On 4/17/2023 4:31 PM, Andrii Nakryiko wrote: > On Thu, Apr 13, 2023 at 9:27 AM Casey Schaufler <casey@xxxxxxxxxxxxxxxx> wrote: >> On 4/12/2023 6:43 PM, Andrii Nakryiko wrote: >>> On Wed, Apr 12, 2023 at 12:07 PM Paul Moore <paul@xxxxxxxxxxxxxx> wrote: >>>> On Wed, Apr 12, 2023 at 2:28 PM Kees Cook <keescook@xxxxxxxxxxxx> 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@xxxxxxxxxxxx> 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@xxxxxxxxxx> 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
