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
61 #ifdef __x86_64__
62 # define REG_IP REG_RIP
63 # define WIDTH "q"
64 # define INT80_CLOBBERS "r8", "r9", "r10", "r11"
65 #else
66 # define REG_IP REG_EIP
67 # define WIDTH "l"
68 # define INT80_CLOBBERS
69 #endif
70
sigtrap(int sig,siginfo_t * info,void * ctx_void)71 static void sigtrap(int sig, siginfo_t *info, void *ctx_void)
72 {
73 ucontext_t *ctx = (ucontext_t*)ctx_void;
74
75 if (get_eflags() & X86_EFLAGS_TF) {
76 set_eflags(get_eflags() & ~X86_EFLAGS_TF);
77 printf("[WARN]\tSIGTRAP handler had TF set\n");
78 _exit(1);
79 }
80
81 sig_traps++;
82
83 if (sig_traps == 10000 || sig_traps == 10001) {
84 printf("[WARN]\tHit %d SIGTRAPs with si_addr 0x%lx, ip 0x%lx\n",
85 (int)sig_traps,
86 (unsigned long)info->si_addr,
87 (unsigned long)ctx->uc_mcontext.gregs[REG_IP]);
88 }
89 }
90
91 static char const * const signames[] = {
92 [SIGSEGV] = "SIGSEGV",
93 [SIGBUS] = "SIBGUS",
94 [SIGTRAP] = "SIGTRAP",
95 [SIGILL] = "SIGILL",
96 };
97
print_and_longjmp(int sig,siginfo_t * si,void * ctx_void)98 static void print_and_longjmp(int sig, siginfo_t *si, void *ctx_void)
99 {
100 ucontext_t *ctx = ctx_void;
101
102 printf("\tGot %s with RIP=%lx, TF=%ld\n", signames[sig],
103 (unsigned long)ctx->uc_mcontext.gregs[REG_IP],
104 (unsigned long)ctx->uc_mcontext.gregs[REG_EFL] & X86_EFLAGS_TF);
105
106 sig_eflags = (unsigned long)ctx->uc_mcontext.gregs[REG_EFL];
107 siglongjmp(jmpbuf, 1);
108 }
109
check_result(void)110 static void check_result(void)
111 {
112 unsigned long new_eflags = get_eflags();
113 set_eflags(new_eflags & ~X86_EFLAGS_TF);
114
115 if (!sig_traps) {
116 printf("[FAIL]\tNo SIGTRAP\n");
117 exit(1);
118 }
119
120 if (!(new_eflags & X86_EFLAGS_TF)) {
121 printf("[FAIL]\tTF was cleared\n");
122 exit(1);
123 }
124
125 printf("[OK]\tSurvived with TF set and %d traps\n", (int)sig_traps);
126 sig_traps = 0;
127 }
128
fast_syscall_no_tf(void)129 static void fast_syscall_no_tf(void)
130 {
131 sig_traps = 0;
132 printf("[RUN]\tFast syscall with TF cleared\n");
133 fflush(stdout); /* Force a syscall */
134 if (get_eflags() & X86_EFLAGS_TF) {
135 printf("[FAIL]\tTF is now set\n");
136 exit(1);
137 }
138 if (sig_traps) {
139 printf("[FAIL]\tGot SIGTRAP\n");
140 exit(1);
141 }
142 printf("[OK]\tNothing unexpected happened\n");
143 }
144
main()145 int main()
146 {
147 #ifdef CAN_BUILD_32
148 int tmp;
149 #endif
150
151 sethandler(SIGTRAP, sigtrap, 0);
152
153 printf("[RUN]\tSet TF and check nop\n");
154 set_eflags(get_eflags() | X86_EFLAGS_TF);
155 asm volatile ("nop");
156 check_result();
157
158 #ifdef __x86_64__
159 printf("[RUN]\tSet TF and check syscall-less opportunistic sysret\n");
160 set_eflags(get_eflags() | X86_EFLAGS_TF);
161 extern unsigned char post_nop[];
162 asm volatile ("pushf" WIDTH "\n\t"
163 "pop" WIDTH " %%r11\n\t"
164 "nop\n\t"
165 "post_nop:"
166 : : "c" (post_nop) : "r11");
167 check_result();
168 #endif
169 #ifdef CAN_BUILD_32
170 printf("[RUN]\tSet TF and check int80\n");
171 set_eflags(get_eflags() | X86_EFLAGS_TF);
172 asm volatile ("int $0x80" : "=a" (tmp) : "a" (SYS_getpid)
173 : INT80_CLOBBERS);
174 check_result();
175 #endif
176
177 /*
178 * This test is particularly interesting if fast syscalls use
179 * SYSENTER: it triggers a nasty design flaw in SYSENTER.
180 * Specifically, SYSENTER does not clear TF, so either SYSENTER
181 * or the next instruction traps at CPL0. (Of course, Intel
182 * mostly forgot to document exactly what happens here.) So we
183 * get a CPL0 fault with usergs (on 64-bit kernels) and possibly
184 * no stack. The only sane way the kernel can possibly handle
185 * it is to clear TF on return from the #DB handler, but this
186 * happens way too early to set TF in the saved pt_regs, so the
187 * kernel has to do something clever to avoid losing track of
188 * the TF bit.
189 *
190 * Needless to say, we've had bugs in this area.
191 */
192 syscall(SYS_getpid); /* Force symbol binding without TF set. */
193 printf("[RUN]\tSet TF and check a fast syscall\n");
194 set_eflags(get_eflags() | X86_EFLAGS_TF);
195 syscall(SYS_getpid);
196 check_result();
197
198 /* Now make sure that another fast syscall doesn't set TF again. */
199 fast_syscall_no_tf();
200
201 /*
202 * And do a forced SYSENTER to make sure that this works even if
203 * fast syscalls don't use SYSENTER.
204 *
205 * Invoking SYSENTER directly breaks all the rules. Just handle
206 * the SIGSEGV.
207 */
208 if (sigsetjmp(jmpbuf, 1) == 0) {
209 unsigned long nr = SYS_getpid;
210 printf("[RUN]\tSet TF and check SYSENTER\n");
211 stack_t stack = {
212 .ss_sp = malloc(sizeof(char) * SIGSTKSZ),
213 .ss_size = SIGSTKSZ,
214 };
215 if (sigaltstack(&stack, NULL) != 0)
216 err(1, "sigaltstack");
217 sethandler(SIGSEGV, print_and_longjmp,
218 SA_RESETHAND | SA_ONSTACK);
219 sethandler(SIGILL, print_and_longjmp, SA_RESETHAND);
220 set_eflags(get_eflags() | X86_EFLAGS_TF);
221 free(stack.ss_sp);
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