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

[Sean Christopherson]

On Thu, Jun 13, 2019 at 01:02:17PM -0400, Stephen Smalley wrote:
> 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@intel.com
> 
> 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?

Andy and/or Cedric, can you please weigh in with a concrete (and practical)
use case that will break if we go with #1?  The auditing issues for #2/#3
are complex to say the least...