[PATCH net-next v2] sctp: sctp_close: fix release of bindings for deferred call_rcu's

[Daniel Borkmann <dborkman@redhat.com>]

It seems due to RCU usage, i.e. within SCTP's address binding list,
a, say, ``behavioral change'' was introduced which does actually
not conform to the RFC anymore. In particular consider the following
(fictional) scenario to demonstrate this:

  do:
    Two SOCK_SEQPACKET-style sockets are openend (S1, S2)
    S1 is bound to 127.0.0.1, port 1024 [server]
    S2 is bound to 127.0.0.1, port 1025 [client]
    listen(2) is invoked on S1
    From S2 we call one sendmsg(2) with msg.msg_name and
       msg.msg_namelen parameters set to the server's
       address
    S1, S2 are closed
    goto do

The first pass of this loop passes sucessful, while the second round
fails during binding of S1 (address still in use). What is happening?
In the first round, the initial handshake is being done, and, at the
time close(2) is called on S1, a nongraceful shutdown is performed via
ABORT since in S1's receive queue an unprocessed packet is present,
thus stating an error condition. This can be considered as a correct
behavior.

During close also all bound addresses are freed, thus nothing *must*
be active anymore. In reference to RFC2960:

  After checking the Verification Tag, the receiving endpoint shall
  remove the association from its record, and shall report the
  termination to its upper layer. (9.1 Abort of an Association)

Also, no half-open states are supported, thus after an ungraceful
shutdown, we leave nothing behind. However, this seems not to be
happening though. In a real-world scenario, this is exactly where
it breaks the lksctp-tools functional test suite, *for instance*:

  ./test_sockopt
  test_sockopt.c  1 PASS : getsockopt(SCTP_STATUS) on a socket with no assoc
  test_sockopt.c  2 PASS : getsockopt(SCTP_STATUS)
  test_sockopt.c  3 PASS : getsockopt(SCTP_STATUS) with invalid associd
  test_sockopt.c  4 PASS : getsockopt(SCTP_STATUS) with NULL associd
  test_sockopt.c  5 BROK : bind: Address already in use

The underlying problem is that sctp_endpoint_destroy() hasn't been
triggered yet while the next bind attempt is being done. It will be
triggered eventually (but too late) by sctp_transport_destroy_rcu()
after one RCU grace period:

  sctp_transport_destroy()
    sctp_transport_destroy_rcu() ----.
      sctp_association_put() [*]  <--+--> sctp_packet_free()
        sctp_association_destroy()          [...]
          sctp_endpoint_put()                 skb->destructor
            sctp_endpoint_destroy()             sctp_wfree()
              sctp_bind_addr_free()               sctp_association_put() [*]

Thus, we move out the condition with sctp_association_put() as well as
the sctp_packet_free() invokation and the issue can be solved.

With this patch, the example above (which simulates a similar scenario
as in the implementation of this test case) and therefore also the test
suite run successfully through. Tested by myself.

Cc: Vlad Yasevich <vyasevich@gmail.com>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
---
 net/sctp/transport.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/net/sctp/transport.c b/net/sctp/transport.c
index 4e45bb6..624edeb 100644
--- a/net/sctp/transport.c
+++ b/net/sctp/transport.c
@@ -168,10 +168,6 @@ static void sctp_transport_destroy_rcu(struct rcu_head *head)
 	struct sctp_transport *transport;
 
 	transport = container_of(head, struct sctp_transport, rcu);
-	if (transport->asoc)
-		sctp_association_put(transport->asoc);
-
-	sctp_packet_free(&transport->packet);
 
 	dst_release(transport->dst);
 	kfree(transport);
@@ -186,6 +182,11 @@ static void sctp_transport_destroy(struct sctp_transport *transport)
 	SCTP_ASSERT(transport->dead, "Transport is not dead", return);
 
 	call_rcu(&transport->rcu, sctp_transport_destroy_rcu);
+
+	if (transport->asoc)
+		sctp_association_put(transport->asoc);
+
+	sctp_packet_free(&transport->packet);
 }
 
 /* Start T3_rtx timer if it is not already running and update the heartbeat
-- 
1.7.11.7