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, &regs) != 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, &regs2) != 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, &regs) != 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