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 20:58:29 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
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, 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.

But then the logic in unix_stream_connect() does not match, but it also
holds the other's socket lock durign sk_receive_queue modification.

Hmm, something is wrong here, that is clear. :/

Bye,
Hannes

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