linux-kernel - Re: [PATCH] NFS4: Fix NULL deref in nfsd4_lock() by makeing unhash

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Date:	Sun, 22 Jul 2012 23:01:44 +0200 (CEST)
From:	Jesper Juhl <jj@...osbits.net>
To:	"J. Bruce Fields" <bfields@...ldses.org>
cc:	linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org,
	Kendrick Smith <kmsmith@...ch.edu>,
	Andy Adamson <kandros@...ch.edu>,
	Trond Myklebust <Trond.Myklebust@...app.com>
Subject: Re: [PATCH] NFS4: Fix NULL deref in nfsd4_lock() by makeing
 unhash_lockowner() safe to call with NULL arg

On Mon, 30 Jan 2012, J. Bruce Fields wrote:

> On Sun, Jan 29, 2012 at 10:29:24PM +0100, Jesper Juhl wrote:
> > The Coverity checker noticed a path through nfsd4_lock() where we call
> > release_lockowner(lock_sop); (at the 'out:' label) where 'lock_sop' is
> > NULL.
> > That goes bad since release_lockowner() calls unhash_lockowner() which
> > dereferences its argument.
> > release_lockowner() also calls nfs4_free_lockowner(), but that's not a
> > problem since that function just calls kfree() and kmem_cache_free(),
> > both of which are safe to call with NULL as argument.
> > 
> > There are several ways to fix the bug.
> >  - rework nfsd4_lock() so the call to release_lockowner(NULL) will never happen.
> >  - let release_lockowner() test for NULL and return if given one.
> >  - let unhash_lockowner() test for NULL and return if given one (which makes both it and release_lockowner() safe).
> > 
> > I chose the last option for this patch.
> > 
> > For information, the path Coverity spotted (in defect report 201504) is this:
> > 
> ...
> 
> 
> > [...]
> > 4098out:
> > At conditional (12): "status" taking the true branch.
> > At conditional (13): "new_state" taking the true branch.
> > 4099        if (status && new_state)
> 
> new_state is initialized false, and referenced otherwise only once,
> when a reference ot is is passed here:
> 
> 
> > 4010                status = lookup_or_create_lock_state(cstate, open_stp, lock,
> > 4011                                                        &lock_stp, &new_state);
> 
> so if new_state is true, then lookup_or_create_lock_state() set it to
> true.  But it sets that only in one spot, at the end:
> 
> 	*new = true;
> 	return nfs_ok;
> 
> Note nfs_ok is zero.  Therefore:
> 
> > At conditional (11): "status" taking the true branch.
> > 4012                if (status)
> > 4013                        goto out;
> 
> this could not have happened.
> 
> So it looks like a Coverity bug.
> 
Sorry about the late reply, I managed to completely miss this mail until I 
wen't to check up on some of my old patches.

Thank you for the feedback. I've dropped my local copy of this patch.

-- 
Jesper Juhl <jj@...osbits.net>       http://www.chaosbits.net/
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