Re: [PATCH v1 bpf-next 0/5] af_unix: Allow BPF LSM to scrub SCM_RIGHTS at sendmsg().

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From: Paul Moore <paul@paul-moore.com>
Date: Mon, 5 May 2025 19:21:25 -0400
> On Mon, May 5, 2025 at 5:58 PM Kuniyuki Iwashima <kuniyu@amazon.com> wrote:
> >
> > 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.
> >
> > 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.
> >
> > 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 because the last fput()
> > could occur on the receiver 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 filter SCM_RIGHTS on the sender side.
> >
> > This series allows BPF LSM to inspect skb at sendmsg() and scrub
> > SCM_RIGHTS fds by kfunc.
> 
> I'll take a closer look later this week, but generally speaking LSM
> hooks are intended for observability and access control, not data
> modification, which means what you are trying to accomplish may not be
> a good fit for a LSM hook.  Have you considered simply inspecting the
> skb at sendmsg() and rejecting the send in the LSM hook if a
> SCM_RIGHTS cmsg is present that doesn't fit within the security policy
> implemented in your BPF program?

I think the simple inspection (accept all or deny) does not cover
a real use case and is not that helpful.

I don't like to add another hook point in AF_UNIX code just because
of it and rather want to reuse the exisiting hook as we have a nice
place.

Also, passing skb makes it possible to build much more flexible
policy as it allows bpf prog to inspect the skb payload with
existing bpf helpers.