netdev - Re: [PATCH] ipv4: fix a race in ip4_datagram_release

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Date:	Wed, 11 Jun 2014 06:12:53 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	dormando <dormando@...ia.net>
Cc:	Alexey Preobrazhensky <preobr@...gle.com>,
	Steffen Klassert <steffen.klassert@...unet.com>,
	David Miller <davem@...emloft.net>, paulmck@...ux.vnet.ibm.com,
	netdev@...r.kernel.org, Kostya Serebryany <kcc@...gle.com>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	Lars Bull <larsbull@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Bruce Curtis <brutus@...gle.com>,
	Maciej Żenczykowski <maze@...gle.com>,
	Alexei Starovoitov <alexei.starovoitov@...il.com>
Subject: Re: [PATCH] ipv4: fix a race in ip4_datagram_release_cb()

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;
 }


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