sunrpc: fix handling of server side tls alerts

Scott Mayhew discovered a security exploit in NFS over TLS in
tls_alert_recv() due to its assumption it can read data from
the msg iterator's kvec..

kTLS implementation splits TLS non-data record payload between
the control message buffer (which includes the type such as TLS
aler or TLS cipher change) and the rest of the payload (say TLS
alert's level/description) which goes into the msg payload buffer.

This patch proposes to rework how control messages are setup and
used by sock_recvmsg().

If no control message structure is setup, kTLS layer will read and
process TLS data record types. As soon as it encounters a TLS control
message, it would return an error. At that point, NFS can setup a
kvec backed msg buffer and read in the control message such as a
TLS alert. Msg iterator can advance the kvec pointer as a part of
the copy process thus we need to revert the iterator before calling
into the tls_alert_recv.

Reported-by: Scott Mayhew <smayhew@redhat.com>
Fixes: 5e052dda12 ("SUNRPC: Recognize control messages in server-side TCP socket code")
Suggested-by: Trond Myklebust <trondmy@hammerspace.com>
Cc: stable@vger.kernel.org
Signed-off-by: Olga Kornievskaia <okorniev@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

net/sunrpc/svcsock.c

@@ -257,20 +257,47 @@ svc_tcp_sock_process_cmsg(struct socket *sock, struct msghdr *msg,
 }
 
 static int
-svc_tcp_sock_recv_cmsg(struct svc_sock *svsk, struct msghdr *msg)
+svc_tcp_sock_recv_cmsg(struct socket *sock, unsigned int *msg_flags)
 {
 	union {
 		struct cmsghdr	cmsg;
 		u8		buf[CMSG_SPACE(sizeof(u8))];
 	} u;
-	struct socket *sock = svsk->sk_sock;
+	u8 alert[2];
+	struct kvec alert_kvec = {
+		.iov_base = alert,
+		.iov_len = sizeof(alert),
+	};
+	struct msghdr msg = {
+		.msg_flags = *msg_flags,
+		.msg_control = &u,
+		.msg_controllen = sizeof(u),
+	};
 	int ret;
 
-	msg->msg_control = &u;
+	iov_iter_kvec(&msg.msg_iter, ITER_DEST, &alert_kvec, 1,
-	msg->msg_controllen = sizeof(u);
+		      alert_kvec.iov_len);
+	ret = sock_recvmsg(sock, &msg, MSG_DONTWAIT);
+	if (ret > 0 &&
+	    tls_get_record_type(sock->sk, &u.cmsg) == TLS_RECORD_TYPE_ALERT) {
+		iov_iter_revert(&msg.msg_iter, ret);
+		ret = svc_tcp_sock_process_cmsg(sock, &msg, &u.cmsg, -EAGAIN);
+	}
+	return ret;
+}
+
+static int
+svc_tcp_sock_recvmsg(struct svc_sock *svsk, struct msghdr *msg)
+{
+	int ret;
+	struct socket *sock = svsk->sk_sock;
+
 	ret = sock_recvmsg(sock, msg, MSG_DONTWAIT);
-	if (unlikely(msg->msg_controllen != sizeof(u)))
+	if (msg->msg_flags & MSG_CTRUNC) {
-		ret = svc_tcp_sock_process_cmsg(sock, msg, &u.cmsg, ret);
+		msg->msg_flags &= ~(MSG_CTRUNC | MSG_EOR);
+		if (ret == 0 || ret == -EIO)
+			ret = svc_tcp_sock_recv_cmsg(sock, &msg->msg_flags);
+	}
 	return ret;
 }
 
@@ -321,7 +348,7 @@ static ssize_t svc_tcp_read_msg(struct svc_rqst *rqstp, size_t buflen,
 		iov_iter_advance(&msg.msg_iter, seek);
 		buflen -= seek;
 	}
-	len = svc_tcp_sock_recv_cmsg(svsk, &msg);
+	len = svc_tcp_sock_recvmsg(svsk, &msg);
 	if (len > 0)
 		svc_flush_bvec(bvec, len, seek);
 
@@ -1018,7 +1045,7 @@ static ssize_t svc_tcp_read_marker(struct svc_sock *svsk,
 		iov.iov_base = ((char *)&svsk->sk_marker) + svsk->sk_tcplen;
 		iov.iov_len  = want;
 		iov_iter_kvec(&msg.msg_iter, ITER_DEST, &iov, 1, want);
-		len = svc_tcp_sock_recv_cmsg(svsk, &msg);
+		len = svc_tcp_sock_recvmsg(svsk, &msg);
 		if (len < 0)
 			return len;
 		svsk->sk_tcplen += len;