sctp: validate chunk len before actually using it

Andrey Konovalov reported that KASAN detected that SCTP was using a slab
beyond the boundaries. It was caused because when handling out of the
blue packets in function sctp_sf_ootb() it was checking the chunk len
only after already processing the first chunk, validating only for the
2nd and subsequent ones.

The fix is to just move the check upwards so it's also validated for the
1st chunk.
Reported-by: Andrey Konovalov <andreyknvl@google.com>
Tested-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Reviewed-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sctp/sm_statefuns.c

@@ -3422,6 +3422,12 @@ sctp_disposition_t sctp_sf_ootb(struct net *net,
 			return sctp_sf_violation_chunklen(net, ep, asoc, type, arg,
 						  commands);
 
+		/* Report violation if chunk len overflows */
+		ch_end = ((__u8 *)ch) + SCTP_PAD4(ntohs(ch->length));
+		if (ch_end > skb_tail_pointer(skb))
+			return sctp_sf_violation_chunklen(net, ep, asoc, type, arg,
+						  commands);
+
 		/* Now that we know we at least have a chunk header,
 		 * do things that are type appropriate.
 		 */
@@ -3453,12 +3459,6 @@ sctp_disposition_t sctp_sf_ootb(struct net *net,
 			}
 		}
 
-		/* Report violation if chunk len overflows */
-		ch_end = ((__u8 *)ch) + SCTP_PAD4(ntohs(ch->length));
-		if (ch_end > skb_tail_pointer(skb))
-			return sctp_sf_violation_chunklen(net, ep, asoc, type, arg,
-						  commands);
-
 		ch = (sctp_chunkhdr_t *) ch_end;
 	} while (ch_end < skb_tail_pointer(skb));