landlock: Add UDP send+connect access control

Add support for a second fine-grained UDP access right.
LANDLOCK_ACCESS_NET_CONNECT_SEND_UDP controls the ability to set the
remote port of a socket (via connect()) and to specify an explicit
destination when sending a datagram, to override any remote peer set on
a UDP socket (e.g. in sendto() or sendmsg()).  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.

Add detection of when autobind is about to be required, and deny the
operation if the process would not be allowed to call bind(0)
explicitly. Autobind can only be performed in udp_lib_get_port() from
code paths already controlled by LSM hooks: when connect()ing, sending a
first datagram, and in some splice() EOF edge case which, afaiu, can
only happen after a remote peer has been set. This invariant needs to be
preserved to keep bind policies actually enforced.

Signed-off-by: Matthieu Buffet <matthieu@buffet.re>
Link: https://patch.msgid.link/20260611162107.49278-3-matthieu@buffet.re
[mic: Add quick return for non-sandboxed tasks, fix sa_family
dereferencing, fix comment formatting]
Signed-off-by: Mickaël Salaün <mic@digikod.net>

1 files changed, 23 insertions, 0 deletions

diff --git a/include/uapi/linux/landlock.h b/include/uapi/linux/landlock.h
index f2927681e92d..811ec77f9105 100644
--- a/include/uapi/linux/landlock.h
+++ b/include/uapi/linux/landlock.h
@@ -378,11 +378,34 @@ struct landlock_net_port_attr {
  *
  * - %LANDLOCK_ACCESS_NET_BIND_UDP: Bind UDP sockets to the given local
  *   port. Support added in Landlock ABI version 10.
+ * - %LANDLOCK_ACCESS_NET_CONNECT_SEND_UDP: Set the remote port of UDP
+ *   sockets to the given port, or send datagrams to the given remote port
+ *   ignoring any destination pre-set on a socket. Support added in
+ *   Landlock ABI version 10.
+ *
+ * .. note:: Setting a remote address or sending a first datagram
+ *   auto-binds UDP sockets to an ephemeral local source port if not
+ *   already bound. To allow this if both %LANDLOCK_ACCESS_NET_BIND_UDP
+ *   and %LANDLOCK_ACCESS_NET_CONNECT_SEND_UDP are handled, you need to
+ *   either:
+ *
+ *   - use a socket already bound to a port before the ruleset started
+ *     being enforced;
+ *   - or grant %LANDLOCK_ACCESS_NET_BIND_UDP on port 0, meaning "any
+ *     port in the ephemeral port range";
+ *   - or grant %LANDLOCK_ACCESS_NET_BIND_UDP on a specific port, and
+ *     call :manpage:`bind(2)` on that port before trying to
+ *     :manpage:`connect(2)` or send datagrams.
+ *
+ * .. note:: Sending datagrams to an ``AF_UNSPEC`` destination address
+ *   family is not supported for IPv6 UDP sockets: you will need to use a
+ *   ``NULL`` address instead.
  */
 /* clang-format off */
 #define LANDLOCK_ACCESS_NET_BIND_TCP			(1ULL << 0)
 #define LANDLOCK_ACCESS_NET_CONNECT_TCP			(1ULL << 1)
 #define LANDLOCK_ACCESS_NET_BIND_UDP			(1ULL << 2)
+#define LANDLOCK_ACCESS_NET_CONNECT_SEND_UDP		(1ULL << 3)
 /* clang-format on */
 
 /**