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

From: Sean Christopherson <sean.j.christopherson@intel.com>
To: Stephen Smalley <sds@tycho.nsa.gov>
Cc: "Xing, Cedric" <cedric.xing@intel.com>,
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	<linux-security-module@vger.kernel.org>,
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	"Roberts, William C" <william.c.roberts@intel.com>,
	"Tricca, Philip B" <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 12:48:33 -0700	[thread overview]
Message-ID: <20190613194833.GB18385@linux.intel.com> (raw)
In-Reply-To: <b7f3decf-b1d2-01b1-6294-ccf9ba2d5df4@tycho.nsa.gov>

On Thu, Jun 13, 2019 at 02:00:29PM -0400, Stephen Smalley wrote:
> On 6/11/19 6:55 PM, Xing, Cedric wrote:
> >You are right that there are SGX specific stuff. More precisely, SGX
> >enclaves don't have access to anything except memory, so there are only 3
> >questions that need to be answered for each enclave page: 1) whether X is
> >allowed; 2) whether W->X is allowed and 3 whether WX is allowed. This
> >proposal tries to cache the answers to those questions upon creation of each
> >enclave page, meaning it involves a) figuring out the answers and b)
> >"remember" them for every page. #b is generic, mostly captured in
> >intel_sgx.c, and could be shared among all LSM modules; while #a is SELinux
> >specific. I could move intel_sgx.c up one level in the directory hierarchy
> >if that's what you'd suggest.
> >
> >By "SGX", did you mean the SGX subsystem being upstreamed? It doesn’t track
> >that state. In practice, there's no way for SGX to track it because there's
> >no vm_ops->may_mprotect() callback. It doesn't follow the philosophy of
> >Linux either, as mprotect() doesn't track it for regular memory. And it
> >doesn't have a use without LSM, so I believe it makes more sense to track it
> >inside LSM.
> 
> Yes, the SGX driver/subsystem.  I had the impression from Sean that it does
> track this kind of per-page state already in some manner, but possibly he
> means it does under a given proposal and not in the current driver.

Yeah, under a given proposal.  SGX has per-page tracking, adding new flags
is fairly easy.  Philosophical objections aside, adding .may_mprotect() is
trivial.

> Even the #b remembering might end up being SELinux-specific if we also have
> to remember the original inputs used to compute the answer so that we can
> audit that information when access is denied later upon mprotect().  At the
> least we'd need it to save some opaque data and pass it to a callback into
> SELinux to perform that auditing.