Chroot bug (was: sys_chroot+sys_fchdir Fix)

From: David Newall
Date: Tue Sep 25 2007 - 11:10:43 EST

Miloslav Semler pointed out that a root process can chdir("..") out of its chroot. Although this is documented in the man page, it conflicts with the essential function, which is to change the root directory of the process. In addition to any creative uses, for example Philipp Marek's loading dynamic libraries, it seems clear that the prime purpose of chroot is to aid security. Being able to cd your way out is handy for the bad guys, but the good guys don't need it; there are a thousand better, safer solutions.

If there truly is a need to be able to pop in and out of a chroot, then the solution should be obvious, such as with real versus effective user and group ids. An important quality of a solution would be a way to fix that essential function: to set the root in such a way that you can no longer pop out. But that is a separate question.

The question: is chroot buggy? I'm pleased to turn to SCO for an independent definition for chroot, from which I get the following:

The *..* entry in the root directory is interpreted to mean the root directory itself. Thus, *..* cannot be used to access files outside the subtree rooted at the root directory.

I argue chroot is buggy. Miloslav's patch might not be the right solution, but he has the right idea (i.e. fix it.)
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