SCTP, possible bug in peer authentication key

[Yann WANWANSCAPPEL]

Hi all,

I think I found a bug in the SCTP authentication code, in
sctp_load_addresses_from_init() in sctp_pcb.c

keylen = sizeof(*p_random) + random_len + sizeof(*chunks) + num_chunks +
sizeof(*hmacs) + hmacs_len;

The keylen calculation assumes the Chunk List Parameter (CHUNKS)
vl-param was present in the received INIT packet, which can be false if
peer SCTP does not require any chunk to be authenticated (this typically
occurs if peer does not support ASCONF).

>From RFC 4895, 6.1

* An SCTP endpoint has a list of chunks it only accepts if they are
* received in an authenticated way.  This list is included in the INIT
* and INIT-ACK, and MAY be omitted if it is empty.  Since this list
* does not change during the lifetime of the SCTP endpoint there is no
* problem in case of INIT collision.

This case is properly handled later in the build of the key

	/* append in the AUTH chunks */
	if (chunks != NULL) {
	.....
	}

I think the calculated keylen should be something like this :

keylen = sizeof(*p_random) + random_len + sizeof(*hmacs) + hmacs_len;

if (chunks != NULL) {
   keylen += sizeof(*chunks) + num_chunks
}

This problem results in authenticated packets sent from peer SCTP to be
discarded.

The problem does not occurs if peer SCTP is modified to send an empty
Chunk List Parameter, (eg num_chunks = 0 in the decoding).

Br,
Yann