1 /*
2 * single_step_syscall.c - single-steps various x86 syscalls
3 * Copyright (c) 2014-2015 Andrew Lutomirski
4 *
5 * This program is free software; you can redistribute it and/or modify
6 * it under the terms and conditions of the GNU General Public License,
7 * version 2, as published by the Free Software Foundation.
8 *
9 * This program is distributed in the hope it will be useful, but
10 * WITHOUT ANY WARRANTY; without even the implied warranty of
11 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
12 * General Public License for more details.
13 *
14 * This is a very simple series of tests that makes system calls with
15 * the TF flag set. This exercises some nasty kernel code in the
16 * SYSENTER case: SYSENTER does not clear TF, so SYSENTER with TF set
17 * immediately issues #DB from CPL 0. This requires special handling in
18 * the kernel.
19 */
20
21 #define _GNU_SOURCE
22
23 #include <sys/time.h>
24 #include <time.h>
25 #include <stdlib.h>
26 #include <sys/syscall.h>
27 #include <unistd.h>
28 #include <stdio.h>
29 #include <string.h>
30 #include <inttypes.h>
31 #include <sys/mman.h>
32 #include <sys/signal.h>
33 #include <sys/ucontext.h>
34 #include <asm/ldt.h>
35 #include <err.h>
36 #include <setjmp.h>
37 #include <stddef.h>
38 #include <stdbool.h>
39 #include <sys/ptrace.h>
40 #include <sys/user.h>
41
42 static void sethandler(int sig, void (*handler)(int, siginfo_t *, void *),
43 int flags)
44 {
45 struct sigaction sa;
46 memset(&sa, 0, sizeof(sa));
47 sa.sa_sigaction = handler;
48 sa.sa_flags = SA_SIGINFO | flags;
49 sigemptyset(&sa.sa_mask);
50 if (sigaction(sig, &sa, 0))
51 err(1, "sigaction");
52 }
53
54 static volatile sig_atomic_t sig_traps;
55
56 #ifdef __x86_64__
57 # define REG_IP REG_RIP
58 # define WIDTH "q"
59 # define INT80_CLOBBERS "r8", "r9", "r10", "r11"
60 #else
61 # define REG_IP REG_EIP
62 # define WIDTH "l"
63 # define INT80_CLOBBERS
64 #endif
65
66 static unsigned long get_eflags(void)
67 {
68 unsigned long eflags;
69 asm volatile ("pushf" WIDTH "\n\tpop" WIDTH " %0" : "=rm" (eflags));
70 return eflags;
71 }
72
73 static void set_eflags(unsigned long eflags)
74 {
75 asm volatile ("push" WIDTH " %0\n\tpopf" WIDTH
76 : : "rm" (eflags) : "flags");
77 }
78
79 #define X86_EFLAGS_TF (1UL << 8)
80
81 static void sigtrap(int sig, siginfo_t *info, void *ctx_void)
82 {
83 ucontext_t *ctx = (ucontext_t*)ctx_void;
84
85 if (get_eflags() & X86_EFLAGS_TF) {
86 set_eflags(get_eflags() & ~X86_EFLAGS_TF);
87 printf("[WARN]\tSIGTRAP handler had TF set\n");
88 _exit(1);
89 }
90
91 sig_traps++;
92
93 if (sig_traps == 10000 || sig_traps == 10001) {
94 printf("[WARN]\tHit %d SIGTRAPs with si_addr 0x%lx, ip 0x%lx\n",
95 (int)sig_traps,
96 (unsigned long)info->si_addr,
97 (unsigned long)ctx->uc_mcontext.gregs[REG_IP]);
98 }
99 }
100
101 static void check_result(void)
102 {
103 unsigned long new_eflags = get_eflags();
104 set_eflags(new_eflags & ~X86_EFLAGS_TF);
105
106 if (!sig_traps) {
107 printf("[FAIL]\tNo SIGTRAP\n");
108 exit(1);
109 }
110
111 if (!(new_eflags & X86_EFLAGS_TF)) {
112 printf("[FAIL]\tTF was cleared\n");
113 exit(1);
114 }
115
116 printf("[OK]\tSurvived with TF set and %d traps\n", (int)sig_traps);
117 sig_traps = 0;
118 }
119
120 int main()
121 {
122 #ifdef CAN_BUILD_32
123 int tmp;
124 #endif
125
126 sethandler(SIGTRAP, sigtrap, 0);
127
128 printf("[RUN]\tSet TF and check nop\n");
129 set_eflags(get_eflags() | X86_EFLAGS_TF);
130 asm volatile ("nop");
131 check_result();
132
133 #ifdef __x86_64__
134 printf("[RUN]\tSet TF and check syscall-less opportunistic sysret\n");
135 set_eflags(get_eflags() | X86_EFLAGS_TF);
136 extern unsigned char post_nop[];
137 asm volatile ("pushf" WIDTH "\n\t"
138 "pop" WIDTH " %%r11\n\t"
139 "nop\n\t"
140 "post_nop:"
141 : : "c" (post_nop) : "r11");
142 check_result();
143 #endif
144 #ifdef CAN_BUILD_32
145 printf("[RUN]\tSet TF and check int80\n");
146 set_eflags(get_eflags() | X86_EFLAGS_TF);
147 asm volatile ("int $0x80" : "=a" (tmp) : "a" (SYS_getpid)
148 : INT80_CLOBBERS);
149 check_result();
150 #endif
151
152 /*
153 * This test is particularly interesting if fast syscalls use
154 * SYSENTER: it triggers a nasty design flaw in SYSENTER.
155 * Specifically, SYSENTER does not clear TF, so either SYSENTER
156 * or the next instruction traps at CPL0. (Of course, Intel
157 * mostly forgot to document exactly what happens here.) So we
158 * get a CPL0 fault with usergs (on 64-bit kernels) and possibly
159 * no stack. The only sane way the kernel can possibly handle
160 * it is to clear TF on return from the #DB handler, but this
161 * happens way too early to set TF in the saved pt_regs, so the
162 * kernel has to do something clever to avoid losing track of
163 * the TF bit.
164 *
165 * Needless to say, we've had bugs in this area.
166 */
167 syscall(SYS_getpid); /* Force symbol binding without TF set. */
168 printf("[RUN]\tSet TF and check a fast syscall\n");
169 set_eflags(get_eflags() | X86_EFLAGS_TF);
170 syscall(SYS_getpid);
171 check_result();
172
173 /* Now make sure that another fast syscall doesn't set TF again. */
174 printf("[RUN]\tFast syscall with TF cleared\n");
175 fflush(stdout); /* Force a syscall */
176 if (get_eflags() & X86_EFLAGS_TF) {
177 printf("[FAIL]\tTF is now set\n");
178 exit(1);
179 }
180 if (sig_traps) {
181 printf("[FAIL]\tGot SIGTRAP\n");
182 exit(1);
183 }
184 printf("[OK]\tNothing unexpected happened\n");
185
186 return 0;
187 }
188
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