Re: [intel-sgx-kernel-dev] [PATCH v11 13/13] intel_sgx: in-kernel launch enclave

[Jethro Beekman <jethro@fortanix.com>]

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On 2018-06-20 09:28, Nathaniel McCallum wrote:
> As I understand it, the current policy models under discussion look like this:
> 
> 1. SGX w/o FLC (not being merged) looks like this:
>    Intel CPU => (Intel signed) launch enclave => enclaves

I think you mean:

      Intel CPU => kernel => (Intel signed) launch enclave => enclaves

> 
> 2. SGX w/ FLC, looks like this:
>    Intel CPU => kernel => launch enclave => enclaves
> 
> 3. Andy is proposing this:
>    Intel CPU => kernel => enclaves
> 
> This proposal is based on the fact that if the kernel can write to the
> MSRs then a kernel compromise allows an attacker to run their own
> launch enclave and therefore having an independent launch enclave adds
> only complexity but not security.
> 
> Is it possible to restrict the ability of the kernel to change the
> MSRs? For example, could a UEFI module manage the MSRs? Could the
> launch enclave live entirely within that UEFI module?

On 2017-03-17 09:15, Jethro Beekman wrote:
 > While collecting my thoughts about the issue I read through the
 > documentation again and it seems that it will not be possible for a
 > platform to lock in a non-Intel hash at all. From Section 39.1.4 of the
 > latest Intel SDM:
 >
 >  > IA32_SGXLEPUBKEYHASH defaults to digest of Intel’s launch enclave
 >  > signing key after reset.
 >  >
 >  > IA32_FEATURE_CONTROL bit 17 controls the permissions on the
 >  > IA32_SGXLEPUBKEYHASH MSRs when CPUID.(EAX=12H, ECX=00H):EAX[0] = 1.
 >  > If IA32_FEATURE_CONTROL is locked with bit 17 set,
 >  > IA32_SGXLEPUBKEYHASH MSRs are reconfigurable (writeable). If either
 >  > IA32_FEATURE_CONTROL is not locked or bit 17 is clear, the MSRs are
 >  > read only.
 >
 > This last bit is also repeated in different words in Table 35-2 and
 > Section 42.2.2. The MSRs are *not writable* before the write-lock bit
 > itself is locked. Meaning the MSRs are either locked with Intel's key
 > hash, or not locked at all.

> 
> 4. I am suggesting this:
>    Intel CPU => UEFI module => enclaves
> 
> Under this architecture, the kernel isn't involved in policy at all
> and users get exactly the same freedoms they already have with Secure
> Boot. Further, the launch enclave can be independently updated and
> could be distributed in linux-firmware. The UEFI module can also be
> shared across operating systems. If I want to have my own enclave
> policy, then I can build the UEFI module myself, with my
> modifications, and I can disable Secure Boot. Alternatively,
> distributions that want to set their own policies can build their own
> UEFI module and sign it with their vendor key.

Jethro Beekman | Fortanix


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