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. > > > 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. > > > 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
