xref: /linux/samples/seccomp/bpf-direct.c (revision 6ed7ffddcf61f668114edb676417e5fb33773b59)
1 /*
2  * Seccomp filter example for x86 (32-bit and 64-bit) with BPF macros
3  *
4  * Copyright (c) 2012 The Chromium OS Authors <chromium-os-dev@chromium.org>
5  * Author: Will Drewry <wad@chromium.org>
6  *
7  * The code may be used by anyone for any purpose,
8  * and can serve as a starting point for developing
9  * applications using prctl(PR_SET_SECCOMP, 2, ...).
10  */
11 #if defined(__i386__) || defined(__x86_64__)
12 #define SUPPORTED_ARCH 1
13 #endif
14 
15 #if defined(SUPPORTED_ARCH)
16 #define __USE_GNU 1
17 #define _GNU_SOURCE 1
18 
19 #include <linux/types.h>
20 #include <linux/filter.h>
21 #include <linux/seccomp.h>
22 #include <linux/unistd.h>
23 #include <signal.h>
24 #include <stdio.h>
25 #include <stddef.h>
26 #include <string.h>
27 #include <sys/prctl.h>
28 #include <unistd.h>
29 
30 #define syscall_arg(_n) (offsetof(struct seccomp_data, args[_n]))
31 #define syscall_nr (offsetof(struct seccomp_data, nr))
32 
33 #if defined(__i386__)
34 #define REG_RESULT	REG_EAX
35 #define REG_SYSCALL	REG_EAX
36 #define REG_ARG0	REG_EBX
37 #define REG_ARG1	REG_ECX
38 #define REG_ARG2	REG_EDX
39 #define REG_ARG3	REG_ESI
40 #define REG_ARG4	REG_EDI
41 #define REG_ARG5	REG_EBP
42 #elif defined(__x86_64__)
43 #define REG_RESULT	REG_RAX
44 #define REG_SYSCALL	REG_RAX
45 #define REG_ARG0	REG_RDI
46 #define REG_ARG1	REG_RSI
47 #define REG_ARG2	REG_RDX
48 #define REG_ARG3	REG_R10
49 #define REG_ARG4	REG_R8
50 #define REG_ARG5	REG_R9
51 #endif
52 
53 #ifndef PR_SET_NO_NEW_PRIVS
54 #define PR_SET_NO_NEW_PRIVS 38
55 #endif
56 
57 #ifndef SYS_SECCOMP
58 #define SYS_SECCOMP 1
59 #endif
60 
61 static void emulator(int nr, siginfo_t *info, void *void_context)
62 {
63 	ucontext_t *ctx = (ucontext_t *)(void_context);
64 	int syscall;
65 	char *buf;
66 	ssize_t bytes;
67 	size_t len;
68 	if (info->si_code != SYS_SECCOMP)
69 		return;
70 	if (!ctx)
71 		return;
72 	syscall = ctx->uc_mcontext.gregs[REG_SYSCALL];
73 	buf = (char *) ctx->uc_mcontext.gregs[REG_ARG1];
74 	len = (size_t) ctx->uc_mcontext.gregs[REG_ARG2];
75 
76 	if (syscall != __NR_write)
77 		return;
78 	if (ctx->uc_mcontext.gregs[REG_ARG0] != STDERR_FILENO)
79 		return;
80 	/* Redirect stderr messages to stdout. Doesn't handle EINTR, etc */
81 	ctx->uc_mcontext.gregs[REG_RESULT] = -1;
82 	if (write(STDOUT_FILENO, "[ERR] ", 6) > 0) {
83 		bytes = write(STDOUT_FILENO, buf, len);
84 		ctx->uc_mcontext.gregs[REG_RESULT] = bytes;
85 	}
86 	return;
87 }
88 
89 static int install_emulator(void)
90 {
91 	struct sigaction act;
92 	sigset_t mask;
93 	memset(&act, 0, sizeof(act));
94 	sigemptyset(&mask);
95 	sigaddset(&mask, SIGSYS);
96 
97 	act.sa_sigaction = &emulator;
98 	act.sa_flags = SA_SIGINFO;
99 	if (sigaction(SIGSYS, &act, NULL) < 0) {
100 		perror("sigaction");
101 		return -1;
102 	}
103 	if (sigprocmask(SIG_UNBLOCK, &mask, NULL)) {
104 		perror("sigprocmask");
105 		return -1;
106 	}
107 	return 0;
108 }
109 
110 static int install_filter(void)
111 {
112 	struct sock_filter filter[] = {
113 		/* Grab the system call number */
114 		BPF_STMT(BPF_LD+BPF_W+BPF_ABS, syscall_nr),
115 		/* Jump table for the allowed syscalls */
116 		BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_rt_sigreturn, 0, 1),
117 		BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
118 #ifdef __NR_sigreturn
119 		BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_sigreturn, 0, 1),
120 		BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
121 #endif
122 		BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_exit_group, 0, 1),
123 		BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
124 		BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_exit, 0, 1),
125 		BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
126 		BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_read, 1, 0),
127 		BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_write, 3, 2),
128 
129 		/* Check that read is only using stdin. */
130 		BPF_STMT(BPF_LD+BPF_W+BPF_ABS, syscall_arg(0)),
131 		BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, STDIN_FILENO, 4, 0),
132 		BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL),
133 
134 		/* Check that write is only using stdout */
135 		BPF_STMT(BPF_LD+BPF_W+BPF_ABS, syscall_arg(0)),
136 		BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, STDOUT_FILENO, 1, 0),
137 		/* Trap attempts to write to stderr */
138 		BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, STDERR_FILENO, 1, 2),
139 
140 		BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
141 		BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_TRAP),
142 		BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL),
143 	};
144 	struct sock_fprog prog = {
145 		.len = (unsigned short)(sizeof(filter)/sizeof(filter[0])),
146 		.filter = filter,
147 	};
148 
149 	if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
150 		perror("prctl(NO_NEW_PRIVS)");
151 		return 1;
152 	}
153 
154 
155 	if (prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &prog)) {
156 		perror("prctl");
157 		return 1;
158 	}
159 	return 0;
160 }
161 
162 #define payload(_c) (_c), sizeof((_c))
163 int main(int argc, char **argv)
164 {
165 	char buf[4096];
166 	ssize_t bytes = 0;
167 	if (install_emulator())
168 		return 1;
169 	if (install_filter())
170 		return 1;
171 	syscall(__NR_write, STDOUT_FILENO,
172 		payload("OHAI! WHAT IS YOUR NAME? "));
173 	bytes = syscall(__NR_read, STDIN_FILENO, buf, sizeof(buf));
174 	syscall(__NR_write, STDOUT_FILENO, payload("HELLO, "));
175 	syscall(__NR_write, STDOUT_FILENO, buf, bytes);
176 	syscall(__NR_write, STDERR_FILENO,
177 		payload("Error message going to STDERR\n"));
178 	return 0;
179 }
180 #else	/* SUPPORTED_ARCH */
181 /*
182  * This sample is x86-only.  Since kernel samples are compiled with the
183  * host toolchain, a non-x86 host will result in using only the main()
184  * below.
185  */
186 int main(void)
187 {
188 	return 1;
189 }
190 #endif	/* SUPPORTED_ARCH */
191