Re: [PATCH v5 11/11] intel_sgx: driver documentation

From: Jarkko Sakkinen
Date: Tue Nov 21 2017 - 07:39:03 EST

On Tue, Nov 21, 2017 at 12:46:23AM +0200, Jarkko Sakkinen wrote:
> On Wed, Nov 15, 2017 at 12:54:12PM +0100, Peter Zijlstra wrote:
> > On Tue, Nov 14, 2017 at 10:49:48PM +0200, Jarkko Sakkinen wrote:
> > > In these cases IA32_FEATURE_CONTROL[17] would be zeroed before locking
> > > the feature control, which would mean that the kernel could not write
> > > new values with wrmsr for the root key hash.
> >
> > > The question is whether we want to allow this or not. If the answer is
> > > no, a check can be added to the driver initialization code whether 17 is
> > > set, and if not, it driver would fail to initialize.
> >
> > That has my vote; I would not trust a firmware/BIOS key.
> Please note that it does not have a key to look inside the enclave. The
> enclave is protected by two means:
> 1. The CPU asserts the memory accesses to it.
> 2. The CPU encrypts/decrypts in L1 in order to protect from physical
> attacks and peripherals that have potential spy the bus.

The encryption key is generated for every boot cycle and it is *never*
leaked out of the CPU package.

After thinking what you and Borislav said I position myself to the point
of view that even if the MSRs would be read-only the kernel could allow
running enclaves *with a condition*.

I propose adding an additional check to the driver initialization:

Try to start LE. If it doesn't start i.e. is signed with a different
root key than the one inside MSRs, then fail the initialization.