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

[Casey Schaufler <casey@schaufler-ca.com>]

On 4/17/2023 4:31 PM, Andrii Nakryiko wrote:
> On Thu, Apr 13, 2023 at 9:27 AM Casey Schaufler <casey@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@paul-moore.com> wrote:
>>>> On Wed, Apr 12, 2023 at 2:28 PM Kees Cook <keescook@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@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@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.

>>>  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.

>>>  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