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

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Date:	Fri, 27 Mar 2009 00:04:20 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Pavel Machek <pavel@....cz>
Cc:	Miklos Szeredi <miklos@...redi.hu>, serue@...ibm.com,
	bfields@...ldses.org, linux-kernel@...r.kernel.org,
	viro@...iv.linux.org.uk, linux-fsdevel@...r.kernel.org
Subject: Re: unprivileged mounts vs. rmdir (was: VFS, NFS security bug? ...)

Pavel Machek <pavel@....cz> writes:

> On Mon 2009-03-23 14:21:30, Miklos Szeredi wrote:
>> [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?
>
> Well... traditionally if you have an open file or cwd inside mounted
> tree... that blocks unmount, right?
>
> What will you do with processes that have open (deleted) files inside
> the mount? What about cwd?

That is a backwards understanding, of the problem.

Currently I can not delete my mount point if I have something mounted on it in another
mount namespace.

Generally lazy unmounts solve the deleted inodes problem, your were talking about.

Eric


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