From: Eric Dumazet <edumazet@google.com>
Date: Tue, 10 Nov 2015 01:51:23 +0000 (-0800)
Subject: net: fix a race in dst_release()
X-Git-Tag: v3.16.35~1060
X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=86c908b4d5346bf270e84d8dee1a1bc41a91eb57;p=thirdparty%2Fkernel%2Fstable.git

net: fix a race in dst_release()

commit d69bbf88c8d0b367cf3e3a052f6daadf630ee566 upstream.

Only cpu seeing dst refcount going to 0 can safely
dereference dst->flags.

Otherwise an other cpu might already have freed the dst.

Fixes: 27b75c95f10d ("net: avoid RCU for NOCACHE dst")
Reported-by: Greg Thelen <gthelen@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[ luis: backported to 3.16: adjusted context ]
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
---

diff --git a/net/core/dst.c b/net/core/dst.c
index a028409ee438c..a80e92346b9bf 100644
--- a/net/core/dst.c
+++ b/net/core/dst.c
@@ -285,7 +285,7 @@ void dst_release(struct dst_entry *dst)
 
 		newrefcnt = atomic_dec_return(&dst->__refcnt);
 		WARN_ON(newrefcnt < 0);
-		if (unlikely(dst->flags & DST_NOCACHE) && !newrefcnt)
+		if (!newrefcnt && unlikely(dst->flags & DST_NOCACHE))
 			call_rcu(&dst->rcu_head, dst_destroy_rcu);
 	}
 }