[PATCH bpf-next 0/8] New BPF map and BTF security LSM hooks

[Andrii Nakryiko]

On Mon, Apr 17, 2023 at 4:53 PM Casey Schaufler <casey at schaufler-ca.com> wrote:
>
> On 4/17/2023 4:31 PM, Andrii Nakryiko wrote:
> > On Thu, Apr 13, 2023 at 9:27 AM Casey Schaufler <casey at schaufler-ca.com> wrote:
> >> On 4/12/2023 6:43 PM, Andrii Nakryiko wrote:
> >>> On Wed, Apr 12, 2023 at 12:07 PM Paul Moore <paul at paul-moore.com> wrote:
> >>>> On Wed, Apr 12, 2023 at 2:28 PM Kees Cook <keescook at chromium.org> wrote:
> >>>>> On Wed, Apr 12, 2023 at 02:06:23PM -0400, Paul Moore wrote:
> >>>>>> On Wed, Apr 12, 2023 at 1:47 PM Kees Cook <keescook at chromium.org> wrote:
> >>>>>>> On Wed, Apr 12, 2023 at 12:49:06PM -0400, Paul Moore wrote:
> >>>>>>>> On Wed, Apr 12, 2023 at 12:33 AM Andrii Nakryiko <andrii at kernel.org> wrote:
> >>>>>>>>> Add new LSM hooks, bpf_map_create_security and bpf_btf_load_security, which
> >>>>>>>>> are meant to allow highly-granular LSM-based control over the usage of BPF
> >>>>>>>>> subsytem. Specifically, to control the creation of BPF maps and BTF data
> >>>>>>>>> objects, which are fundamental building blocks of any modern BPF application.
> >>>>>>>>>
> >>>>>>>>> These new hooks are able to override default kernel-side CAP_BPF-based (and
> >>>>>>>>> sometimes CAP_NET_ADMIN-based) permission checks. It is now possible to
> >>>>>>>>> implement LSM policies that could granularly enforce more restrictions on
> >>>>>>>>> a per-BPF map basis (beyond checking coarse CAP_BPF/CAP_NET_ADMIN
> >>>>>>>>> capabilities), but also, importantly, allow to *bypass kernel-side
> >>>>>>>>> enforcement* of CAP_BPF/CAP_NET_ADMIN checks for trusted applications and use
> >>>>>>>>> cases.
> >>>>>>>> One of the hallmarks of the LSM has always been that it is
> >>>>>>>> non-authoritative: it cannot unilaterally grant access, it can only
> >>>>>>>> restrict what would have been otherwise permitted on a traditional
> >>>>>>>> Linux system.  Put another way, a LSM should not undermine the Linux
> >>>>>>>> discretionary access controls, e.g. capabilities.
> >>>>>>>>
> >>>>>>>> If there is a problem with the eBPF capability-based access controls,
> >>>>>>>> that problem needs to be addressed in how the core eBPF code
> >>>>>>>> implements its capability checks, not by modifying the LSM mechanism
> >>>>>>>> to bypass these checks.
> >>>>>>> I think semantics matter here. I wouldn't view this as _bypassing_
> >>>>>>> capability enforcement: it's just more fine-grained access control.
> >>> Exactly. One of the motivations for this work was the need to move
> >>> some production use cases that are only needing extra privileges so
> >>> that they can use BPF into a more restrictive environment. Granting
> >>> CAP_BPF+CAP_PERFMON+CAP_NET_ADMIN to all such use cases that need them
> >>> for BPF usage is too coarse grained. These caps would allow those
> >>> applications way more than just BPF usage. So the idea here is more
> >>> finer-grained control of BPF-specific operations, granting *effective*
> >>> CAP_BPF+CAP_PERFMON+CAP_NET_ADMIN caps dynamically based on custom
> >>> production logic that would validate the use case.
> >> That's an authoritative model which is in direct conflict with the
> >> design and implementation of both capabilities and LSM.
> >>
> >>> This *is* an attempt to achieve a more secure production approach.
> >>>
> >>>>>>> For example, in many places we have things like:
> >>>>>>>
> >>>>>>>         if (!some_check(...) && !capable(...))
> >>>>>>>                 return -EPERM;
> >>>>>>>
> >>>>>>> I would expect this is a similar logic. An operation can succeed if the
> >>>>>>> access control requirement is met. The mismatch we have through-out the
> >>>>>>> kernel is that capability checks aren't strictly done by LSM hooks. And
> >>>>>>> this series conceptually, I think, doesn't violate that -- it's changing
> >>>>>>> the logic of the capability checks, not the LSM (i.e. there no LSM hooks
> >>>>>>> yet here).
> >>>>>> Patch 04/08 creates a new LSM hook, security_bpf_map_create(), which
> >>>>>> when it returns a positive value "bypasses kernel checks".  The patch
> >>>>>> isn't based on either Linus' tree or the LSM tree, I'm guessing it is
> >>>>>> based on a eBPF tree, so I can't say with 100% certainty that it is
> >>>>>> bypassing a capability check, but the description claims that to be
> >>>>>> the case.
> >>>>>>
> >>>>>> Regardless of how you want to spin this, I'm not supportive of a LSM
> >>>>>> hook which allows a LSM to bypass a capability check.  A LSM hook can
> >>>>>> be used to provide additional access control restrictions beyond a
> >>>>>> capability check, but a LSM hook should never be allowed to overrule
> >>>>>> an access denial due to a capability check.
> >>>>>>
> >>>>>>> The reason CAP_BPF was created was because there was nothing else that
> >>>>>>> would be fine-grained enough at the time.
> >>>>>> The LSM layer predates CAP_BPF, and one could make a very solid
> >>>>>> argument that one of the reasons LSMs exist is to provide
> >>>>>> supplementary controls due to capability-based access controls being a
> >>>>>> poor fit for many modern use cases.
> >>>>> I generally agree with what you say, but we DO have this code pattern:
> >>>>>
> >>>>>          if (!some_check(...) && !capable(...))
> >>>>>                  return -EPERM;
> >>>> I think we need to make this more concrete; we don't have a pattern in
> >>>> the upstream kernel where 'some_check(...)' is a LSM hook, right?
> >>>> Simply because there is another kernel access control mechanism which
> >>>> allows a capability check to be skipped doesn't mean I want to allow a
> >>>> LSM hook to be used to skip a capability check.
> >>> This work is an attempt to tighten the security of production systems
> >>> by allowing to drop too coarse-grained and permissive capabilities
> >>> (like CAP_BPF, CAP_PERFMON, CAP_NET_ADMIN, which inevitable allow more
> >>> than production use cases are meant to be able to do)
> >> The BPF developers are in complete control of what CAP_BPF controls.
> >> You can easily address the granularity issue by adding addition restrictions
> >> on processes that have CAP_BPF. That is the intended use of LSM.
> >> The whole point of having multiple capabilities is so that you can
> >> grant just those that are required by the system security policy, and
> >> do so safely. That leads to differences of opinion regarding the definition
> >> of the system security policy. BPF chose to set itself up as an element
> >> of security policy (you need CAP_BPF) rather than define elements such that
> >> existing capabilities (CAP_FOWNER, CAP_KILL, CAP_MAC_OVERRIDE, ...) would
> >> control.
> > Please see my reply to Paul, where I explain CAP_BPF's system-wide
> > nature and problem with user namespaces. I don't think the problem is
> > in the granularity of CAP_BPF, it's more of a "non-namespaceable"
> > nature of the BPF subsystem in general.
>
> Paul is approaching this from a different angle. Your response to Paul
> does not address the issue I have raised.

