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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 
emulator(int nr,siginfo_t * info,void * void_context)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 
install_emulator(void)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 
install_filter(void)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))
main(int argc,char ** argv)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  */
main(void)187 int main(void)
188 {
189 	return 1;
190 }
191 #endif	/* SUPPORTED_ARCH */
192