sctp: add vtag check in sctp_sf_do_8_5_1_E_sa

sctp_sf_do_8_5_1_E_sa() is called when processing SHUTDOWN_ACK chunk
in cookie_wait and cookie_echoed state.

The vtag in the chunk's sctphdr should be verified, otherwise, as
later in chunk length check, it may send abort with the existent
asoc's vtag, which can be exploited by one to cook a malicious
chunk to terminate a SCTP asoc.

Note that when fails to verify the vtag from SHUTDOWN-ACK chunk,
SHUTDOWN COMPLETE message will still be sent back to peer, but
with the vtag from SHUTDOWN-ACK chunk, as said in 5) of
rfc4960#section-8.4.

While at it, also remove the unnecessary chunk length check from
sctp_sf_shut_8_4_5(), as it's already done in both places where
it calls sctp_sf_shut_8_4_5().

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/sctp/sm_statefuns.c

@@ -3803,12 +3803,6 @@ static enum sctp_disposition sctp_sf_shut_8_4_5(
 
 	SCTP_INC_STATS(net, SCTP_MIB_OUTCTRLCHUNKS);
 
-	/* If the chunk length is invalid, we don't want to process
-	 * the reset of the packet.
-	 */
-	if (!sctp_chunk_length_valid(chunk, sizeof(struct sctp_chunkhdr)))
-		return sctp_sf_pdiscard(net, ep, asoc, type, arg, commands);
-
 	/* We need to discard the rest of the packet to prevent
 	 * potential boomming attacks from additional bundled chunks.
 	 * This is documented in SCTP Threats ID.
@@ -3836,6 +3830,9 @@ enum sctp_disposition sctp_sf_do_8_5_1_E_sa(struct net *net,
 {
 	struct sctp_chunk *chunk = arg;
 
+	if (!sctp_vtag_verify(chunk, asoc))
+		asoc = NULL;
+
 	/* Make sure that the SHUTDOWN_ACK chunk has a valid length. */
 	if (!sctp_chunk_length_valid(chunk, sizeof(struct sctp_chunkhdr)))
 		return sctp_sf_violation_chunklen(net, ep, asoc, type, arg,