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

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Date:	Sun, 15 Nov 2015 13:17:27 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	hannes@...essinduktion.org
Cc:	netdev@...r.kernel.org, dvyukov@...gle.com, eric.dumazet@...il.com
Subject: Re: [PATCH net v3] af-unix: fix use-after-free with concurrent
 readers while splicing

From: Hannes Frederic Sowa <hannes@...essinduktion.org>
Date: Tue, 10 Nov 2015 16:23:15 +0100

> During splicing an af-unix socket to a pipe we have to drop all
> af-unix socket locks. While doing so we allow another reader to enter
> unix_stream_read_generic which can read, copy and finally free another
> skb. If exactly this skb is just in process of being spliced we get a
> use-after-free report by kasan.
> 
> First, we must make sure to not have a free while the skb is used during
> the splice operation. We simply increment its use counter before unlocking
> the reader lock.
> 
> Stream sockets have the nice characteristic that we don't care about
> zero length writes and they never reach the peer socket's queue. That
> said, we can take the UNIXCB.consumed field as the indicator if the
> skb was already freed from the socket's receive queue. If the skb was
> fully consumed after we locked the reader side again we know it has been
> dropped by a second reader. We indicate a short read to user space and
> abort the current splice operation.
> 
> This bug has been found with syzkaller
> (http://github.com/google/syzkaller) by Dmitry Vyukov.
> 
> Fixes: 2b514574f7e8 ("net: af_unix: implement splice for stream af_unix sockets")
> Reported-by: Dmitry Vyukov <dvyukov@...gle.com>
> Cc: Dmitry Vyukov <dvyukov@...gle.com>
> Cc: Eric Dumazet <eric.dumazet@...il.com>
> Acked-by: Eric Dumazet <edumazet@...gle.com>
> Signed-off-by: Hannes Frederic Sowa <hannes@...essinduktion.org>

Applied and queued up for -stable, thanks.
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