I see, I definitely missed this. Re-reading your reply, I still am not
clear on what you are proposing, tbh. Can you please elaborate what
you have in mind?

>
> >>>  and then grant
> >>> specific BPF operations on specific BPF programs/maps based on custom
> >>> LSM security policy,
> >> This is backwards. The correct implementation is to require CAP_BPF and
> >> further restrict BPF operations based on a custom LSM security policy.
> >> That's how LSM is designed.
> > Please see my reply to Paul, we can't grant real CAP_BPF for
> > applications in user namespace (unless there is some trick that I
> > don't know, so please do point it out). Let's converge the discussion
> > in that email thread branch to not discuss the same topic multiple
> > times.
>
> I saw your reply to Paul. Paul's points are not my points. If they where,
> I wouldn't have taken my or your time to present them.

Sure, sorry about that. What do you have in mind then?

>
> >>>  which validates application trustworthiness using
> >>> custom production-specific logic.
> >>>
> >>> Isn't this goal in line with LSMs mission to enhance system security?
> >> We're not arguing the goal, we're discussing the implementation.
> >>
> >>>>> It looks to me like this series can be refactored to do the same. I
> >>>>> wouldn't consider that to be a "bypass", but I would agree the current
> >>>>> series looks too much like "bypass", and makes reasoning about the
> >>>>> effect of the LSM hooks too "special". :)
> >>> Sorry, I didn't realize that the current code layout is making things
> >>> more confusing. I'll address feedback to make the intent a bit
> >>> clearer.
> >>>
> >>>> --
> >>>> paul-moore.com