rxrpc: Fix untrusted unsigned subtract

Fix the following Smatch static checker warning:

net/rxrpc/rxgk_app.c:65 rxgk_yfs_decode_ticket()
warn: untrusted unsigned subtract. 'ticket_len - 10 * 4'

by prechecking the length of what we're trying to extract in two places in
the token and decoding for a response packet.

Also use sizeof() on the struct we're extracting rather specifying the size
numerically to be consistent with the other related statements.

Fixes: 9d1d2b59341f ("rxrpc: rxgk: Implement the yfs-rxgk security class (GSSAPI)")
Reported-by: Dan Carpenter <[e-mail]>
Closes: https://lists.infradead.org/pipermail/linux-afs/2025-September/010135.html
Signed-off-by: David Howells <[e-mail]>
cc: Marc Dionne <[e-mail]>
cc: [e-mail]
Reviewed-by: Simon Horman <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/rxrpc/rxgk_app.c

···
 
 	_enter("");
 
+	if (ticket_len < 10 * sizeof(__be32))
+		return rxrpc_abort_conn(conn, skb, RXGK_INCONSISTENCY, -EPROTO,
+					rxgk_abort_resp_short_yfs_tkt);
+
 	/* Get the session key length */
 	ret = skb_copy_bits(skb, ticket_offset, tmp, sizeof(tmp));
 	if (ret < 0)
···
 		__be32 token_len;
 	} container;
 
+	if (token_len < sizeof(container))
+		goto short_packet;
+
 	/* Decode the RXGK_TokenContainer object.  This tells us which server
 	 * key we should be using.  We can then fetch the key, get the secret
 	 * and set up the crypto to extract the token.
 	 */
 	if (skb_copy_bits(skb, token_offset, &container, sizeof(container)) < 0)
-		return rxrpc_abort_conn(conn, skb, RXGK_PACKETSHORT, -EPROTO,
-					rxgk_abort_resp_tok_short);
+		goto short_packet;
 
 	kvno		= ntohl(container.kvno);
 	enctype		= ntohl(container.enctype);
 	ticket_len	= ntohl(container.token_len);
 	ticket_offset	= token_offset + sizeof(container);
 
-	if (xdr_round_up(ticket_len) > token_len - 3 * 4)
-		return rxrpc_abort_conn(conn, skb, RXGK_PACKETSHORT, -EPROTO,
-					rxgk_abort_resp_tok_short);
+	if (xdr_round_up(ticket_len) > token_len - sizeof(container))
+		goto short_packet;
 
 	_debug("KVNO %u", kvno);
 	_debug("ENC  %u", enctype);
···
 	 * also come out this way if the ticket decryption fails.
 	 */
 	return ret;
+
+short_packet:
+	return rxrpc_abort_conn(conn, skb, RXGK_PACKETSHORT, -EPROTO,
+				rxgk_abort_resp_tok_short);
 }