> From: Christopherson, Sean J > Sent: Thursday, June 13, 2019 5:46 PM > > 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.christopherso > > >n@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... How does enclave loader provide per-page ALLOW_* flags? And a related question is why they are necessary for enclaves but unnecessary for regular executables or shared objects. What's the story for SGX2 if mmap()'ing non-existing pages is not allowed? What's the story for auditing? After everything above has been taken care of properly, will #1 still be simpler than #2/#3?
