To: Pavel Machek <pavel@ucw.cz>
Cc: Miklos Szeredi <miklos@szeredi.hu>, serue@us.ibm.com, bfields@fieldses.org,
       linux-kernel@vger.kernel.org, viro@zeniv.linux.org.uk,
       linux-fsdevel@vger.kernel.org
Subject: Re: unprivileged mounts vs. rmdir (was: VFS, NFS security bug? ...)
References: <f44001920903110553t52ce0ba7l4ba97213e1c51873@mail.gmail.com>
	<20090311232356.GP13540@fieldses.org>
	<20090312161047.GA15209@us.ibm.com>
	<517f3f820903121321sf6d2014q8165b925d5d44db7@mail.gmail.com>
	<20090313175848.GB27891@fieldses.org>
	<cfd18e0f0903141220u66230ceer22ef0cc6aed1d046@mail.gmail.com>
	<f44001920903160716i488e3642o869f626a5c3327d0@mail.gmail.com>
	<20090316163611.GB10959@fieldses.org>
	<20090316170433.GA2996@us.ibm.com>
	<E1Llk5q-0000kd-IR@pomaz-ex.szeredi.hu> <20090326124338.GA1466@ucw.cz>
From: ebiederm@xmission.com (Eric W. Biederman)
Date: Fri, 27 Mar 2009 00:04:20 -0700
In-Reply-To: <20090326124338.GA1466@ucw.cz> (Pavel Machek's message of "Thu\, 26 Mar 2009 13\:43\:38 +0100")
Message-ID: <m163hvmop7.fsf@fess.ebiederm.org>
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Pavel Machek <pavel@ucw.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@fieldses.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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