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Linux/samples/seccomp/bpf-direct.c

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  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 

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