dropper.c

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/*
 * Naive system call dropper built on seccomp_filter.
 *
 * 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, ...).
 *
 * When run, returns the specified errno for the specified
 * system call number against the given architecture.
 *
 * Run this one as root as PR_SET_NO_NEW_PRIVS is not called.
 */

#include <errno.h>
#include <linux/audit.h>
#include <linux/filter.h>
#include <linux/seccomp.h>
#include <linux/unistd.h>
#include <stdio.h>
#include <stddef.h>
#include <stdlib.h>
#include <sys/prctl.h>
#include <unistd.h>

static int install_filter(int nr, int arch, int error)
{
    struct sock_filter filter[] = {
        BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
             (offsetof(struct seccomp_data, arch))),
        BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, arch, 0, 3),
        BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
             (offsetof(struct seccomp_data, nr))),
        BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, nr, 0, 1),
        BPF_STMT(BPF_RET+BPF_K,
             SECCOMP_RET_ERRNO|(error & SECCOMP_RET_DATA)),
        BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
    };
    struct sock_fprog prog = {
        .len = (unsigned short)(sizeof(filter)/sizeof(filter[0])),
        .filter = filter,
    };
    if (prctl(PR_SET_SECCOMP, 2, &prog)) {
        perror("prctl");
        return 1;
    }
    return 0;
}

int main(int argc, char **argv)
{
    if (argc < 5) {
        fprintf(stderr, "Usage:\n"
            "dropper <syscall_nr> <arch> <errno> <prog> [<args>]\n"
            "Hint:  AUDIT_ARCH_I386: 0x%X\n"
            "   AUDIT_ARCH_X86_64: 0x%X\n"
            "\n", AUDIT_ARCH_I386, AUDIT_ARCH_X86_64);
        return 1;
    }
    if (install_filter(strtol(argv[1], NULL, 0), strtol(argv[2], NULL, 0),
               strtol(argv[3], NULL, 0)))
        return 1;
    execv(argv[4], &argv[4]);
    printf("Failed to execv\n");
    return 255;
}