[PATCH net] sctp: fix dst reference leak in sctp_v4_get_dst

[Tommi Rantala <tommi.t.rantala@nokia.com>]

Fix dst reference leak in sctp_v4_get_dst() introduced in commit
410f03831 ("sctp: add routing output fallback"):

When walking the address_list, successive ip_route_output_key() calls
may return the same rt->dst with the reference incremented on each call.

The code would not decrement the dst refcount when the dst pointer was
identical from the previous iteration, causing the dst leak.

Testcase:
  ip netns add TEST
  ip netns exec TEST ip link set lo up
  ip link add dummy0 type dummy
  ip link add dummy1 type dummy
  ip link add dummy2 type dummy
  ip link set dev dummy0 netns TEST
  ip link set dev dummy1 netns TEST
  ip link set dev dummy2 netns TEST
  ip netns exec TEST ip addr add 192.168.1.1/24 dev dummy0
  ip netns exec TEST ip link set dummy0 up
  ip netns exec TEST ip addr add 192.168.1.2/24 dev dummy1
  ip netns exec TEST ip link set dummy1 up
  ip netns exec TEST ip addr add 192.168.1.3/24 dev dummy2
  ip netns exec TEST ip link set dummy2 up
  ip netns exec TEST sctp_test -H 192.168.1.2 -P 20002 -h 192.168.1.1 -p 20000 -s -B 192.168.1.3
  ip netns del TEST

In 4.4 and 4.9 kernels this results to:
  [  354.179591] unregister_netdevice: waiting for lo to become free. Usage count = 1
  [  364.419674] unregister_netdevice: waiting for lo to become free. Usage count = 1
  [  374.663664] unregister_netdevice: waiting for lo to become free. Usage count = 1
  [  384.903717] unregister_netdevice: waiting for lo to become free. Usage count = 1
  [  395.143724] unregister_netdevice: waiting for lo to become free. Usage count = 1
  [  405.383645] unregister_netdevice: waiting for lo to become free. Usage count = 1
  ...

Fixes: 410f03831 ("sctp: add routing output fallback")
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Cc: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Cc: Alexey Kodanev <alexey.kodanev@oracle.com>
Signed-off-by: Tommi Rantala <tommi.t.rantala@nokia.com>
---
 net/sctp/protocol.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/net/sctp/protocol.c b/net/sctp/protocol.c
index 8b4ff315695e..aadb9e7f60e2 100644
--- a/net/sctp/protocol.c
+++ b/net/sctp/protocol.c
@@ -508,21 +508,20 @@ static void sctp_v4_get_dst(struct sctp_transport *t, union sctp_addr *saddr,
 		if (IS_ERR(rt))
 			continue;
 
-		if (!dst)
-			dst = &rt->dst;
-
 		/* Ensure the src address belongs to the output
 		 * interface.
 		 */
 		odev = __ip_dev_find(sock_net(sk), laddr->a.v4.sin_addr.s_addr,
 				     false);
 		if (!odev || odev->ifindex != fl4->flowi4_oif) {
-			if (&rt->dst != dst)
+			if (!dst)
+				dst = &rt->dst;
+			else
 				dst_release(&rt->dst);
 			continue;
 		}
 
-		if (dst != &rt->dst)
+		if (dst && dst != &rt->dst)
 			dst_release(dst);
 		dst = &rt->dst;
 		break;
-- 
2.15.1