linux-kernel - Re: [RFC][PATCH 0/5] Mount, Filesystem and Keyrings notifications

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Date:   Tue, 24 Jul 2018 17:00:35 +0100
From:   David Howells <dhowells@...hat.com>
To:     Casey Schaufler <casey@...aufler-ca.com>
Cc:     dhowells@...hat.com, viro@...iv.linux.org.uk,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        raven@...maw.net, keyrings@...r.kernel.org,
        linux-security-module@...r.kernel.org
Subject: Re: [RFC][PATCH 0/5] Mount, Filesystem and Keyrings notifications

Casey Schaufler <casey@...aufler-ca.com> wrote:

> >  (1) Mount topology and reconfiguration change events.
> 
> With the possibility of unprivileged mounting you're going to have to
> address access control on events.  If root in a user namespace mounts a
> filesystem you may have a case where the "real" user wouldn't want the
> listener to receive a notification.

Can you clarify who the listener is in this case?

Note that mount topology events don't leak outside of the mount namespace
they're generated in.

That said, if you, a random user, put a watchpoint on "/" you can see the
mount events triggered by another random user in the same mount namespace.  I
don't see a way to control this except by resorting to the LSM since UNIX
doesn't have 'notify' permission bits.

But for each event, I can associate an object label, derived from the source,
and use f_cred on the notification queue to provide a subject label.

> >  (2) Superblocks EIO, ENOSPC and EDQUOT events (not complete yet).
> 
> Here, too. If SELinux (for example) policy says you can't see
> anything on a filesystem you shouldn't get notifications about
> things that happen to that filesystem.

Yep.  Sounds like I need to refer that to the LSM as above.

It's a bit easier for specifically nominated sb sources since you might only
need to do the check once at sb_notify() time.  If there's a general queue
that all sbs contribute to, however, then things become more complicated as
the checks have to be done at do-we-write-into-this-queue? time.

> >  (3) Key/keyring changes events
> 
> And again, I should only get notifications about keys and
> keyrings I have access to.

Currently, you can only watch keys that grant you View permission, which might
suffice.

> I expect that you intentionally left off
> 
>    (4) User injected events
> 
> at this point, but it's an obvious extension. That is going
> to require access controls (remember kdbus) so I think you'd
> do well to design them in now rather than have some security
> module hack like me come along later and "fix" it. 

Yeah - the thought had occurred to me, but there needs to be some way to
define a 'source' and a way to connect them.  Also, would you want a general
source that anyone can contribute through, specific sources where you have to
directly connect or namespace-restricted sources?

David