[PATCH 7/8] SUNRPC: reject a client-side TLS alert record that is not two octets

tls_alert_recv() reads two octets from the kvec it is handed and does
not check the length (net/handshake/alert.c). xs_sock_process_cmsg()
calls it for any alert record, and the alert[] buffer that
xs_sock_recv_cmsg() supplies carries no initializer. A one-octet alert
body leaves the description read from uninitialized stack and reported
through trace_tls_alert_recv().

The peer controls that length. Neither tls_rx_msg_size() nor
tls_rx_one_record() enforces the two-octet Alert payload. A TLS 1.3
record carrying only the inner content-type octet decrypts to a
zero-length payload. RFC 8446 Section 5.1 requires a record with an
Alert type to carry exactly one message, so any other length is
malformed. RFC 9289 Section 5 bars RPC-with-TLS from negotiating a
version below TLS 1.3, so no other alert framing applies.

Require exactly two octets before parsing and return -EACCES
otherwise. xs_stream_data_receive() already treats -EACCES as a fatal
alert and reports it to the pending tasks. Gate the path on a control
message rather than a positive count so that a zero-length record
reaches the check.

Fixes: cc5d59081fa2 ("sunrpc: fix client side handling of tls alerts")
Signed-off-by: Chuck Lever <cel@kernel.org>
---
 net/sunrpc/xprtsock.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/net/sunrpc/xprtsock.c b/net/sunrpc/xprtsock.c
index 7178db70c1ff..5527f7f8a185 100644
--- a/net/sunrpc/xprtsock.c
+++ b/net/sunrpc/xprtsock.c

@@ -417,9 +417,25 @@ xs_sock_recv_cmsg(struct socket *sock, unsigned int *msg_flags, int flags)
 	iov_iter_kvec(&msg.msg_iter, ITER_DEST, &alert_kvec, 1,
 		      alert_kvec.iov_len);
 	ret = sock_recvmsg(sock, &msg, flags);
-	if (ret > 0) {
-		if (tls_get_record_type(sock->sk, &u.cmsg) == TLS_RECORD_TYPE_ALERT)
+	/* put_cmsg() shrinks msg_controllen, so a short one means
+	 * kTLS filled in u.cmsg.
+	 */
+	if (ret >= 0 && msg.msg_controllen < sizeof(u)) {
+		if (tls_get_record_type(sock->sk, &u.cmsg) ==
+		    TLS_RECORD_TYPE_ALERT) {
+			/* RFC 8446 Section 5.1 requires a record with an
+			 * Alert type to carry exactly one message. An alert
+			 * is two octets. tls_alert_recv() reads both without
+			 * checking the length. alert_kvec caps the count at
+			 * two, so a longer record fills it as well. kTLS
+			 * sets MSG_EOR only once the record has been
+			 * drained.
+			 */
+			if (ret != sizeof(alert) ||
+			    !(msg.msg_flags & MSG_EOR))
+				return -EACCES;
 			iov_iter_revert(&msg.msg_iter, ret);
+		}
 		ret = xs_sock_process_cmsg(sock, &msg, msg_flags, &u.cmsg,
 					   -EAGAIN);
 	}

-- 
2.54.0