Messages in this thread | | | Subject | Re: [PATCH bpf-next v1 00/13] MAC and Audit policy using eBPF (KRSI) | From | Stephen Smalley <> | Date | Thu, 9 Jan 2020 14:47:53 -0500 |
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On 1/9/20 2:43 PM, KP Singh wrote: > On 10-Jan 06:07, James Morris wrote: >> On Thu, 9 Jan 2020, Stephen Smalley wrote: >> >>> On 1/9/20 1:11 PM, James Morris wrote: >>>> On Wed, 8 Jan 2020, Stephen Smalley wrote: >>>> >>>>> The cover letter subject line and the Kconfig help text refer to it as a >>>>> BPF-based "MAC and Audit policy". It has an enforce config option that >>>>> enables the bpf programs to deny access, providing access control. IIRC, >>>>> in >>>>> the earlier discussion threads, the BPF maintainers suggested that Smack >>>>> and >>>>> other LSMs could be entirely re-implemented via it in the future, and that >>>>> such an implementation would be more optimal. >>>> >>>> In this case, the eBPF code is similar to a kernel module, rather than a >>>> loadable policy file. It's a loadable mechanism, rather than a policy, in >>>> my view. >>> >>> I thought you frowned on dynamically loadable LSMs for both security and >>> correctness reasons? > > Based on the feedback from the lists we've updated the design for v2. > > In v2, LSM hook callbacks are allocated dynamically using BPF > trampolines, appended to a separate security_hook_heads and run > only after the statically allocated hooks. > > The security_hook_heads for all the other LSMs (SELinux, AppArmor etc) > still remains __lsm_ro_after_init and cannot be modified. We are still > working on v2 (not ready for review yet) but the general idea can be > seen here: > > https://github.com/sinkap/linux-krsi/blob/patch/v1/trampoline_prototype/security/bpf/lsm.c > >> >> Evaluating the security impact of this is the next step. My understanding >> is that eBPF via BTF is constrained to read only access to hook >> parameters, and that its behavior would be entirely restrictive. >> >> I'd like to understand the security impact more fully, though. Can the >> eBPF code make arbitrary writes to the kernel, or read anything other than >> the correctly bounded LSM hook parameters? >> > > As mentioned, the BPF verifier does not allow writes to BTF types. > >>> And a traditional security module would necessarily fall >>> under GPL; is the eBPF code required to be likewise? If not, KRSI is a >>> gateway for proprietary LSMs... >> >> Right, we do not want this to be a GPL bypass. > > This is not intended to be a GPL bypass and the BPF verifier checks > for license compatibility of the loaded program with GPL.
IIUC, it checks that the program is GPL compatible if it uses a function marked GPL-only. But what specifically is marked GPL-only that is required for eBPF programs using KRSI?
> > - KP > >> >> If these issues can be resolved, this may be a "safe" way to support >> loadable LSM applications. >> >> Again, I'd be interested in knowing how this is is handled in the >> networking stack (keeping in mind that LSM is a much more invasive API, >> and may not be directly comparable). >> >> -- >> James Morris >> <jmorris@namei.org> >>
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