af_unix: Introduce SO_PASSRIGHTS.

As long as recvmsg() or recvmmsg() is used with cmsg, it is not
possible to avoid receiving file descriptors via SCM_RIGHTS.

This behaviour has occasionally been flagged as problematic, as
it can be (ab)used to trigger DoS during close(), for example, by
passing a FUSE-controlled fd or a hung NFS fd.

For instance, as noted on the uAPI Group page [0], an untrusted peer
could send a file descriptor pointing to a hung NFS mount and then
close it.  Once the receiver calls recvmsg() with msg_control, the
descriptor is automatically installed, and then the responsibility
for the final close() now falls on the receiver, which may result
in blocking the process for a long time.

Regarding this, systemd calls cmsg_close_all() [1] after each
recvmsg() to close() unwanted file descriptors sent via SCM_RIGHTS.

However, this cannot work around the issue at all, because the final
fput() may still occur on the receiver's side once sendmsg() with
SCM_RIGHTS succeeds.  Also, even filtering by LSM at recvmsg() does
not work for the same reason.

Thus, we need a better way to refuse SCM_RIGHTS at sendmsg().

Let's introduce SO_PASSRIGHTS to disable SCM_RIGHTS.

Note that this option is enabled by default for backward
compatibility.

Link: https://uapi-group.org/kernel-features/#disabling-reception-of-scm_rights-for-af_unix-sockets #[0]
Link: https://github.com/systemd/systemd/blob/v257.5/src/basic/fd-util.c#L612-L628 #[1]
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 14 insertions, 0 deletions

diff --git a/net/core/sock.c b/net/core/sock.c
index 381abf8f25b7..0cb52e590094 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -1571,6 +1571,13 @@ set_sndbuf:
 			ret = -EOPNOTSUPP;
 		break;
 
+	case SO_PASSRIGHTS:
+		if (sk_is_unix(sk))
+			sk->sk_scm_rights = valbool;
+		else
+			ret = -EOPNOTSUPP;
+		break;
+
 	case SO_INCOMING_CPU:
 		reuseport_update_incoming_cpu(sk, val);
 		break;
@@ -1879,6 +1886,13 @@ int sk_getsockopt(struct sock *sk, int level, int optname,
 		v.val = sk->sk_scm_pidfd;
 		break;
 
+	case SO_PASSRIGHTS:
+		if (!sk_is_unix(sk))
+			return -EOPNOTSUPP;
+
+		v.val = sk->sk_scm_rights;
+		break;
+
 	case SO_PEERCRED:
 	{
 		struct ucred peercred;