linux-kernel - Re: [PATCH 4/4] Allow unprivileged chroot when safe

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Date:	Mon, 16 Jan 2012 12:15:09 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	Casey Schaufler <casey@...aufler-ca.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jamie Lokier <jamie@...reable.org>,
	Will Drewry <wad@...omium.org>, linux-kernel@...r.kernel.org,
	keescook@...omium.org, john.johansen@...onical.com,
	serge.hallyn@...onical.com, coreyb@...ux.vnet.ibm.com,
	pmoore@...hat.com, eparis@...hat.com, djm@...drot.org,
	segoon@...nwall.com, rostedt@...dmis.org, jmorris@...ei.org,
	scarybeasts@...il.com, avi@...hat.com, penberg@...helsinki.fi,
	mingo@...e.hu, akpm@...ux-foundation.org, khilman@...com,
	borislav.petkov@....com, amwang@...hat.com, oleg@...hat.com,
	ak@...ux.intel.com, eric.dumazet@...il.com, gregkh@...e.de,
	dhowells@...hat.com, daniel.lezcano@...e.fr,
	linux-fsdevel@...r.kernel.org,
	linux-security-module@...r.kernel.org, olofj@...omium.org,
	mhalcrow@...gle.com, dlaor@...hat.com, corbet@....net,
	alan@...rguk.ukuu.org.uk
Subject: Re: [PATCH 4/4] Allow unprivileged chroot when safe

On Mon, Jan 16, 2012 at 12:06 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Sun, Jan 15, 2012 at 04:37:21PM -0800, Andy Lutomirski wrote:
>
>> +     is_chrooted = (fs->root.mnt->mnt_mountpoint !=
>> +                    fs->root.mnt->mnt_parent->mnt_root ||
>> +                    fs->root.dentry != fs->root.mnt->mnt_root);
>
> Folks, is it _that_ hard to at least try to compile your patches?  Hint:
> this one will *not*.  That sad detail aside, this test would have been
> a pile of garbage even on the kernels that used to have mnt_mountpoint
> in struct vfsmount.  What *are* you trying to test here?  The last part
> at least does make some sense - it's "process root happens to be the
> root of some vfsmount".  The first part, though, makes no sense whatsoever;
> it's "... and that vfsmount is mounted on top of root of some other
> vfsmount".

I compiled it, booted it, and tested it.  I based it off an oldish
kernel, though, so I can rebase.

The first approach I tried was (from memory -- may not compile at all
on any version) fs->root.mnt != fs->root.mnt->mnt_parent.  That didn't
work.  The issue is that on dracut-based distros, AFAICT, the root (in
the sense of the root of the tree of struct vfsmounts) is rootfs.  The
apparent root (the filesystem containing /, /usr, etc) is mounted on
top of (rootfs)/.  Dracut then does something with the effect of
chroot("/").  So you end up with the vfsmount that contains "/" not
being the actual root vfsmount.  But there's nothing hidden by the
chroot -- even if fs->root.mnt pointed at rootfs, "/" would still
follow the mountpoint into the actual filesystem.

An different approach would be to have fs_struct keep track of a hard
and a soft root.  chroot would stay CAP_SYS_ADMIN only and change both
roots.  A new unprivileged_chroot would change only the soft root.
follow_dotdot would check both, so unprivileged_chroot wouldn't be
useful for breaking chroot.  The big downside would be an extra branch
on every follow_dotdot.

--Andy
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