1 // SPDX-License-Identifier: GPL-2.0
2 /*
3 * Seccomp filter example for x86 (32-bit and 64-bit) with BPF macros
4 *
5 * Copyright (c) 2012 The Chromium OS Authors <chromium-os-dev@chromium.org>
6 * Author: Will Drewry <wad@chromium.org>
7 *
8 * The code may be used by anyone for any purpose,
9 * and can serve as a starting point for developing
10 * applications using prctl(PR_SET_SECCOMP, 2, ...).
11 */
12 #if defined(__i386__) || defined(__x86_64__)
13 #define SUPPORTED_ARCH 1
14 #endif
15
16 #if defined(SUPPORTED_ARCH)
17 #define __USE_GNU 1
18 #define _GNU_SOURCE 1
19
20 #include <linux/types.h>
21 #include <linux/filter.h>
22 #include <linux/seccomp.h>
23 #include <linux/unistd.h>
24 #include <signal.h>
25 #include <stdio.h>
26 #include <stddef.h>
27 #include <string.h>
28 #include <sys/prctl.h>
29 #include <unistd.h>
30
31 #define syscall_arg(_n) (offsetof(struct seccomp_data, args[_n]))
32 #define syscall_nr (offsetof(struct seccomp_data, nr))
33
34 #if defined(__i386__)
35 #define REG_RESULT REG_EAX
36 #define REG_SYSCALL REG_EAX
37 #define REG_ARG0 REG_EBX
38 #define REG_ARG1 REG_ECX
39 #define REG_ARG2 REG_EDX
40 #define REG_ARG3 REG_ESI
41 #define REG_ARG4 REG_EDI
42 #define REG_ARG5 REG_EBP
43 #elif defined(__x86_64__)
44 #define REG_RESULT REG_RAX
45 #define REG_SYSCALL REG_RAX
46 #define REG_ARG0 REG_RDI
47 #define REG_ARG1 REG_RSI
48 #define REG_ARG2 REG_RDX
49 #define REG_ARG3 REG_R10
50 #define REG_ARG4 REG_R8
51 #define REG_ARG5 REG_R9
52 #endif
53
54 #ifndef PR_SET_NO_NEW_PRIVS
55 #define PR_SET_NO_NEW_PRIVS 38
56 #endif
57
58 #ifndef SYS_SECCOMP
59 #define SYS_SECCOMP 1
60 #endif
61
62 static void emulator(int nr, siginfo_t *info, void *void_context)
63 {
64 ucontext_t *ctx = (ucontext_t *)(void_context);
65 int syscall;
66 char *buf;
67 ssize_t bytes;
68 size_t len;
69 if (info->si_code != SYS_SECCOMP)
70 return;
71 if (!ctx)
72 return;
73 syscall = ctx->uc_mcontext.gregs[REG_SYSCALL];
74 buf = (char *) ctx->uc_mcontext.gregs[REG_ARG1];
75 len = (size_t) ctx->uc_mcontext.gregs[REG_ARG2];
76
77 if (syscall != __NR_write)
78 return;
79 if (ctx->uc_mcontext.gregs[REG_ARG0] != STDERR_FILENO)
80 return;
81 /* Redirect stderr messages to stdout. Doesn't handle EINTR, etc */
82 ctx->uc_mcontext.gregs[REG_RESULT] = -1;
83 if (write(STDOUT_FILENO, "[ERR] ", 6) > 0) {
84 bytes = write(STDOUT_FILENO, buf, len);
85 ctx->uc_mcontext.gregs[REG_RESULT] = bytes;
86 }
87 return;
88 }
89
90 static int install_emulator(void)
91 {
92 struct sigaction act;
93 sigset_t mask;
94 memset(&act, 0, sizeof(act));
95 sigemptyset(&mask);
96 sigaddset(&mask, SIGSYS);
97
98 act.sa_sigaction = &emulator;
99 act.sa_flags = SA_SIGINFO;
100 if (sigaction(SIGSYS, &act, NULL) < 0) {
101 perror("sigaction");
102 return -1;
103 }
104 if (sigprocmask(SIG_UNBLOCK, &mask, NULL)) {
105 perror("sigprocmask");
106 return -1;
107 }
108 return 0;
109 }
110
111 static int install_filter(void)
112 {
113 struct sock_filter filter[] = {
114 /* Grab the system call number */
115 BPF_STMT(BPF_LD+BPF_W+BPF_ABS, syscall_nr),
116 /* Jump table for the allowed syscalls */
117 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_rt_sigreturn, 0, 1),
118 BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
119 #ifdef __NR_sigreturn
120 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_sigreturn, 0, 1),
121 BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
122 #endif
123 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_exit_group, 0, 1),
124 BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
125 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_exit, 0, 1),
126 BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
127 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_read, 1, 0),
128 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_write, 3, 2),
129
130 /* Check that read is only using stdin. */
131 BPF_STMT(BPF_LD+BPF_W+BPF_ABS, syscall_arg(0)),
132 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, STDIN_FILENO, 4, 0),
133 BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL),
134
135 /* Check that write is only using stdout */
136 BPF_STMT(BPF_LD+BPF_W+BPF_ABS, syscall_arg(0)),
137 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, STDOUT_FILENO, 1, 0),
138 /* Trap attempts to write to stderr */
139 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, STDERR_FILENO, 1, 2),
140
141 BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
142 BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_TRAP),
143 BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL),
144 };
145 struct sock_fprog prog = {
146 .len = (unsigned short)(sizeof(filter)/sizeof(filter[0])),
147 .filter = filter,
148 };
149
150 if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
151 perror("prctl(NO_NEW_PRIVS)");
152 return 1;
153 }
154
155
156 if (prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &prog)) {
157 perror("prctl");
158 return 1;
159 }
160 return 0;
161 }
162
163 #define payload(_c) (_c), sizeof((_c))
164 int main(int argc, char **argv)
165 {
166 char buf[4096];
167 ssize_t bytes = 0;
168 if (install_emulator())
169 return 1;
170 if (install_filter())
171 return 1;
172 syscall(__NR_write, STDOUT_FILENO,
173 payload("OHAI! WHAT IS YOUR NAME? "));
174 bytes = syscall(__NR_read, STDIN_FILENO, buf, sizeof(buf));
175 syscall(__NR_write, STDOUT_FILENO, payload("HELLO, "));
176 syscall(__NR_write, STDOUT_FILENO, buf, bytes);
177 syscall(__NR_write, STDERR_FILENO,
178 payload("Error message going to STDERR\n"));
179 return 0;
180 }
181 #else /* SUPPORTED_ARCH */
182 /*
183 * This sample is x86-only. Since kernel samples are compiled with the
184 * host toolchain, a non-x86 host will result in using only the main()
185 * below.
186 */
187 int main(void)
188 {
189 return 1;
190 }
191 #endif /* SUPPORTED_ARCH */
192
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