1 /* SPDX-License-Identifier: GPL-2.0 */
2 /*
3  * mov_ss_trap.c: Exercise the bizarre side effects of a watchpoint on MOV SS
4  *
5  * This does MOV SS from a watchpointed address followed by various
6  * types of kernel entries.  A MOV SS that hits a watchpoint will queue
7  * up a #DB trap but will not actually deliver that trap.  The trap
8  * will be delivered after the next instruction instead.  The CPU's logic
9  * seems to be:
10  *
11  *  - Any fault: drop the pending #DB trap.
12  *  - INT $N, INT3, INTO, SYSCALL, SYSENTER: enter the kernel and then
13  *    deliver #DB.
14  *  - ICEBP: enter the kernel but do not deliver the watchpoint trap
15  *  - breakpoint: only one #DB is delivered (phew!)
16  *
17  * There are plenty of ways for a kernel to handle this incorrectly.  This
18  * test tries to exercise all the cases.
19  *
20  * This should mostly cover CVE-2018-1087 and CVE-2018-8897.
21  */
22 #define _GNU_SOURCE
23 
24 #include <stdlib.h>
25 #include <sys/ptrace.h>
26 #include <sys/types.h>
27 #include <sys/wait.h>
28 #include <sys/user.h>
29 #include <sys/syscall.h>
30 #include <unistd.h>
31 #include <errno.h>
32 #include <stddef.h>
33 #include <stdio.h>
34 #include <err.h>
35 #include <string.h>
36 #include <setjmp.h>
37 #include <sys/prctl.h>
38 
39 #define X86_EFLAGS_RF (1UL << 16)
40 
41 #if __x86_64__
42 # define REG_IP REG_RIP
43 #else
44 # define REG_IP REG_EIP
45 #endif
46 
47 unsigned short ss;
48 extern unsigned char breakpoint_insn[];
49 sigjmp_buf jmpbuf;
50 static unsigned char altstack_data[SIGSTKSZ];
51 
52 static void enable_watchpoint(void)
53 {
54 	pid_t parent = getpid();
55 	int status;
56 
57 	pid_t child = fork();
58 	if (child < 0)
59 		err(1, "fork");
60 
61 	if (child) {
62 		if (waitpid(child, &status, 0) != child)
63 			err(1, "waitpid for child");
64 	} else {
65 		unsigned long dr0, dr1, dr7;
66 
67 		dr0 = (unsigned long)&ss;
68 		dr1 = (unsigned long)breakpoint_insn;
69 		dr7 = ((1UL << 1) |	/* G0 */
70 		       (3UL << 16) |	/* RW0 = read or write */
71 		       (1UL << 18) |	/* LEN0 = 2 bytes */
72 		       (1UL << 3));	/* G1, RW1 = insn */
73 
74 		if (ptrace(PTRACE_ATTACH, parent, NULL, NULL) != 0)
75 			err(1, "PTRACE_ATTACH");
76 
77 		if (waitpid(parent, &status, 0) != parent)
78 			err(1, "waitpid for child");
79 
80 		if (ptrace(PTRACE_POKEUSER, parent, (void *)offsetof(struct user, u_debugreg[0]), dr0) != 0)
81 			err(1, "PTRACE_POKEUSER DR0");
82 
83 		if (ptrace(PTRACE_POKEUSER, parent, (void *)offsetof(struct user, u_debugreg[1]), dr1) != 0)
84 			err(1, "PTRACE_POKEUSER DR1");
85 
86 		if (ptrace(PTRACE_POKEUSER, parent, (void *)offsetof(struct user, u_debugreg[7]), dr7) != 0)
87 			err(1, "PTRACE_POKEUSER DR7");
88 
89 		printf("\tDR0 = %lx, DR1 = %lx, DR7 = %lx\n", dr0, dr1, dr7);
90 
91 		if (ptrace(PTRACE_DETACH, parent, NULL, NULL) != 0)
92 			err(1, "PTRACE_DETACH");
93 
94 		exit(0);
95 	}
96 }
97 
98 static void sethandler(int sig, void (*handler)(int, siginfo_t *, void *),
99 		       int flags)
100 {
101 	struct sigaction sa;
102 	memset(&sa, 0, sizeof(sa));
103 	sa.sa_sigaction = handler;
104 	sa.sa_flags = SA_SIGINFO | flags;
105 	sigemptyset(&sa.sa_mask);
106 	if (sigaction(sig, &sa, 0))
107 		err(1, "sigaction");
108 }
109 
