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
  61static 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
  89static 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
 110static 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))
 163int 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 */
 186int main(void)
 187{
 188        return 1;
 189}
 190#endif  /* SUPPORTED_ARCH */
 191
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