Message-ID: <8d99d8fb-a921-286a-8cf0-cd522e09b37c@tycho.nsa.gov>
Date:   Thu, 13 Jun 2019 13:02:17 -0400
From:   Stephen Smalley <sds@...ho.nsa.gov>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
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 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.christopherson@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?

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