[PATCH v2 4/4] bpf, lsm: Allow editing capabilities in BPF-LSM hooks

[Paul Moore]

On Wed, Jun 12, 2024 at 11:54 PM John Johansen
<john.johansen at canonical.com> wrote:
> On 6/12/24 10:29, Paul Moore wrote:
> > On Wed, Jun 12, 2024 at 4:15 AM Jonathan Calmels <jcalmels at 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 at 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.

Ah, okay, yes, that seems reasonable; although like any other change,
until we have an in-tree user we should just leave it as-is.

-- 
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