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

[Miklos Szeredi]

[CCs trimmed]

On Mon, 16 Mar 2009, Serge E. Hallyn wrote:
> Quoting J. Bruce Fields (bfields@xxxxxxxxxxxx):
> > 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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