From mboxrd@z Thu Jan  1 00:00:00 1970
From: Eric Dumazet <eric.dumazet@gmail.com>
Subject: Re: [PATCH] ipv4: fix a race in ip4_datagram_release_cb()
Date: Wed, 11 Jun 2014 06:12:53 -0700
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Cc: Alexey Preobrazhensky <preobr@google.com>,
	Steffen Klassert <steffen.klassert@secunet.com>,
	David Miller <davem@davemloft.net>, paulmck@linux.vnet.ibm.com,
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	Dmitry Vyukov <dvyukov@google.com>,
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To: dormando <dormando@rydia.net>
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On Wed, 2014-06-11 at 05:41 -0700, Eric Dumazet wrote:

> 
> OK then we probably have another bug in UDP, which is that we call
> sk_dst_set(sk, dst_clone(&rt->dst)); with a dst having DST_NOCACHE set
> 
> Its a problem, because sk_dst_get() cannot deal safely with such dst.

You could try this in top of other patches.

diff --git a/include/net/sock.h b/include/net/sock.h
index 21569cf456ed..427ac7cc50fc 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -1728,8 +1728,8 @@ sk_dst_get(struct sock *sk)
 
 	rcu_read_lock();
 	dst = rcu_dereference(sk->sk_dst_cache);
-	if (dst)
-		dst_hold(dst);
+	if (dst && !atomic_inc_not_zero(&dst->__refcnt))
+		dst = NULL;
 	rcu_read_unlock();
 	return dst;
 }