[SCTP]: Allow spillover of receive buffer to avoid deadlock.

This patch fixes a deadlock situation in the receive path by allowing
temporary spillover of the receive buffer.

- If the chunk we receive has a tsn that immediately follows the ctsn,
  accept it even if we run out of receive buffer space and renege data with
  higher TSNs.
- Once we accept one chunk in a packet, accept all the remaining chunks
  even if we run out of receive buffer space.

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Acked-by: Mark Butler <butlerm@middle.net>
Acked-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: Sridhar Samudrala <sri@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 37 insertions, 10 deletions

diff --git a/net/sctp/inqueue.c b/net/sctp/inqueue.c
index 297b8951463e..cf0c767d43ae 100644
--- a/net/sctp/inqueue.c
+++ b/net/sctp/inqueue.c
@@ -149,6 +149,7 @@ struct sctp_chunk *sctp_inq_pop(struct sctp_inq *queue)
                 /* This is the first chunk in the packet.  */
                 chunk->singleton = 1;
                 ch = (sctp_chunkhdr_t *) chunk->skb->data;
+                chunk->data_accepted = 0;
         }
 
         chunk->chunk_hdr = ch;
diff --git a/net/sctp/sm_statefuns.c b/net/sctp/sm_statefuns.c
index 2b9a832b29a7..f5d131f52a70 100644
--- a/net/sctp/sm_statefuns.c
+++ b/net/sctp/sm_statefuns.c
@@ -5151,7 +5151,9 @@ static int sctp_eat_data(const struct sctp_association *asoc,
         int tmp;
         __u32 tsn;
         int account_value;
+        struct sctp_tsnmap *map = (struct sctp_tsnmap *)&asoc->peer.tsn_map;
         struct sock *sk = asoc->base.sk;
+        int rcvbuf_over = 0;
 
         data_hdr = chunk->subh.data_hdr = (sctp_datahdr_t *)chunk->skb->data;
         skb_pull(chunk->skb, sizeof(sctp_datahdr_t));
@@ -5162,10 +5164,16 @@ static int sctp_eat_data(const struct sctp_association *asoc,
         /* ASSERT:  Now skb->data is really the user data.  */
 
         /*
-         * if we are established, and we have used up our receive
-         * buffer memory, drop the frame
-         */
-        if (asoc->state == SCTP_STATE_ESTABLISHED) {
+         * If we are established, and we have used up our receive buffer
+         * memory, think about droping the frame.
+         * Note that we have an opportunity to improve performance here.
+         * If we accept one chunk from an skbuff, we have to keep all the
+         * memory of that skbuff around until the chunk is read into user
+         * space. Therefore, once we accept 1 chunk we may as well accept all
+         * remaining chunks in the skbuff. The data_accepted flag helps us do
+         * that.
+         */
+        if ((asoc->state == SCTP_STATE_ESTABLISHED) && (!chunk->data_accepted)) {
                 /*
                  * If the receive buffer policy is 1, then each
                  * association can allocate up to sk_rcvbuf bytes
@@ -5176,9 +5184,25 @@ static int sctp_eat_data(const struct sctp_association *asoc,
                         account_value = atomic_read(&asoc->rmem_alloc);
                 else
                         account_value = atomic_read(&sk->sk_rmem_alloc);
-
-                if (account_value > sk->sk_rcvbuf)
-                        return SCTP_IERROR_IGNORE_TSN;
+                if (account_value > sk->sk_rcvbuf) {
+                        /*
+                         * We need to make forward progress, even when we are
+                         * under memory pressure, so we always allow the
+                         * next tsn after the ctsn ack point to be accepted.
+                         * This lets us avoid deadlocks in which we have to
+                         * drop frames that would otherwise let us drain the
+                         * receive queue.
+                         */
+                        if ((sctp_tsnmap_get_ctsn(map) + 1) != tsn)
+                                return SCTP_IERROR_IGNORE_TSN;
+
+                        /*
+                         * We're going to accept the frame but we should renege
+                         * to make space for it. This will send us down that
+                         * path later in this function.
+                         */
+                        rcvbuf_over = 1;
+                }
         }
 
         /* Process ECN based congestion.
@@ -5226,6 +5250,7 @@ static int sctp_eat_data(const struct sctp_association *asoc,
         datalen -= sizeof(sctp_data_chunk_t);
 
         deliver = SCTP_CMD_CHUNK_ULP;
+        chunk->data_accepted = 1;
 
         /* Think about partial delivery. */
         if ((datalen >= asoc->rwnd) && (!asoc->ulpq.pd_mode)) {
@@ -5242,7 +5267,8 @@ static int sctp_eat_data(const struct sctp_association *asoc,
          * large spill over.
          */
         if (!asoc->rwnd || asoc->rwnd_over ||
-            (datalen > asoc->rwnd + asoc->frag_point)) {
+            (datalen > asoc->rwnd + asoc->frag_point) ||
+            rcvbuf_over) {
 
                 /* If this is the next TSN, consider reneging to make
                  * room.   Note: Playing nice with a confused sender.  A
@@ -5250,8 +5276,8 @@ static int sctp_eat_data(const struct sctp_association *asoc,
                  * space and in the future we may want to detect and
                  * do more drastic reneging.
                  */
-                if (sctp_tsnmap_has_gap(&asoc->peer.tsn_map) &&
-                    (sctp_tsnmap_get_ctsn(&asoc->peer.tsn_map) + 1) == tsn) {
+                if (sctp_tsnmap_has_gap(map) &&
+                    (sctp_tsnmap_get_ctsn(map) + 1) == tsn) {
                         SCTP_DEBUG_PRINTK("Reneging for tsn:%u\n", tsn);
                         deliver = SCTP_CMD_RENEGE;
                 } else {