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

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Date:   Tue, 11 Jun 2019 15:02:43 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Stephen Smalley <sds@...ho.nsa.gov>
Cc:     Cedric Xing <cedric.xing@...el.com>,
        linux-security-module@...r.kernel.org, selinux@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-sgx@...r.kernel.org,
        jarkko.sakkinen@...ux.intel.com, luto@...nel.org,
        jmorris@...ei.org, serge@...lyn.com, paul@...l-moore.com,
        eparis@...isplace.org, jethro@...tanix.com, dave.hansen@...el.com,
        tglx@...utronix.de, torvalds@...ux-foundation.org,
        akpm@...ux-foundation.org, nhorman@...hat.com,
        pmccallum@...hat.com, serge.ayoun@...el.com,
        shay.katz-zamir@...el.com, haitao.huang@...el.com,
        andriy.shevchenko@...ux.intel.com, kai.svahn@...el.com,
        bp@...en8.de, josh@...htriplett.org, kai.huang@...el.com,
        rientjes@...gle.com, william.c.roberts@...el.com,
        philip.b.tricca@...el.com
Subject: Re: [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