linux-kernel - Re: [PATCH 1/4] proc: fix NULL ->i

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Date:	Tue, 20 Nov 2007 10:05:05 -0500
From:	Stephen Smalley <sds@...ho.nsa.gov>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Alexey Dobriyan <adobriyan@...ru>, akpm@...l.org,
	linux-kernel@...r.kernel.org, viro@...iv.linux.org.uk,
	devel@...nvz.org, James Morris <jmorris@...ei.org>,
	Eric Paris <eparis@...isplace.org>
Subject: Re: [PATCH 1/4] proc: fix NULL ->i_fop oops

On Mon, 2007-11-19 at 12:51 +0000, Christoph Hellwig wrote:
> On Fri, Nov 16, 2007 at 06:06:51PM +0300, Alexey Dobriyan wrote:
> > proc_kill_inodes() can clear ->i_fop in the middle of vfs_readdir resulting in
> > NULL dereference during "file->f_op->readdir(file, buf, filler)".
> > 
> > The solution is to remove proc_kill_inodes() completely:
> > a) we don't have tricky modules implementing their tricky readdir hooks which
> >    could keeping this revoke from hell.
> > b) In a situation when module is gone but PDE still alive, standard readdir
> >    will return only "." and "..", because pde->next was cleared by
> >    remove_proc_entry().
> > c) the race proc_kill_inode() destined to prevent is not completely fixed, just
> >    race window made smaller, because vfs_readdir() is run without sb_lock held and
> >    without file_list_lock held. Effectively, ->i_fop is cleared at random moment,
> >    which can't fix properly anything.
> 
> Nice, getting rid of this is a very good step formwards.  Unfortunately
> we have another copy of this junk in
> security/selinux/selinuxfs.c:sel_remove_entries() which would need the
> same treatment.

Can't just be dropped completely for selinux - we need a way to drop
obsolete entries from the prior policy when we load a new policy.

Is the only real problem here the clearing of f_op?  If so, we can
likely remove that from sel_remove_entries() without harm, and fix the
checks for it to use something more reliable.

-- 
Stephen Smalley
National Security Agency

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