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

From: Stephen Smalley <sds@tycho.nsa.gov>
To: Sean Christopherson <sean.j.christopherson@intel.com>
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,
	jarkko.sakkinen@linux.intel.com, luto@kernel.org,
	jmorris@namei.org, serge@hallyn.com, paul@paul-moore.com,
	eparis@parisplace.org, jethro@fortanix.com,
	dave.hansen@intel.com, tglx@linutronix.de,
	torvalds@linux-foundation.org, akpm@linux-foundation.org,
	nhorman@redhat.com, pmccallum@redhat.com, serge.ayoun@intel.com,
	shay.katz-zamir@intel.com, haitao.huang@intel.com,
	andriy.shevchenko@linux.intel.com, kai.svahn@intel.com,
	bp@alien8.de, josh@joshtriplett.org, kai.huang@intel.com,
	rientjes@google.com, william.c.roberts@intel.com,
	philip.b.tricca@intel.com
Subject: Re: [RFC PATCH v1 2/3] LSM/x86/sgx: Implement SGX specific hooks in SELinux
Date: Thu, 13 Jun 2019 13:02:17 -0400	[thread overview]
Message-ID: <8d99d8fb-a921-286a-8cf0-cd522e09b37c@tycho.nsa.gov> (raw)
In-Reply-To: <20190611220243.GB3416@linux.intel.com>

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?