netdev - Re: [PATCH net] mptcp: fix use-after-free on tcp fallback

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Date:   Wed, 05 Feb 2020 14:08:35 +0100 (CET)
From:   David Miller <davem@...emloft.net>
To:     fw@...len.de
Cc:     netdev@...r.kernel.org, cpaasch@...le.com, pabeni@...hat.com
Subject: Re: [PATCH net] mptcp: fix use-after-free on tcp fallback

From: Florian Westphal <fw@...len.de>
Date: Tue,  4 Feb 2020 18:12:30 +0100

> When an mptcp socket connects to a tcp peer or when a middlebox interferes
> with tcp options, mptcp needs to fall back to plain tcp.
> Problem is that mptcp is trying to be too clever in this case:
> 
> It attempts to close the mptcp meta sk and transparently replace it with
> the (only) subflow tcp sk.
> 
> Unfortunately, this is racy -- the socket is already exposed to userspace.
> Any parallel calls to send/recv/setsockopt etc. can cause use-after-free:
> 
> BUG: KASAN: use-after-free in atomic_try_cmpxchg include/asm-generic/atomic-instrumented.h:693 [inline]
 ...
> While the use-after-free can be resolved, there is another problem:
> sock->ops and sock->sk assignments are not atomic, i.e. we may get calls
> into mptcp functions with sock->sk already pointing at the subflow socket,
> or calls into tcp functions with a mptcp meta sk.
> 
> Remove the fallback code and call the relevant functions for the (only)
> subflow in case the mptcp socket is connected to tcp peer.
> 
> Reported-by: Christoph Paasch <cpaasch@...le.com>
> Diagnosed-by: Paolo Abeni <pabeni@...hat.com>
> Signed-off-by: Florian Westphal <fw@...len.de>

Applied, thanks Florian.