[ 32/48] l2tp: fix a race in l2tp_ip_sendmsg()

[Ben Hutchings]

3.2-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Eric Dumazet <edumazet@xxxxxxxxxx>

[ Upstream commit 4399a4df98a63e30fd16e9d0cecc46ea92269e8f ]

Commit 081b1b1bb27f (l2tp: fix l2tp_ip_sendmsg() route handling) added
a race, in case IP route cache is disabled.

In this case, we should not do the dst_release(&rt->dst), since it'll
free the dst immediately, instead of waiting a RCU grace period.

Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Cc: James Chapman <jchapman@xxxxxxxxxxx>
Cc: Denys Fedoryshchenko <denys@xxxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 net/l2tp/l2tp_ip.c |    9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/net/l2tp/l2tp_ip.c b/net/l2tp/l2tp_ip.c
index 2fbbe1f..6c7e609 100644
--- a/net/l2tp/l2tp_ip.c
+++ b/net/l2tp/l2tp_ip.c
@@ -515,10 +515,12 @@ static int l2tp_ip_sendmsg(struct kiocb *iocb, struct sock *sk, struct msghdr *m
 					   sk->sk_bound_dev_if);
 		if (IS_ERR(rt))
 			goto no_route;
-		if (connected)
+		if (connected) {
 			sk_setup_caps(sk, &rt->dst);
-		else
-			dst_release(&rt->dst); /* safe since we hold rcu_read_lock */
+		} else {
+			skb_dst_set(skb, &rt->dst);
+			goto xmit;
+		}
 	}
 
 	/* We dont need to clone dst here, it is guaranteed to not disappear.
@@ -526,6 +528,7 @@ static int l2tp_ip_sendmsg(struct kiocb *iocb, struct sock *sk, struct msghdr *m
 	 */
 	skb_dst_set_noref(skb, &rt->dst);
 
+xmit:
 	/* Queue the packet to IP for output */
 	rc = ip_queue_xmit(skb, &inet->cork.fl);
 	rcu_read_unlock();


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