110 static char const * const signames[] = {
111 	[SIGSEGV] = "SIGSEGV",
112 	[SIGBUS] = "SIBGUS",
113 	[SIGTRAP] = "SIGTRAP",
114 	[SIGILL] = "SIGILL",
115 };
116 
117 static void sigtrap(int sig, siginfo_t *si, void *ctx_void)
118 {
119 	ucontext_t *ctx = ctx_void;
120 
121 	printf("\tGot SIGTRAP with RIP=%lx, EFLAGS.RF=%d\n",
122 	       (unsigned long)ctx->uc_mcontext.gregs[REG_IP],
123 	       !!(ctx->uc_mcontext.gregs[REG_EFL] & X86_EFLAGS_RF));
124 }
125 
126 static void handle_and_return(int sig, siginfo_t *si, void *ctx_void)
127 {
128 	ucontext_t *ctx = ctx_void;
129 
130 	printf("\tGot %s with RIP=%lx\n", signames[sig],
131 	       (unsigned long)ctx->uc_mcontext.gregs[REG_IP]);
132 }
133 
134 static void handle_and_longjmp(int sig, siginfo_t *si, void *ctx_void)
135 {
136 	ucontext_t *ctx = ctx_void;
137 
138 	printf("\tGot %s with RIP=%lx\n", signames[sig],
139 	       (unsigned long)ctx->uc_mcontext.gregs[REG_IP]);
140 
141 	siglongjmp(jmpbuf, 1);
142 }
143 
144 int main()
145 {
146 	unsigned long nr;
147 
148 	asm volatile ("mov %%ss, %[ss]" : [ss] "=m" (ss));
149 	printf("\tSS = 0x%hx, &SS = 0x%p\n", ss, &ss);
150 
151 	if (prctl(PR_SET_PTRACER, PR_SET_PTRACER_ANY, 0, 0, 0) == 0)
152 		printf("\tPR_SET_PTRACER_ANY succeeded\n");
153 
154 	printf("\tSet up a watchpoint\n");
155 	sethandler(SIGTRAP, sigtrap, 0);
156 	enable_watchpoint();
157 
158 	printf("[RUN]\tRead from watched memory (should get SIGTRAP)\n");
159 	asm volatile ("mov %[ss], %[tmp]" : [tmp] "=r" (nr) : [ss] "m" (ss));
160 
161 	printf("[RUN]\tMOV SS; INT3\n");
162 	asm volatile ("mov %[ss], %%ss; int3" :: [ss] "m" (ss));
163 
164 	printf("[RUN]\tMOV SS; INT 3\n");
165 	asm volatile ("mov %[ss], %%ss; .byte 0xcd, 0x3" :: [ss] "m" (ss));
166 
167 	printf("[RUN]\tMOV SS; CS CS INT3\n");
168 	asm volatile ("mov %[ss], %%ss; .byte 0x2e, 0x2e; int3" :: [ss] "m" (ss));
169 
170 	printf("[RUN]\tMOV SS; CSx14 INT3\n");
171 	asm volatile ("mov %[ss], %%ss; .fill 14,1,0x2e; int3" :: [ss] "m" (ss));
172 
173 	printf("[RUN]\tMOV SS; INT 4\n");
174 	sethandler(SIGSEGV, handle_and_return, SA_RESETHAND);
175 	asm volatile ("mov %[ss], %%ss; int $4" :: [ss] "m" (ss));
176 
177 #ifdef __i386__
178 	printf("[RUN]\tMOV SS; INTO\n");
179 	sethandler(SIGSEGV, handle_and_return, SA_RESETHAND);
180 	nr = -1;
181 	asm volatile ("add $1, %[tmp]; mov %[ss], %%ss; into"
182 		      : [tmp] "+r" (nr) : [ss] "m" (ss));
183 #endif
184 
185 	if (sigsetjmp(jmpbuf, 1) == 0) {
186 		printf("[RUN]\tMOV SS; ICEBP\n");
187 
188 		/* Some emulators (e.g. QEMU TCG) don't emulate ICEBP. */
189 		sethandler(SIGILL, handle_and_longjmp, SA_RESETHAND);
190 
191 		asm volatile ("mov %[ss], %%ss; .byte 0xf1" :: [ss] "m" (ss));
192 	}
193 
194 	if (sigsetjmp(jmpbuf, 1) == 0) {
195 		printf("[RUN]\tMOV SS; CLI\n");
196 		sethandler(SIGSEGV, handle_and_longjmp, SA_RESETHAND);
197 		asm volatile ("mov %[ss], %%ss; cli" :: [ss] "m" (ss));
198 	}
199 
200 	if (sigsetjmp(jmpbuf, 1) == 0) {
201 		printf("[RUN]\tMOV SS; #PF\n");
202 		sethandler(SIGSEGV, handle_and_longjmp, SA_RESETHAND);
203 		asm volatile ("mov %[ss], %%ss; mov (-1), %[tmp]"
204 			      : [tmp] "=r" (nr) : [ss] "m" (ss));
205 	}
206 
207 	/*
208 	 * INT $1: if #DB has DPL=3 and there isn't special handling,
209 	 * then the kernel will die.
