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

[Andy Lutomirski <luto@kernel.org>]

On Fri, Jun 14, 2019 at 10:16 AM Xing, Cedric <cedric.xing@intel.com> wrote:
>
> > 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@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?
>

I think it just works.  Either you can't mmap() the page until you
have explicitly EAUG-ed it, or you add a new ioctl() that is
effectively "EAUG lazily".  The latter would declare that address and
request that it get allocated and EAUGed when faulted, but it wouldn't
actually do the EAUG.

--Andy