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