[PATCH bpf-next v1 00/13] MAC and Audit policy using eBPF (KRSI)

[Stephen Smalley]

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 at namei.org>
>>