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