Subject: Re: [PATCH v4 2/7] landlock: Add UDP connect() access control

From: Mickaël Salaün <[email-masked]>
Date: Fri, 22 May 2026 23:18:06 +0200

On Sat, May 02, 2026 at 02:43:01PM +0200, Matthieu Buffet wrote:
> Add support for a second fine-grained UDP access right.
> This first half of LANDLOCK_ACCESS_NET_CONNECT_SEND_UDP controls the
> ability to set the remote port of a socket (via connect()). It will be
> useful for applications that send datagrams, and for some servers too
> (those creating per-client sockets, which want to receive traffic only
> from a specific address).
> 
> Similarly as for bind(), this access control is performed when
> configuring sockets, not in hot code paths.
> 
> Include detection of when autobind is about to be required, and check if
> the process would be allowed to call bind(0) explicitly. Autobind can
> only be performed when sending a first datagram, when connect()ing, and
> in some splice() EOF edge case which, afaiu, can only happen after a
> remote peer has been set (which is already covered).
> 
> Signed-off-by: Matthieu Buffet <[email-masked]>
> ---
>  include/uapi/linux/landlock.h               | 19 +++++
>  security/landlock/audit.c                   |  2 +
>  security/landlock/limits.h                  |  2 +-
>  security/landlock/net.c                     | 79 +++++++++++++++++----
>  tools/testing/selftests/landlock/net_test.c |  5 +-
>  5 files changed, 92 insertions(+), 15 deletions(-)

> diff --git a/security/landlock/net.c b/security/landlock/net.c
> index f9ccb52e7d45..045881f81295 100644
> --- a/security/landlock/net.c
> +++ b/security/landlock/net.c
> @@ -68,16 +68,17 @@ static int current_check_access_socket(struct socket *const sock,
>  
>  	switch (address->sa_family) {
>  	case AF_UNSPEC:
> -		if (access_request == LANDLOCK_ACCESS_NET_CONNECT_TCP) {
> +		if (access_request == LANDLOCK_ACCESS_NET_CONNECT_TCP ||
> +		    access_request == LANDLOCK_ACCESS_NET_CONNECT_SEND_UDP) {
>  			/*
>  			 * Connecting to an address with AF_UNSPEC dissolves
> -			 * the TCP association, which have the same effect as
> -			 * closing the connection while retaining the socket
> -			 * object (i.e., the file descriptor).  As for dropping
> -			 * privileges, closing connections is always allowed.
> -			 *
> -			 * For a TCP access control system, this request is
> -			 * legitimate. Let the network stack handle potential
> +			 * the remote association while retaining the socket
> +			 * object (i.e., the file descriptor). For TCP, it has
> +			 * the same effect as closing the connection. For UDP,
> +			 * it removes any preset remote address. As for
> +			 * dropping privileges, these actions are always
> +			 * allowed.
> +			 * Let the network stack handle potential
>  			 * inconsistencies and return -EINVAL if needed.
>  			 */
>  			return 0;
> @@ -134,7 +135,8 @@ static int current_check_access_socket(struct socket *const sock,
>  		addr4 = (struct sockaddr_in *)address;
>  		port = addr4->sin_port;
>  
> -		if (access_request == LANDLOCK_ACCESS_NET_CONNECT_TCP) {
> +		if (access_request == LANDLOCK_ACCESS_NET_CONNECT_TCP ||
> +		    access_request == LANDLOCK_ACCESS_NET_CONNECT_SEND_UDP) {
>  			audit_net.dport = port;
>  			audit_net.v4info.daddr = addr4->sin_addr.s_addr;
>  		} else if (access_request == LANDLOCK_ACCESS_NET_BIND_TCP ||
> @@ -157,7 +159,8 @@ static int current_check_access_socket(struct socket *const sock,
>  		addr6 = (struct sockaddr_in6 *)address;
>  		port = addr6->sin6_port;
>  
> -		if (access_request == LANDLOCK_ACCESS_NET_CONNECT_TCP) {
> +		if (access_request == LANDLOCK_ACCESS_NET_CONNECT_TCP ||
> +		    access_request == LANDLOCK_ACCESS_NET_CONNECT_SEND_UDP) {
>  			audit_net.dport = port;
>  			audit_net.v6info.daddr = addr6->sin6_addr;
>  		} else if (access_request == LANDLOCK_ACCESS_NET_BIND_TCP ||
> @@ -213,6 +216,50 @@ static int current_check_access_socket(struct socket *const sock,
>  	return -EACCES;
>  }
>  
> +static int current_check_autobind_udp_socket(struct socket *const sock)
> +{
> +	struct sockaddr_storage port0 = { 0 };

struct sockaddr_storage port0 = {};

> +
> +	/*
> +	 * On UDP sockets, if a local port has not already been bound,
> +	 * calling connect() or sending a first datagram has the side
> +	 * effect of autobinding an ephemeral port: we also have to check
> +	 * that the process would have had the right to bind(0) explicitly.
> +	 * Note: socket is not locked, so another thread could do an
> +	 * explicit bind(!=0) on this socket, changing inet_num to non-zero
> +	 * after we read it, but this would only have us enforce an
> +	 * additional bind(0) access check and would not bypass policy.
> +	 */
> +	if (inet_sk(sock->sk)->inet_num != 0)
> +		return 0;
> +
> +	/*
> +	 * Construct a struct sockaddr* with port 0 to pretend the
> +	 * process tried to bind() on that address.
> +	 */
> +	port0.ss_family = sock->sk->__sk_common.skc_family;
> +	switch (port0.ss_family) {
> +	case AF_INET: {
> +		((struct sockaddr_in *)&port0)->sin_port = 0;

Why is this useful? The struct is already initialized to 0.

> +		break;
> +	}
> +
> +#if IS_ENABLED(CONFIG_IPV6)
> +	case AF_INET6: {
> +		((struct sockaddr_in6 *)&port0)->sin6_port = 0;

Same question.

> +		break;
> +	}
> +#endif /* IS_ENABLED(CONFIG_IPV6) */
> +
> +	default:
> +		return 0;
> +	}
> +
> +	return current_check_access_socket(sock, (struct sockaddr *)&port0,
> +					   sizeof(port0),
> +					   LANDLOCK_ACCESS_NET_BIND_UDP);
> +}