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

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