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Date:   Thu, 09 Mar 2017 18:21:27 -0800 (PST)
From:   David Miller <>
Subject: Re: [PATCH net] dccp/tcp: fix routing redirect race

From: Jon Maxwell <>
Date: Thu,  9 Mar 2017 12:15:21 +1100

> We have seen a few incidents lately where a dst_enty has been freed
> with a dangling TCP socket reference (sk->sk_dst_cache) pointing to that
> dst_entry. If the conditions/timings are right a crash then ensues when the
> freed dst_entry is referenced later on. A Common crashing back trace is:
> A closer look at the tcp_v4_err() handler revealed that do_redirect() will run
> regardless of whether user space has the socket locked. This can result in a 
> race condition where the same dst_entry cached in sk->sk_dst_entry can be 
> decremented twice for the same socket via: 
> do_redirect()->__sk_dst_check()-> dst_release(). 
> Which leads to the dst_entry being prematurely freed with another socket 
> pointing to it via sk->sk_dst_cache and a subsequent crash.
> To fix this skip do_redirect() if usespace has the socket locked. Instead let 
> the redirect take place later when user space does not have the socket 
> locked.
> The dccp code is very similar in this respect, so fixing it there too. 
> As Eric Garver pointed out the following commit now invalidates routes. Which
> can set the dst->obsolete flag so that ipv4_dst_check() returns null and 
> triggers the dst_release().
> Fixes: ceb3320610d6 ("ipv4: Kill routes during PMTU/redirect updates.")
> Cc: Eric Garver <>
> Cc: Hannes Sowa <>
> Signed-off-by: Jon Maxwell <>

Please add the ipv6 side of this fix to this patch and repost.

Thank you.

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