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Date: Wed, 11 Nov 2015 10:58:51 -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 Tue, 2015-11-10 at 16:23 +0100, Hannes Frederic Sowa wrote:
> 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>
> ---
> v2: add missing consume_skb in error path of recv_actor
> v3: move skb_get to separate line as proposed by Eric Dumazet (thanks!)
>
I believe there is another bug in unix_stream_sendpage()
skb = skb_peek_tail(&other->sk_receive_queue);
if (tail && tail == skb) {
Is clearly not safe ?
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