[PATCH v2 2/8] SUNRPC: reject a 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). svc_tcp_sock_recv_cmsg()
calls it for any positive receive, and the alert[] buffer it 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.

Require exactly two octets before parsing and return -EBADMSG
otherwise. That closes the transport rather than acting on a partly
uninitialized alert. Gate the path on a control message rather than a
positive count so that a zero-length record reaches the check.

Fixes: bee47cb026e7 ("sunrpc: fix handling of server side tls alerts")
Signed-off-by: Chuck Lever <cel@kernel.org>
---
 net/sunrpc/svcsock.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/net/sunrpc/svcsock.c b/net/sunrpc/svcsock.c
index 8e1009302e3b..2e5107a1fb89 100644
--- a/net/sunrpc/svcsock.c
+++ b/net/sunrpc/svcsock.c

@@ -289,13 +289,23 @@ svc_tcp_sock_recv_cmsg(struct socket *sock, unsigned int *msg_flags)
 	iov_iter_kvec(&msg.msg_iter, ITER_DEST, &alert_kvec, 1,
 		      alert_kvec.iov_len);
 	ret = sock_recvmsg(sock, &msg, MSG_DONTWAIT);
-	if (ret > 0) {
+	/* put_cmsg() shrinks msg_controllen, so a short one means
+	 * kTLS filled in u.cmsg.
+	 */
+	if (ret >= 0 && msg.msg_controllen < sizeof(u)) {
 		/* Returning the count would credit the RPC stream with
 		 * octets that never reached the caller's buffer.
 		 */
 		if (tls_get_record_type(sock->sk, &u.cmsg) !=
 		    TLS_RECORD_TYPE_ALERT)
 			return -EAGAIN;
+		/* An Alert record carries exactly one two-octet message
+		 * (RFC 8446 Section 5.1). alert_kvec caps the receive at two,
+		 * so a longer record produces the same count. MSG_EOR appears
+		 * only once kTLS has drained the whole record.
+		 */
+		if (ret != sizeof(alert) || !(msg.msg_flags & MSG_EOR))
+			return -EBADMSG;
 		iov_iter_revert(&msg.msg_iter, ret);
 		ret = svc_tcp_sock_process_cmsg(sock, &msg, &u.cmsg, -EAGAIN);
 	}

-- 
2.54.0