210 	 */
211 	if (sigsetjmp(jmpbuf, 1) == 0) {
212 		printf("[RUN]\tMOV SS; INT 1\n");
213 		sethandler(SIGSEGV, handle_and_longjmp, SA_RESETHAND);
214 		asm volatile ("mov %[ss], %%ss; int $1" :: [ss] "m" (ss));
215 	}
216 
217 #ifdef __x86_64__
218 	/*
219 	 * In principle, we should test 32-bit SYSCALL as well, but
220 	 * the calling convention is so unpredictable that it's
221 	 * not obviously worth the effort.
222 	 */
223 	if (sigsetjmp(jmpbuf, 1) == 0) {
224 		printf("[RUN]\tMOV SS; SYSCALL\n");
225 		sethandler(SIGILL, handle_and_longjmp, SA_RESETHAND);
226 		nr = SYS_getpid;
227 		/*
228 		 * Toggle the high bit of RSP to make it noncanonical to
229 		 * strengthen this test on non-SMAP systems.
230 		 */
231 		asm volatile ("btc $63, %%rsp\n\t"
232 			      "mov %[ss], %%ss; syscall\n\t"
233 			      "btc $63, %%rsp"
234 			      : "+a" (nr) : [ss] "m" (ss)
235 			      : "rcx"
236 #ifdef __x86_64__
237 				, "r11"
238 #endif
239 			);
240 	}
241 #endif
242 
243 	printf("[RUN]\tMOV SS; breakpointed NOP\n");
244 	asm volatile ("mov %[ss], %%ss; breakpoint_insn: nop" :: [ss] "m" (ss));
245 
246 	/*
247 	 * Invoking SYSENTER directly breaks all the rules.  Just handle
248 	 * the SIGSEGV.
249 	 */
250 	if (sigsetjmp(jmpbuf, 1) == 0) {
251 		printf("[RUN]\tMOV SS; SYSENTER\n");
252 		stack_t stack = {
253 			.ss_sp = altstack_data,
254 			.ss_size = SIGSTKSZ,
255 		};
256 		if (sigaltstack(&stack, NULL) != 0)
257 			err(1, "sigaltstack");
258 		sethandler(SIGSEGV, handle_and_longjmp, SA_RESETHAND | SA_ONSTACK);
259 		nr = SYS_getpid;
260 		asm volatile ("mov %[ss], %%ss; SYSENTER" : "+a" (nr)
261 			      : [ss] "m" (ss) : "flags", "rcx"
262 #ifdef __x86_64__
263 				, "r11"
264 #endif
265 			);
266 
267 		/* We're unreachable here.  SYSENTER forgets RIP. */
268 	}
269 
270 	if (sigsetjmp(jmpbuf, 1) == 0) {
271 		printf("[RUN]\tMOV SS; INT $0x80\n");
272 		sethandler(SIGSEGV, handle_and_longjmp, SA_RESETHAND);
273 		nr = 20;	/* compat getpid */
274 		asm volatile ("mov %[ss], %%ss; int $0x80"
275 			      : "+a" (nr) : [ss] "m" (ss)
276 			      : "flags"
277 #ifdef __x86_64__
278 				, "r8", "r9", "r10", "r11"
279 #endif
280 			);
281 	}
282 
283 	printf("[OK]\tI aten't dead\n");
284 	return 0;
285 }
286