sctp: validate chunk size in __rcv_asconf_lookup

In one of the fallbacks that SCTP has for identifying an association for an
incoming packet, it looks for AddIp chunk (from ASCONF) and take a peek.
Thing is, at this stage nothing was validating that the chunk actually had
enough content for that, allowing the peek to happen over uninitialized
memory.

Similar check already exists in actual asconf handling in
sctp_verify_asconf().
Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sctp/input.c

@@ -1168,6 +1168,9 @@ static struct sctp_association *__sctp_rcv_asconf_lookup(
 	union sctp_addr_param *param;
 	union sctp_addr paddr;
 
+	if (ntohs(ch->length) < sizeof(*asconf) + sizeof(struct sctp_paramhdr))
+		return NULL;
+
 	/* Skip over the ADDIP header and find the Address parameter */
 	param = (union sctp_addr_param *)(asconf + 1);