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Date:   Tue, 11 Jun 2019 15:02:43 -0700
From:   Sean Christopherson <sean.j.christopherson@intel.com>
To:     Stephen Smalley <sds@tycho.nsa.gov>
Cc:     Cedric Xing <cedric.xing@intel.com>,
        linux-security-module@vger.kernel.org, selinux@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-sgx@vger.kernel.org,
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        philip.b.tricca@intel.com
Subject: Re: [RFC PATCH v1 2/3] LSM/x86/sgx: Implement SGX specific hooks in
 SELinux
Message-ID: <20190611220243.GB3416@linux.intel.com>
References: <cover.1560131039.git.cedric.xing@intel.com>
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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