SYS_gettid);
166
167 printf("\tTracee will take a nap until signaled\n");
168 syscall(SYS_tgkill, pid, tid, SIGSTOP);
169
170 printf("\tTracee was resumed. Will re-check segment.\n");
171
172 val = dereference_seg_base();
173 if (val != EXPECTED_VALUE) {
174 printf("[FAIL]\tseg[0] == %x; should be %x\n", val, EXPECTED_VALUE);
175 exit(1);
176 }
177
178 printf("[OK]\tThe segment points to the right place.\n");
179 exit(0);
180 }
181
182 int status;
183
184 /* Wait for SIGSTOP. */
185 if (waitpid(chld, &status, 0) != chld || !WIFSTOPPED(status))
186 err(1, "waitpid");
187
188 struct user_regs_struct regs;
189
190 if (ptrace(PTRACE_GETREGS, chld, NULL, ®s) != 0)
191 err(1, "PTRACE_GETREGS");
192
193 #ifdef __x86_64__
194 printf("\tChild GS=0x%lx, GSBASE=0x%lx\n", (unsigned long)regs.gs, (unsigned long)regs.gs_base);
195 #else
196 printf("\tChild FS=0x%lx\n", (unsigned long)regs.xfs);
197 #endif
198
199 struct user_regs_struct regs2 = regs;
200 #ifdef __x86_64__
201 regs2.rip = (unsigned long)tracee_zap_segment;
202 regs2.rsp -= 128; /* Don't clobber the redzone. */
203 #else
204 regs2.eip = (unsigned long)tracee_zap_segment;
205 #endif
206
207 printf("\tTracer: redirecting tracee to tracee_zap_segment()\n");
208 if (ptrace(PTRACE_SETREGS, chld, NULL, ®s2) != 0)
209 err(1, "PTRACE_GETREGS");
210 if (ptrace(PTRACE_CONT, chld, NULL, NULL) != 0)
211 err(1, "PTRACE_GETREGS");
212
213 /* Wait for SIGSTOP. */
214 if (waitpid(chld, &status, 0) != chld || !WIFSTOPPED(status))
215 err(1, "waitpid");
216
217 printf("\tTracer: restoring tracee state\n");
218 if (ptrace(PTRACE_SETREGS, chld, NULL, ®s) != 0)
219 err(1, "PTRACE_GETREGS");
220 if (ptrace(PTRACE_DETACH, chld, NULL, NULL) != 0)
221 err(1, "PTRACE_GETREGS");
222
223 /* Wait for SIGSTOP. */
224 if (waitpid(chld, &status, 0) != chld)
225 err(1, "waitpid");
226
227 if (WIFSIGNALED(status)) {
228 printf("[FAIL]\tTracee crashed\n");
229 return 1;
230 }
231
232 if (!WIFEXITED(status)) {
233 printf("[FAIL]\tTracee stopped for an unexpected reason: %d\n", status);
234 return 1;
235 }
236
237 int exitcode = WEXITSTATUS(status);
238 if (exitcode != 0) {
239 printf("[FAIL]\tTracee reported failure\n");
240 return 1;
241 }
242
243 printf("[OK]\tAll is well.\n");
244 return 0;
245 }
246