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Date: Thu, 13 Jun 2024 01:50:29 -0700
From: Jonathan Calmels <jcalmels@3xx0.net>
To: John Johansen <john.johansen@canonical.com>
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Subject: Re: [PATCH v2 4/4] bpf,lsm: Allow editing capabilities in BPF-LSM
 hooks
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On Wed, Jun 12, 2024 at 08:54:28PM GMT, John Johansen wrote:
> On 6/12/24 10:29, Paul Moore wrote:
> > On Wed, Jun 12, 2024 at 4:15 AM Jonathan Calmels <jcalmels@3xx0.net> wrote:
> > > On Tue, Jun 11, 2024 at 06:38:31PM GMT, Paul Moore wrote:
> > > > On Tue, Jun 11, 2024 at 6:15 PM Jonathan Calmels <jcalmels@3xx0.net> wrote:
> > 
> > ...
> > 
> > > > > Arguably, if we do want fine-grained userns policies, we need LSMs to
> > > > > influence the userns capset at some point.
> > > > 
> > > > One could always use, or develop, a LSM that offers additional
> > > > controls around exercising capabilities.  There are currently four
> > > > in-tree LSMs, including the capabilities LSM, which supply a
> > > > security_capable() hook that is used by the capability-based access
> > > > controls in the kernel; all of these hook implementations work
> > > > together within the LSM framework and provide an additional level of
> > > > control/granularity beyond the existing capabilities.
> > > 
> > > Right, but the idea was to have a simple and easy way to reuse/trigger
> > > as much of the commoncap one as possible from BPF. If we're saying we
> > > need to reimplement and/or use a whole new framework, then there is
> > > little value.
> > 
> > I can appreciate how allowing direct manipulation of capability bits
> > from a BPF LSM looks attractive, but my hope is that our discussion
> > here revealed that as you look deeper into making it work there are a
> > number of pitfalls which prevent this from being a safe option for
> > generalized systems.
> > 
> > > TBH, I don't feel strongly about this, which is why it is absent from
> > > v1. However, as John pointed out, we should at least be able to modify
> > > the blob if we want flexible userns caps policies down the road.
> > 
> > As discussed in this thread, there are existing ways to provide fine
> > grained control over exercising capabilities that can be safely used
> > within the LSM framework.  I don't want to speak to what John is
> > envisioning, but he should be aware of these mechanisms, and if I
> > recall he did voice a level of concern about the same worries I
> > mentioned.
> > 
> 
> sorry, I should have been more clear. I envision LSMs being able to
> update their own state in the userns hook.
> 
> Basically the portion of the patch that removes const from the
> userns hook.

Yes, pretty sure we'll need this regardless.

> An LSM updating the capset is worrysome for all the reasons you
> pointed out, and I think a few more. I haven't had a chance to really
> look at v2 yet, so I didn't want to speak directly on the bpf part of
> the patch without first giving a good once over.
> 
> > I'm happy to discuss ways in which we can adjust the LSM hooks/layer
> > to support different approaches to capability controls, but one LSM
> > directly manipulating the state of another is going to be a no vote
> > from me.
> > 
> I might not be as hard no as Paul here, I am always willing to listen
> to arguments, but it would have to be a really good argument to
> modify the capset, when there are multiple LSMs in play on a system.

The way I see it, it's more about enhancing the capability LSM with BPF
hooks and have it modify its own state dynamically, not so much
crosstalk between two distinct LSM frameworks (say one where the BPF
LSM implements a lot of things like capable()).

In this context and with enough safeguards (say we only allow dropping
caps) this could be a net positive. Sure, ordering could come into play
in very specific scenarios, but at this point I would expect the
admin/LSM author to be conscious about it.

If we think there is no way we can come up with something that's safe
enough, and that the risks outweigh the benefits, fine by me, we can
drop this patch from the series.