bpf-direct.c

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/*
 * Seccomp filter example for x86 (32-bit and 64-bit) with BPF macros
 *
 * Copyright (c) 2012 The Chromium OS Authors <[email protected]>
 * Author: Will Drewry <[email protected]>
 *
 * 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 */