// SPDX-License-Identifier: GPL-2.0 /* * Seccomp filter example for x86 (32-bit and 64-bit) with BPF macros * * Copyright (c) 2012 The Chromium OS Authors <chromium-os-dev@chromium.org> * Author: Will Drewry <wad@chromium.org> * * The code may be used by anyone for any purpose, * and can serve as a starting point for developing * applications using prctl(PR_SET_SECCOMP, 2, ...). */ #if defined(__i386__) || defined(__x86_64__) #define SUPPORTED_ARCH 1 #endif #if defined(SUPPORTED_ARCH) #define __USE_GNU 1 #define _GNU_SOURCE 1 #include <linux/types.h> #include <linux/filter.h> #include <linux/seccomp.h> #include <linux/unistd.h> #include <signal.h> #include <stdio.h> #include <stddef.h> #include <string.h> #include <sys/prctl.h> #include <unistd.h> #define syscall_arg(_n) (offsetof(struct seccomp_data, args[_n])) #define syscall_nr (offsetof(struct seccomp_data, nr)) #if defined(__i386__) #define REG_RESULT REG_EAX #define REG_SYSCALL REG_EAX #define REG_ARG0 REG_EBX #define REG_ARG1 REG_ECX #define REG_ARG2 REG_EDX #define REG_ARG3 REG_ESI #define REG_ARG4 REG_EDI #define REG_ARG5 REG_EBP #elif defined(__x86_64__) #define REG_RESULT REG_RAX #define REG_SYSCALL REG_RAX #define REG_ARG0 REG_RDI #define REG_ARG1 REG_RSI #define REG_ARG2 REG_RDX #define REG_ARG3 REG_R10 #define REG_ARG4 REG_R8 #define REG_ARG5 REG_R9 #endif #ifndef PR_SET_NO_NEW_PRIVS #define PR_SET_NO_NEW_PRIVS 38 #endif #ifndef SYS_SECCOMP #define SYS_SECCOMP 1 #endif static void emulator(int nr, siginfo_t *info, void *void_context) { ucontext_t *ctx = (ucontext_t *)(void_context); int syscall; char *buf; ssize_t bytes; size_t len; if (info->si_code != SYS_SECCOMP) return; if (!ctx) return; syscall = ctx->uc_mcontext.gregs[REG_SYSCALL]; buf = (char *) ctx->uc_mcontext.gregs[REG_ARG1]; len = (size_t) ctx->uc_mcontext.gregs[REG_ARG2]; if (syscall != __NR_write) return; if (ctx->uc_mcontext.gregs[REG_ARG0] != STDERR_FILENO) return; /* Redirect stderr messages to stdout. Doesn't handle EINTR, etc */ ctx->uc_mcontext.gregs[REG_RESULT] = -1; if (write(STDOUT_FILENO, "[ERR] ", 6) > 0) { bytes = write(STDOUT_FILENO, buf, len); ctx->uc_mcontext.gregs[REG_RESULT] = bytes; } return; } static int install_emulator(void) { struct sigaction act; sigset_t mask; memset(&act, 0, sizeof(act)); sigemptyset(&mask); sigaddset(&mask, SIGSYS); act.sa_sigaction = &emulator; act.sa_flags = SA_SIGINFO; if (sigaction(SIGSYS, &act, NULL) < 0) { perror("sigaction"); return -1; } if (sigprocmask(SIG_UNBLOCK, &mask, NULL)) { perror("sigprocmask"); return -1; } return 0; } static int install_filter(void) { struct sock_filter filter[] = { /* Grab the system call number */ BPF_STMT(BPF_LD+BPF_W+BPF_ABS, syscall_nr), /* Jump table for the allowed syscalls */ BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_rt_sigreturn, 0, 1), BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW), #ifdef __NR_sigreturn BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_sigreturn, 0, 1), BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW), #endif BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_exit_group, 0, 1), BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW), BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_exit, 0, 1), BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW), BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_read, 1, 0), BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_write, 3, 2), /* Check that read is only using stdin. */ BPF_STMT(BPF_LD+BPF_W+BPF_ABS, syscall_arg(0)), BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, STDIN_FILENO, 4, 0), BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL), /* Check that write is only using stdout */ BPF_STMT(BPF_LD+BPF_W+BPF_ABS, syscall_arg(0)), BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, STDOUT_FILENO, 1, 0), /* Trap attempts to write to stderr */ BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, STDERR_FILENO, 1, 2), BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW), BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_TRAP), BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL), }; struct sock_fprog prog = { .len = (unsigned short)(sizeof(filter)/sizeof(filter[0])), .filter = filter, }; if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) { perror("prctl(NO_NEW_PRIVS)"); return 1; } if (prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &prog)) { perror("prctl"); return 1; } return 0; } #define payload(_c) (_c), sizeof((_c)) int main(int argc, char **argv) { char buf[4096]; ssize_t bytes = 0; if (install_emulator()) return 1; if (install_filter()) return 1; syscall(__NR_write, STDOUT_FILENO, payload("OHAI! WHAT IS YOUR NAME? ")); bytes = syscall(__NR_read, STDIN_FILENO, buf, sizeof(buf)); syscall(__NR_write, STDOUT_FILENO, payload("HELLO, ")); syscall(__NR_write, STDOUT_FILENO, buf, bytes); syscall(__NR_write, STDERR_FILENO, payload("Error message going to STDERR\n")); return 0; } #else /* SUPPORTED_ARCH */ /* * This sample is x86-only. Since kernel samples are compiled with the * host toolchain, a non-x86 host will result in using only the main() * below. */ int main(void) { return 1; } #endif /* SUPPORTED_ARCH */