sctp: Fix out-of-bounds reading in sctp_asoc_get_hmac()

The sctp_asoc_get_hmac() function iterates through a peer's hmac_ids
array and attempts to ensure that only a supported hmac entry is
returned.  The current code fails to do this properly - if the last id
in the array is out of range (greater than SCTP_AUTH_HMAC_ID_MAX), the
id integer remains set after exiting the loop, and the address of an
out-of-bounds entry will be returned and subsequently used in the parent
function, causing potentially ugly memory corruption.  This patch resets
the id integer to 0 on encountering an invalid id so that NULL will be
returned after finishing the loop if no valid ids are found.
Signed-off-by: Dan Rosenberg <drosenberg@vsecurity.com>
Acked-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sctp/auth.c

@@ -543,16 +543,20 @@ struct sctp_hmac *sctp_auth_asoc_get_hmac(const struct sctp_association *asoc)
 		id = ntohs(hmacs->hmac_ids[i]);
 
 		/* Check the id is in the supported range */
-		if (id > SCTP_AUTH_HMAC_ID_MAX)
+		if (id > SCTP_AUTH_HMAC_ID_MAX) {
+			id = 0;
 			continue;
+		}
 
 		/* See is we support the id.  Supported IDs have name and
 		 * length fields set, so that we can allocated and use
 		 * them.  We can safely just check for name, for without the
 		 * name, we can't allocate the TFM.
 		 */
-		if (!sctp_hmac_list[id].hmac_name)
+		if (!sctp_hmac_list[id].hmac_name) {
+			id = 0;
 			continue;
+		}
 
 		break;
 	}