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@intel.com