Re: Chroot bug

From: Kyle Moffett
Date: Wed Sep 26 2007 - 08:54:56 EST

On Sep 26, 2007, at 06:27:38, David Newall wrote:
Kyle Moffett wrote:
David, please do tell myself and Adrian how "locking down" chroot () the way you want will avoid letting root break out through any of the above ways?

As has been said, there are thousands of ways to break out of a chroot. It's just that one of them should not be that chroot lets you walk out. I can't explain it clearer than that. If you don't see it now you probably never will.

Let me put it this way: You *CANNOT* enforce chroot() the way you want to without a completely unacceptable performance penalty. Let's start with the simplest example of:

fd = open("/", O_DIRECTORY);

If you had ever actually looked at the Linux VFS, it is completely *impossible* to tell whether "fd" at the time of the chroot is inside or outside of "/foo" without tracking an enormous amount of extra state. Even then, any such determination may not be valid since an FD may be opened to an inode which is hardlinked at multiple locations in the directory tree. It could also be bind-mounted at multiple locations, or it may not even be mounted at all in this namespace (CDROM that was lazy-unmounted). That FD may be later passed over an open UNIX-domain socket from another process. Moreover, arbitrarily closing FDs would break a huge number of programs. Furthermore, since you can't fix the "trivial" case of 'fchdir()', then there's no point in even *attempting* to fix the "cwd is outside of chroot" problem, although that is basically equivalent in difficulty to fixing the "dir-fd is outside of chroot" problem.

As for the nested-chroot() bit, the root user inside of a chroot is always allowed to chroot(). This is necessary for test-suites for various distro installers, chroot once to enter the installer playpen, installer chroots again to configure the test-installed- system. Once you allow a second chroot, you're back at the "can't reliably and efficiently track directory sub-tree members" problem.

So if you think it can and should be fixed, then PROVIDE THE CODE.

Kyle Moffett

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