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

[Xing, Cedric]

> 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 at 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...

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?