[RFC PATCH 1/2] landlock: fix TCP Fast Open connection bypass

[Matthieu Buffet <matthieu@buffet.re>]

The documentation of the socket_connect() LSM hook states that it
controls connecting a socket to a remote address. It has not been the
case since the addition of TCP Fast Open (RFC 7413) support, which allows
opening a TCP connection (thus, setting a socket's destination address)
via the MSG_FASTOPEN flag passed to sendto()/sendmsg()/sendmmsg(). The
problem then got duplicated into MPTCP.

Landlock did not take it into account when its TCP support was added,
leaving a bypass of TCP connect policy.

Ideally a call to the LSM hook would be added in the fastopen code path,
in order to fix this generically. But connect() hooks are designed to run
with the socket locked, unlike sendmsg() hooks.

Closes: https://github.com/landlock-lsm/linux/issues/41
Fixes: fff69fb03dde ("landlock: Support network rules with TCP bind and connect")
Signed-off-by: Matthieu Buffet <matthieu@buffet.re>
---
 security/landlock/net.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/security/landlock/net.c b/security/landlock/net.c
index 4ee4002a8f56..a2375762c18b 100644
--- a/security/landlock/net.c
+++ b/security/landlock/net.c
@@ -246,9 +246,26 @@ static int hook_socket_connect(struct socket *const sock,
 					   access_request);
 }
 
+static int hook_socket_sendmsg(struct socket *const sock,
+			       struct msghdr *const msg, const int size)
+{
+	struct sockaddr *const address = msg->msg_name;
+	const int addrlen = msg->msg_namelen;
+
+	if (sk_is_tcp(sock->sk) && address != NULL &&
+	    (msg->msg_flags & MSG_FASTOPEN) != 0) {
+		return current_check_access_socket(
+			sock, address, addrlen,
+			LANDLOCK_ACCESS_NET_CONNECT_TCP);
+	}
+
+	return 0;
+}
+
 static struct security_hook_list landlock_hooks[] __ro_after_init = {
 	LSM_HOOK_INIT(socket_bind, hook_socket_bind),
 	LSM_HOOK_INIT(socket_connect, hook_socket_connect),
+	LSM_HOOK_INIT(socket_sendmsg, hook_socket_sendmsg),
 };
 
 __init void landlock_add_net_hooks(void)
-- 
2.47.3