[PATCH 5/5] SUNRPC: Bypass sock_recvmsg() for the TLS control-record receive

svc_tcp_recvfrom() parses the RPC record stream with ->read_sock,
which calls neither security_socket_recvmsg() nor the
sock:sock_recv_length tracepoint. svc_tcp_recv_cmsg() still goes
through sock_recvmsg(), so an LSM mediates only the TLS control
records on a server socket, and sock:sock_recv_length reports only
those. Partial coverage is worse than none. It makes the RPC stream
look mediated and observed when it is not.

Until the record stream moved to ->read_sock, an LSM saw every octet
NFSD read from a TCP socket. An SELinux policy that denies
SOCKET__READ to NFSD blocked the receive. After this change no call
on the server's TCP receive path consults an LSM, so that denial has
no effect.

Dispatch ->recvmsg directly so the whole receive path behaves one
way. sock_recvmsg_nosec() reaches ->recvmsg through
INDIRECT_CALL_INET(), so on a retpoline build the direct dispatch
costs one indirect call per control record. Control records are rare
on an established connection.

Signed-off-by: Chuck Lever <[email protected]>
---
 net/sunrpc/svcsock.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/net/sunrpc/svcsock.c b/net/sunrpc/svcsock.c
index fe307d8314c4..ef7ac080fcd3 100644
--- a/net/sunrpc/svcsock.c
+++ b/net/sunrpc/svcsock.c
@@ -229,10 +229,16 @@ static int svc_one_sock_name(struct svc_sock *svsk, char *buf, int remaining)
 	return len;
 }
 
+/*
+ * The ->read_sock data path invokes neither security_socket_recvmsg()
+ * nor the sock:sock_recv_length tracepoint. Dispatch ->recvmsg
+ * directly so the whole receive path behaves one way.
+ */
 static int svc_tcp_recv_cmsg(struct socket *sock, int flags,
 			     struct kvec *payload, u8 *type,
 			     unsigned int *msg_flags)
 {
+	const struct proto_ops *ops = READ_ONCE(sock->ops);
 	union {
 		struct cmsghdr	cmsg;
 		u8		buf[CMSG_SPACE(sizeof(u8))];
@@ -244,7 +250,7 @@ static int svc_tcp_recv_cmsg(struct socket *sock, int flags,
 	int ret;
 
 	iov_iter_kvec(&msg.msg_iter, ITER_DEST, payload, 1, payload->iov_len);
-	ret = sock_recvmsg(sock, &msg, flags);
+	ret = ops->recvmsg(sock, &msg, msg_data_left(&msg), flags);
 	if (ret < 0)
 		return ret;
 	*msg_flags = msg.msg_flags;

-- 
2.54.0