Commit message: 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>
CVE: CVE-2016-9555
CWE: CWE-125
Vulnerability description: The sctp_sf_ootb function in net/sctp/sm_statefuns.c in the Linux kernel before 4.8.8 lacks chunk-length checking for the first chunk, which allows remote attackers to cause a denial of service (out-of-bounds slab access) or possibly have unspecified other impact via crafted SCTP data.

Project: linux
File: sm_statefuns.c

sctp_disposition_t sctp_sf_ootb(struct net *net,
				const struct sctp_endpoint *ep,
				const struct sctp_association *asoc,
				const sctp_subtype_t type,
				void *arg,
				sctp_cmd_seq_t *commands)
{
	struct sctp_chunk *chunk = arg;
	struct sk_buff *skb = chunk->skb;
	sctp_chunkhdr_t *ch;
	sctp_errhdr_t *err;
	__u8 *ch_end;
	int ootb_shut_ack = 0;
	int ootb_cookie_ack = 0;

	SCTP_INC_STATS(net, SCTP_MIB_OUTOFBLUES);

	ch = (sctp_chunkhdr_t *) chunk->chunk_hdr;
	do {
		/* Report violation if the chunk is less then minimal */
		if (ntohs(ch->length) < sizeof(sctp_chunkhdr_t))
 			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.
 		 */
		if (SCTP_CID_SHUTDOWN_ACK == ch->type)
			ootb_shut_ack = 1;

		/* RFC 2960, Section 3.3.7
		 *   Moreover, under any circumstances, an endpoint that
		 *   receives an ABORT  MUST NOT respond to that ABORT by
		 *   sending an ABORT of its own.
		 */
		if (SCTP_CID_ABORT == ch->type)
			return sctp_sf_pdiscard(net, ep, asoc, type, arg, commands);

		/* RFC 8.4, 7) If the packet contains a "Stale cookie" ERROR
		 * or a COOKIE ACK the SCTP Packet should be silently
		 * discarded.
		 */

		if (SCTP_CID_COOKIE_ACK == ch->type)
			ootb_cookie_ack = 1;

		if (SCTP_CID_ERROR == ch->type) {
			sctp_walk_errors(err, ch) {
				if (SCTP_ERROR_STALE_COOKIE == err->cause) {
					ootb_cookie_ack = 1;
					break;
				}
 			}
 		}
 
 		ch = (sctp_chunkhdr_t *) ch_end;
 	} while (ch_end < skb_tail_pointer(skb));
 
	if (ootb_shut_ack)
		return sctp_sf_shut_8_4_5(net, ep, asoc, type, arg, commands);
	else if (ootb_cookie_ack)
		return sctp_sf_pdiscard(net, ep, asoc, type, arg, commands);
	else
		return sctp_sf_tabort_8_4_8(net, ep, asoc, type, arg, commands);
}