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@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? 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...
