Re: [RFC][PATCH 0/8] Mount, FS, Block and Keyrings notifications [ver #2]

[Andy Lutomirski]

On Tue, Jun 4, 2019 at 1:31 PM Casey Schaufler <casey@xxxxxxxxxxxxxxxx> wrote:
>
> n 6/4/2019 10:43 AM, Andy Lutomirski wrote:
> > On Tue, Jun 4, 2019 at 9:35 AM David Howells <dhowells@xxxxxxxxxx> wrote:
> >>
> >> Hi Al,
> >>
> >> Here's a set of patches to add a general variable-length notification queue
> >> concept and to add sources of events for:
> > I asked before and didn't see a response, so I'll ask again.  Why are
> > you paying any attention at all to the creds that generate an event?
> > It seems like the resulting security model will be vary hard to
> > understand and probably buggy.  Can't you define a sensible model in
> > which only the listener creds matter?
>
> We've spent the last 18 months reeling from the implications
> of what can happen when one process has the ability to snoop
> on another. Introducing yet another mechanism that is trivial
> to exploit is a very bad idea.

If you're talking about Spectre, etc, this is IMO entirely irrelevant.
Among other things, setting these watches can and should require some
degree of privilege.

>
> I will try to explain the problem once again. If process A
> sends a signal (writes information) to process B the kernel
> checks that either process A has the same UID as process B
> or that process A has privilege to override that policy.
> Process B is passive in this access control decision, while
> process A is active.

Are you stating what you see to be a requirement?

> Process A must have write access
> (defined by some policy) to process B's event buffer.

No, stop right here.  Process B is monitoring some aspect of the
system.  Process A is doing something.  Process B should need
permission to monitor whatever it's monitoring, and process A should
have permission to do whatever it's doing.  I don't think it makes
sense to try to ascribe an identity to the actor doing some action to
decide to omit it from the watch -- this has all kinds of correctness
issues.

If you're writing a policy and you don't like letting process B spy on
processes doing various things, then disallow that type of spying.

> To
> implement such a policy requires A's credential,

You may not design a new mechanism that looks at the credential in a
context where looking at a credential is invalid unless you have some
very strong justification for why all of the known reasons that it's a
bad idea don't apply to what you're doing.

So, without a much stronger justification, NAK.