sctp: fix the issue that a __u16 variable may overflow in sctp_ulpq_renege

Now when reneging events in sctp_ulpq_renege(), the variable freed
could be increased by a __u16 value twice while freed is of __u16
type. It means freed may overflow at the second addition.

This patch is to fix it by using __u32 type for 'freed', while at
it, also to remove 'if (chunk)' check, as all renege commands are
generated in sctp_eat_data and it can't be NULL.
Reported-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

CVE-2019-3874

(backported from commit 5c468674)
[tyhicks: Backport to 4.4:
 - Continue to use typedef sctp_data_chunk_t which was later removed by
   commit 9f8d3147 ("sctp: remove the typedef sctp_data_chunk_t")]
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

net/sctp/ulpqueue.c

@@ -1080,29 +1080,21 @@ void sctp_ulpq_partial_delivery(struct sctp_ulpq *ulpq,
 void sctp_ulpq_renege(struct sctp_ulpq *ulpq, struct sctp_chunk *chunk,
 		      gfp_t gfp)
 {
-	struct sctp_association *asoc;
-	__u16 needed, freed;
-
-	asoc = ulpq->asoc;
+	struct sctp_association *asoc = ulpq->asoc;
+	__u32 freed = 0;
+	__u16 needed;
 
-	if (chunk) {
-		needed = ntohs(chunk->chunk_hdr->length);
-		needed -= sizeof(sctp_data_chunk_t);
-	} else
-		needed = SCTP_DEFAULT_MAXWINDOW;
-
-	freed = 0;
+	needed = ntohs(chunk->chunk_hdr->length) -
+		 sizeof(sctp_data_chunk_t);
 
 	if (skb_queue_empty(&asoc->base.sk->sk_receive_queue)) {
 		freed = sctp_ulpq_renege_order(ulpq, needed);
-		if (freed < needed) {
+		if (freed < needed)
 			freed += sctp_ulpq_renege_frags(ulpq, needed - freed);
-		}
 	}
 	/* If able to free enough room, accept this chunk. */
-	if (chunk && (freed >= needed)) {
-		int retval;
-		retval = sctp_ulpq_tail_data(ulpq, chunk, gfp);
+	if (freed >= needed) {
+		int retval = sctp_ulpq_tail_data(ulpq, chunk, gfp);
 		/*
 		 * Enter partial delivery if chunk has not been
 		 * delivered; otherwise, drain the reassembly queue.