linux-kernel - Re: unprivileged mounts git tree

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Message-ID: <m17i9icxme.fsf@frodo.ebiederm.org>
Date:	Thu, 11 Sep 2008 12:04:41 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	serue@...ibm.com, akpm@...ux-foundation.org, hch@...radead.org,
	viro@...IV.linux.org.uk, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: unprivileged mounts git tree

Miklos Szeredi <miklos@...redi.hu> writes:

> On Thu, 11 Sep 2008, ebiederm@...ssion.com (Eric W. Biederman)
>> There is a weird corner case I'm trying to wrap my head around.
>> unlink and rmdir do not work on dentries that are mount points
>> in another mount namespace.
>> 
>> Which is at least needed for the moment so we don't leak mounts.
>> 
>> Once we have unprivileged mounts does that introduce a DOS attack?
>
> Hmm, yes.  That's a tough one...
>
> I think if the dentry has only user mounts, unlink should go ahead and
> on success dissolve any mounts on the dentry.  Does that sound
> workable?

I don't think only user mounts is the right filter.

We have support for lazy unmounts so it is possible to handle
that case.

Technically all we need to do is transform d_mounted from a counter
to a hlist_head and thread yet another list through struct vfs_mount
to track this.

I need to think about the semantics a little more before I have a good
feel of what makes sense.  In particular do we want a full
recursive lazy unmount or do we want to handle submounts in a different
way.

This also intersects in interesting ways with dcache pruning, and
automounting.

Eric
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