linux-kernel - unprivileged mounts vs. rmdir (was: VFS, NFS security bug? ...)

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Message-Id: <E1Llk5q-0000kd-IR@pomaz-ex.szeredi.hu>
Date:	Mon, 23 Mar 2009 14:21:30 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	serue@...ibm.com
CC:	bfields@...ldses.org, linux-kernel@...r.kernel.org,
	viro@...iv.linux.org.uk, ebiederm@...ssion.com,
	linux-fsdevel@...r.kernel.org
Subject: unprivileged mounts vs. rmdir (was: VFS, NFS security bug? ...)

[CCs trimmed]

On Mon, 16 Mar 2009, Serge E. Hallyn wrote:
> Quoting J. Bruce Fields (bfields@...ldses.org):
> > special privilege, so don't consult filesystem permissions (do I have
> > that right?  What happened to the attempt to allow ordinary users to
> > mount?).
> 
> Well, they keep getting stalled because we don't have a good answer for
> what to do about the fact that an unprivileged user can make trees
> undeletable by pinning them with mounts.  (Miklos and Eric cc'd in case
> I didn't explain that well enough).

That's correct.

The best answer I can come up with is to allow rmdir/unlink to
automatically umount trees from their respective dentries.  Obviously
this can't be done for regular (privileged) mounts, which must keep
returning EBUSY in such situations.

But for unprivileged mounts I can't see any fundamental issue with
such an approach.

Does anyone see a problem with this?  Is there a better solution?

Thanks,
Miklos
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