netdev - Re: [PATCH net v3] af-unix: fix use-after-free with concurrent readers while splicing

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Date:	Wed, 11 Nov 2015 12:09:05 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:	netdev@...r.kernel.org, Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH net v3] af-unix: fix use-after-free with concurrent
 readers while splicing

On Wed, 2015-11-11 at 20:58 +0100, Hannes Frederic Sowa wrote:
> 
> On Wed, Nov 11, 2015, at 20:42, Eric Dumazet wrote:
> > On Wed, 2015-11-11 at 20:35 +0100, Hannes Frederic Sowa wrote:
> > > 
> > > On Wed, Nov 11, 2015, at 20:28, Eric Dumazet wrote:
> > > > On Wed, 2015-11-11 at 20:14 +0100, Hannes Frederic Sowa wrote:
> > > > > On Wed, Nov 11, 2015, at 19:58, Eric Dumazet wrote:
> > > > 
> > > > > Can you elaborate?
> > > > > 
> > > > > I use tail as a cookie and check if we already tried to append to the
> > > > > same tail skb with skb_append_pagefrags. If during allocation, which we
> > > > > do outside of the locks, a new skb arrives, we take that and try to
> > > > > append again (and free the old skb), to correctly not create any
> > > > > reordering in the data stream.
> > > > > 
> > > > > You think that tail could be reused in the meanwhile?
> > > > 
> > > > Hmmm, there is some funky stuff at least.
> > > > 
> > > > Are you sure the __skb_queue_tail(&other->sk_receive_queue, newskb)
> > > > is appropriate ?
> > > > 
> > > > (Why not locking sk_receive_queue is safe ?)
> > > 
> > > We hold the other's state lock at that time.
> > 
> > Well, this is not safe enough :(
> > 
> > Look at unix_stream_sendmsg() : It uses skb_queue_tail(), not
> > __skb_queue_tail()
> > 
> > Think of concurrent splice() (or sendfile()) and sendmsg() on the same
> > af_unix socket.
> 
> Well,
> 
> unix_stream_sendmsg:
> 
> unix_state_lock(other);
> skb_queue_tail(&other->sk_receive_queue);
> unix_state_unlock(other);
> 
> unix_stream_sendpage:
> 
> unix_state_lock(other);
> __skb_queue_tail(&other->sk_receive_queue, skb);
> unix_state_unlock(other);
> 
> unix_stream_read_generic:
> 
> I only see the skb_unlink as a dangerous operation because outside of
> other lock and solely taking the sk_receivie_queue lock. Actually I
> think skb_queue_tail can be converted to __skb_queue_tail.

Nope. See unix_inq_len() you definitely need to take the list lock,
or make a full audit.




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