1 // SPDX-License-Identifier: GPL-2.0-only
2 /*
3 * single_step_syscall.c - single-steps various x86 syscalls
4 * Copyright (c) 2014-2015 Andrew Lutomirski
5 *
6 * This is a very simple series of tests that makes system calls with
7 * the TF flag set. This exercises some nasty kernel code in the
8 * SYSENTER case: SYSENTER does not clear TF, so SYSENTER with TF set
9 * immediately issues #DB from CPL 0. This requires special handling in
10 * the kernel.
11 */
12
13 #define _GNU_SOURCE
14
15 #include <sys/time.h>
16 #include <time.h>
17 #include <stdlib.h>
18 #include <sys/syscall.h>
19 #include <unistd.h>
20 #include <stdio.h>
21 #include <string.h>
22 #include <inttypes.h>
23 #include <sys/mman.h>
24 #include <sys/signal.h>
25 #include <sys/ucontext.h>
26 #include <asm/ldt.h>
27 #include <err.h>
28 #include <setjmp.h>
29 #include <stddef.h>
30 #include <stdbool.h>
31 #include <sys/ptrace.h>
32 #include <sys/user.h>
33
34 #include "helpers.h"
35
sethandler(int sig,void (* handler)(int,siginfo_t *,void *),int flags)36 static void sethandler(int sig, void (*handler)(int, siginfo_t *, void *),
37 int flags)
38 {
39 struct sigaction sa;
40 memset(&sa, 0, sizeof(sa));
41 sa.sa_sigaction = handler;
42 sa.sa_flags = SA_SIGINFO | flags;
43 sigemptyset(&sa.sa_mask);
44 if (sigaction(sig, &sa, 0))
45 err(1, "sigaction");
46 }
47
clearhandler(int sig)48 static void clearhandler(int sig)
49 {
50 struct sigaction sa;
51 memset(&sa, 0, sizeof(sa));
52 sa.sa_handler = SIG_DFL;
53 sigemptyset(&sa.sa_mask);
54 if (sigaction(sig, &sa, 0))
55 err(1, "sigaction");
56 }
57
58 static volatile sig_atomic_t sig_traps, sig_eflags;
59 sigjmp_buf jmpbuf;
60 static unsigned char altstack_data[SIGSTKSZ];
61
62 #ifdef __x86_64__
63 # define REG_IP REG_RIP
64 # define WIDTH "q"
65 # define INT80_CLOBBERS "r8", "r9", "r10", "r11"
66 #else
67 # define REG_IP REG_EIP
68 # define WIDTH "l"
69 # define INT80_CLOBBERS
70 #endif
71
sigtrap(int sig,siginfo_t * info,void * ctx_void)72 static void sigtrap(int sig, siginfo_t *info, void *ctx_void)
73 {
74 ucontext_t *ctx = (ucontext_t*)ctx_void;
75
76 if (get_eflags() & X86_EFLAGS_TF) {
77 set_eflags(get_eflags() & ~X86_EFLAGS_TF);
78 printf("[WARN]\tSIGTRAP handler had TF set\n");
79 _exit(1);
80 }
81
82 sig_traps++;
83
84 if (sig_traps == 10000 || sig_traps == 10001) {
85 printf("[WARN]\tHit %d SIGTRAPs with si_addr 0x%lx, ip 0x%lx\n",
86 (int)sig_traps,
87 (unsigned long)info->si_addr,
88 (unsigned long)ctx->uc_mcontext.gregs[REG_IP]);
89 }
90 }
91
92 static char const * const signames[] = {
93 [SIGSEGV] = "SIGSEGV",
94 [SIGBUS] = "SIBGUS",
95 [SIGTRAP] = "SIGTRAP",
96 [SIGILL] = "SIGILL",
97 };
98
print_and_longjmp(int sig,siginfo_t * si,void * ctx_void)99 static void print_and_longjmp(int sig, siginfo_t *si, void *ctx_void)
100 {
101 ucontext_t *ctx = ctx_void;
102
103 printf("\tGot %s with RIP=%lx, TF=%ld\n", signames[sig],
104 (unsigned long)ctx->uc_mcontext.gregs[REG_IP],
105 (unsigned long)ctx->uc_mcontext.gregs[REG_EFL] & X86_EFLAGS_TF);
106
107 sig_eflags = (unsigned long)ctx->uc_mcontext.gregs[REG_EFL];
108 siglongjmp(jmpbuf, 1);
109 }
110
check_result(void)111 static void check_result(void)
112 {
113 unsigned long new_eflags = get_eflags();
114 set_eflags(new_eflags & ~X86_EFLAGS_TF);
115
116 if (!sig_traps) {
117 printf("[FAIL]\tNo SIGTRAP\n");
118 exit(1);
119 }
120
121 if (!(new_eflags & X86_EFLAGS_TF)) {
122 printf("[FAIL]\tTF was cleared\n");
123 exit(1);
124 }
125
126 printf("[OK]\tSurvived with TF set and %d traps\n", (int)sig_traps);
127 sig_traps = 0;
