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. > > 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. -- paul-moore.com
