handshake: Require admin permission for DONE command

[ Upstream commit 81246a6 ]

ACCEPT and DONE are the two downcalls of the handshake genl
family, both intended for use by the trusted handshake agent
(tlshd). ACCEPT already requires GENL_ADMIN_PERM; DONE has
no privilege check at all.

The fd-lookup in handshake_nl_done_doit() only confirms that
some pending handshake request exists for the supplied sockfd;
it does not authenticate the sender. An unprivileged process
that guesses or observes a valid sockfd can therefore submit
a DONE with HANDSHAKE_A_DONE_STATUS == 0, leaving the kernel
consumer to proceed as if the handshake succeeded. A non-zero
status on a forged DONE tears down a legitimate in-flight
handshake before tlshd can report its real result.

Fixes: 3b3009e ("net/handshake: Create a NETLINK service for handling handshake requests")
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Hannes Reinecke <hare@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Link: https://patch.msgid.link/20260609141831.90694-1-cel@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: chucklever

Documentation/netlink/specs/handshake.yaml

@@ -113,6 +113,7 @@ operations:
       name: done
       doc: Handler reports handshake completion
       attribute-set: done
+      flags: [admin-perm]
       do:
         request:
           attributes:

net/handshake/genl.c

@@ -36,7 +36,7 @@ static const struct genl_split_ops handshake_nl_ops[] = {
 		.doit		= handshake_nl_done_doit,
 		.policy		= handshake_done_nl_policy,
 		.maxattr	= HANDSHAKE_A_DONE_REMOTE_AUTH,
-		.flags		= GENL_CMD_CAP_DO,
+		.flags		= GENL_ADMIN_PERM | GENL_CMD_CAP_DO,
 	},
 };