128 }
129
fast_syscall_no_tf(void)130 static void fast_syscall_no_tf(void)
131 {
132 sig_traps = 0;
133 printf("[RUN]\tFast syscall with TF cleared\n");
134 fflush(stdout); /* Force a syscall */
135 if (get_eflags() & X86_EFLAGS_TF) {
136 printf("[FAIL]\tTF is now set\n");
137 exit(1);
138 }
139 if (sig_traps) {
140 printf("[FAIL]\tGot SIGTRAP\n");
141 exit(1);
142 }
143 printf("[OK]\tNothing unexpected happened\n");
144 }
145
main()146 int main()
147 {
148 #ifdef CAN_BUILD_32
149 int tmp;
150 #endif
151
152 sethandler(SIGTRAP, sigtrap, 0);
153
154 printf("[RUN]\tSet TF and check nop\n");
155 set_eflags(get_eflags() | X86_EFLAGS_TF);
156 asm volatile ("nop");
157 check_result();
158
159 #ifdef __x86_64__
160 printf("[RUN]\tSet TF and check syscall-less opportunistic sysret\n");
161 set_eflags(get_eflags() | X86_EFLAGS_TF);
162 extern unsigned char post_nop[];
163 asm volatile ("pushf" WIDTH "\n\t"
164 "pop" WIDTH " %%r11\n\t"
165 "nop\n\t"
166 "post_nop:"
167 : : "c" (post_nop) : "r11");
168 check_result();
169 #endif
170 #ifdef CAN_BUILD_32
171 printf("[RUN]\tSet TF and check int80\n");
172 set_eflags(get_eflags() | X86_EFLAGS_TF);
173 asm volatile ("int $0x80" : "=a" (tmp) : "a" (SYS_getpid)
174 : INT80_CLOBBERS);
175 check_result();
176 #endif
177
178 /*
179 * This test is particularly interesting if fast syscalls use
180 * SYSENTER: it triggers a nasty design flaw in SYSENTER.
181 * Specifically, SYSENTER does not clear TF, so either SYSENTER
182 * or the next instruction traps at CPL0. (Of course, Intel
183 * mostly forgot to document exactly what happens here.) So we
184 * get a CPL0 fault with usergs (on 64-bit kernels) and possibly
185 * no stack. The only sane way the kernel can possibly handle
186 * it is to clear TF on return from the #DB handler, but this
187 * happens way too early to set TF in the saved pt_regs, so the
188 * kernel has to do something clever to avoid losing track of
189 * the TF bit.
190 *
191 * Needless to say, we've had bugs in this area.
192 */
193 syscall(SYS_getpid); /* Force symbol binding without TF set. */
194 printf("[RUN]\tSet TF and check a fast syscall\n");
195 set_eflags(get_eflags() | X86_EFLAGS_TF);
196 syscall(SYS_getpid);
197 check_result();
198
199 /* Now make sure that another fast syscall doesn't set TF again. */
200 fast_syscall_no_tf();
201
202 /*
203 * And do a forced SYSENTER to make sure that this works even if
204 * fast syscalls don't use SYSENTER.
205 *
206 * Invoking SYSENTER directly breaks all the rules. Just handle
207 * the SIGSEGV.
208 */
209 if (sigsetjmp(jmpbuf, 1) == 0) {
210 unsigned long nr = SYS_getpid;
211 printf("[RUN]\tSet TF and check SYSENTER\n");
212 stack_t stack = {
213 .ss_sp = altstack_data,
214 .ss_size = SIGSTKSZ,
215 };
216 if (sigaltstack(&stack, NULL) != 0)
217 err(1, "sigaltstack");
218 sethandler(SIGSEGV, print_and_longjmp,
219 SA_RESETHAND | SA_ONSTACK);
220 sethandler(SIGILL, print_and_longjmp, SA_RESETHAND);
221 set_eflags(get_eflags() | X86_EFLAGS_TF);
222 /* Clear EBP first to make sure we segfault cleanly. */
223 asm volatile ("xorl %%ebp, %%ebp; SYSENTER" : "+a" (nr) :: "flags", "rcx"
224 #ifdef __x86_64__
225 , "r11"
226 #endif
227 );
228
229 /* We're unreachable here. SYSENTER forgets RIP. */
230 }
231 clearhandler(SIGSEGV);
232 clearhandler(SIGILL);
233 if (!(sig_eflags & X86_EFLAGS_TF)) {
234 printf("[FAIL]\tTF was cleared\n");
235 exit(1);
236 }
237
238 /* Now make sure that another fast syscall doesn't set TF again. */
239 fast_syscall_no_tf();
240
241 return 0;
242 }
243