netdev - Re: [PATCH net] tcp/dccp: fix timewait races in timer handling

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Message-Id: <20150921.163245.1066159200155037848.davem@davemloft.net>
Date:	Mon, 21 Sep 2015 16:32:45 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	netdev@...r.kernel.org, ycai@...gle.com
Subject: Re: [PATCH net] tcp/dccp: fix timewait races in timer handling

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Sat, 19 Sep 2015 09:08:34 -0700

> From: Eric Dumazet <edumazet@...gle.com>
> 
> When creating a timewait socket, we need to arm the timer before
> allowing other cpus to find it. The signal allowing cpus to find
> the socket is setting tw_refcnt to non zero value.
> 
> As we set tw_refcnt in __inet_twsk_hashdance(), we therefore need to
> call inet_twsk_schedule() first.
> 
> This also means we need to remove tw_refcnt changes from
> inet_twsk_schedule() and let the caller handle it.
> 
> Note that because we use mod_timer_pinned(), we have the guarantee
> the timer wont expire before we set tw_refcnt as we run in BH context.
> 
> To make things more readable I introduced inet_twsk_reschedule() helper.
> 
> When rearming the timer, we can use mod_timer_pending() to make sure
> we do not rearm a canceled timer.
> 
> Note: This bug can possibly trigger if packets of a flow can hit
> multiple cpus. This does not normally happen, unless flow steering
> is broken somehow. This explains this bug was spotted ~5 months after
> its introduction.
> 
> A similar fix is needed for SYN_RECV sockets in reqsk_queue_hash_req(),
> but will be provided in a separate patch for proper tracking.
> 
> Fixes: 789f558cfb36 ("tcp/dccp: get rid of central timewait timer")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Reported-by: Ying Cai <ycai@...gle.com>

Applied and queued up for -stable.
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