sctp: Enforce retransmission limit during shutdown

When initiating a graceful shutdown while having data chunks
on the retransmission queue with a peer which is in zero
window mode the shutdown is never completed because the
retransmission error count is reset periodically by the
following two rules:

 - Do not timeout association while doing zero window probe.
 - Reset overall error count when a heartbeat request has
   been acknowledged.

The graceful shutdown will wait for all outstanding TSN to
be acknowledged before sending the SHUTDOWN request. This
never happens due to the peer's zero window not acknowledging
the continuously retransmitted data chunks. Although the
error counter is incremented for each failed retransmission,
the receiving of the SACK announcing the zero window clears
the error count again immediately. Also heartbeat requests
continue to be sent periodically. The peer acknowledges these
requests causing the error counter to be reset as well.

This patch changes behaviour to only reset the overall error
counter for the above rules while not in shutdown. After
reaching the maximum number of retransmission attempts, the
T5 shutdown guard timer is scheduled to give the receiver
some additional time to recover. The timer is stopped as soon
as the receiver acknowledges any data.

The issue can be easily reproduced by establishing a sctp
association over the loopback device, constantly queueing
data at the sender while not reading any at the receiver.
Wait for the window to reach zero, then initiate a shutdown
by killing both processes simultaneously. The association
will never be freed and the chunks on the retransmission
queue will be retransmitted indefinitely.

Signed-off-by: Thomas Graf <tgraf@infradead.org>
Acked-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 19 insertions, 1 deletions

diff --git a/net/sctp/outqueue.c b/net/sctp/outqueue.c
index 1c88c89..d036821 100644
--- a/net/sctp/outqueue.c
+++ b/net/sctp/outqueue.c
@@ -1582,6 +1582,8 @@ static void sctp_check_transmitted(struct sctp_outq *q,
 #endif /* SCTP_DEBUG */
 	if (transport) {
 		if (bytes_acked) {
+			struct sctp_association *asoc = transport->asoc;
+
 			/* We may have counted DATA that was migrated
 			 * to this transport due to DEL-IP operation.
 			 * Subtract those bytes, since the were never
@@ -1600,6 +1602,17 @@ static void sctp_check_transmitted(struct sctp_outq *q,
 			transport->error_count = 0;
 			transport->asoc->overall_error_count = 0;
 
+			/*
+			 * While in SHUTDOWN PENDING, we may have started
+			 * the T5 shutdown guard timer after reaching the
+			 * retransmission limit. Stop that timer as soon
+			 * as the receiver acknowledged any data.
+			 */
+			if (asoc->state == SCTP_STATE_SHUTDOWN_PENDING &&
+			    del_timer(&asoc->timers
+				[SCTP_EVENT_TIMEOUT_T5_SHUTDOWN_GUARD]))
+					sctp_association_put(asoc);
+
 			/* Mark the destination transport address as
 			 * active if it is not so marked.
 			 */
@@ -1629,10 +1642,15 @@ static void sctp_check_transmitted(struct sctp_outq *q,
 			 * A sender is doing zero window probing when the
 			 * receiver's advertised window is zero, and there is
 			 * only one data chunk in flight to the receiver.
+			 *
+			 * Allow the association to timeout while in SHUTDOWN
+			 * PENDING or SHUTDOWN RECEIVED in case the receiver
+			 * stays in zero window mode forever.
 			 */
 			if (!q->asoc->peer.rwnd &&
 			    !list_empty(&tlist) &&
-			    (sack_ctsn+2 == q->asoc->next_tsn)) {
+			    (sack_ctsn+2 == q->asoc->next_tsn) &&
+			    q->asoc->state < SCTP_STATE_SHUTDOWN_PENDING) {
 				SCTP_DEBUG_PRINTK("%s: SACK received for zero "
 						  "window probe: %u\n",
 						  __func__, sack_ctsn);