Re: [RFC PATCH v1 2/3] LSM/x86/sgx: Implement SGX specific hooks in SELinux

[Stephen Smalley]

On 6/11/19 6:02 PM, Sean Christopherson wrote:
> On Tue, Jun 11, 2019 at 09:40:25AM -0400, Stephen Smalley wrote:
> > I haven't looked at this code closely, but it feels like a lot of
> > SGX-specific logic embedded into SELinux that will have to be repeated or
> > reused for every security module.  Does SGX not track this state itself?
>
> SGX does track equivalent state.
>
> There are three proposals on the table (I think):
>
>    1. Require userspace to explicitly specificy (maximal) enclave page
>       permissions at build time.  The enclave page permissions are provided
>       to, and checked by, LSMs at enclave build time.
>
>       Pros: Low-complexity kernel implementation, straightforward auditing
>       Cons: Sullies the SGX UAPI to some extent, may increase complexity of
>             SGX2 enclave loaders.
>
>    2. Pre-check LSM permissions and dynamically track mappings to enclave
>       pages, e.g. add an SGX mprotect() hook to restrict W->X and WX
>       based on the pre-checked permissions.
>
>       Pros: Does not impact SGX UAPI, medium kernel complexity
>       Cons: Auditing is complex/weird, requires taking enclave-specific
>             lock during mprotect() to query/update tracking.
>
>    3. Implement LSM hooks in SGX to allow LSMs to track enclave regions
>       from cradle to grave, but otherwise defer everything to LSMs.
>
>       Pros: Does not impact SGX UAPI, maximum flexibility, precise auditing
>       Cons: Most complex and "heaviest" kernel implementation of the three,
>             pushes more SGX details into LSMs.
>
> My RFC series[1] implements #1.  My understanding is that Andy (Lutomirski)
> prefers #2.  Cedric's RFC series implements #3.
>
> Perhaps the easiest way to make forward progress is to rule out the
> options we absolutely *don't* want by focusing on the potentially blocking
> issue with each option:
>
>    #1 - SGX UAPI funkiness
>
>    #2 - Auditing complexity, potential enclave lock contention
>
>    #3 - Pushing SGX details into LSMs and complexity of kernel implementation
>
>
> [1] https://lkml.kernel.org/r/20190606021145.12604-1-sean.j.christopherson@xxxxxxxxx

Given the complexity tradeoff, what is the clear motivating example for 
why #1 isn't the obvious choice? That the enclave loader has no way of 
knowing a priori whether the enclave will require W->X or WX?  But 
aren't we better off requiring enclaves to be explicitly marked as 
needing such so that we can make a more informed decision about whether 
to load them in the first place?