netdev - Re: [RFC] net: remove erroneous sk null assignment in timestamping

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Date:	Fri, 07 Oct 2011 19:40:22 +0200
From:	Johannes Berg <johannes@...solutions.net>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org, richardcochran@...il.com
Subject: Re: [RFC] net: remove erroneous sk null assignment in timestamping

On Fri, 2011-10-07 at 13:33 -0400, David Miller wrote:
> From: Johannes Berg <johannes@...solutions.net>
> Date: Fri, 07 Oct 2011 19:11:41 +0200
> 
> > From: Johannes Berg <johannes.berg@...el.com>
> > 
> > skb->sk is obviously required to be non-NULL
> > when we get into skb_complete_tx_timestamp().
> > sock_queue_err_skb() will call skb_orphan()
> > first thing which sets skb->sk = NULL itself.
> > This may crash if the skb is still charged to
> > the socket (skb->destructor is sk_wfree).
> > 
> > The assignment here thus seems to not only be
> > pointless (due to the skb_orphan() call) but
> > also dangerous (due to the crash).
> > 
> > Signed-off-by: Johannes Berg <johannes.berg@...el.com>
> 
> It looks like skb_clone_tx_timestamp() sets clone->sk without any
> proper refcounting, so I bet this NULL'ing it out is working
> around that bug.

You're right. But this bug can actually trigger another way: The only
user of this is dp83640_txtstamp() which might do kfree_skb() on this
skb, in which case that'll likely crash trying to clean up the sk.

johannes

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