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

[Sean Christopherson]

On Thu, Jun 21, 2018 at 03:11:18PM -0400, Nathaniel McCallum wrote:
> If this is acceptable for everyone, my hope is the following:
> 
> 1. Intel would split the existing code into one of the following
> schemas (I don't care which):
>   A. three parts: UEFI module, FLC-only kernel driver and user-space
> launch enclave
>   B. two parts: UEFI module (including launch enclave) and FLC-only
> kernel driver

To make sure I understand correctly...

The UEFI module would lock the LE MSRs with a public key hardcoded
into both the UEFI module and the kernel at build time?

And for the kernel, it would only load its SGX driver if FLC is
supported and the MSRs are locked to the expected key?

IIUC, this approach will cause problems for virtualization.  Running
VMs with different LE keys would require the bare metal firmware to
configure the LE MSRs to be unlocked, which would effectively make
using SGX in the host OS mutually exlusive with exposing SGX to KVM
guests.  Theoretically it would be possible for KVM to emulate the
guest's LE and use the host's LE to generate EINIT tokens, but
emulating an enclave would likely require a massive amount of code
and/or complexity.

> 2. Intel would release a reproducible build of the GPL UEFI module
> sources signed with a SecureBoot trusted key and provide an
> acceptable[0] binary redistribution license.
> 
> 3. The kernel community would agree to merge the kernel driver given
> the above criteria (and, obviously, acceptable kernel code).
> 
> The question of how to distribute the UEFI module and possible launch
> enclave remains open. I see two options: independent distribution and
> bundling it in linux-firmware. The former may be a better
> technological fit since the UEFI module will likely need to be run
> before the kernel (and the boot loader; and shim). However, the latter
> has the benefit of already being a well-known entity to our downstream
> distributors. I could go either way on this.

Writing and locks the LE MSRs effectively needs to be done before
running the bootloader/kernel/etc...  Delaying activation would
require, at a minimum, leaving IA32_FEATURE_CONTROL unlocked since
IA32_FEATURE_CONTROL's SGX bits can't be set until SGX is activated.

> I know this plan is more work for everyone involved, but I think it
> manages to actually maximize both security and freedom.
> 
> [0]: details here -
> https://git.kernel.org/pub/scm/linux/kernel/git/firmware/linux-firmware.git/tree/README#n19
> On Thu, Jun 21, 2018 at 11:29 AM Neil Horman <nhorman@xxxxxxxxxx> wrote:
> >
> > On Thu, Jun 21, 2018 at 08:32:25AM -0400, Nathaniel McCallum wrote:
> > > On Wed, Jun 20, 2018 at 5:02 PM Sean Christopherson
> > > <sean.j.christopherson@xxxxxxxxx> wrote:
> > > >
> > > > On Wed, Jun 20, 2018 at 11:39:00AM -0700, Jethro Beekman wrote:
> > > > > On 2018-06-20 11:16, Jethro Beekman wrote:
> > > > > > > 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.
> > > > >
> > > > > Actually, this might be a documentation bug. I have some test hardware and I
> > > > > was able to configure the MSRs in the BIOS and then read the MSRs after boot
> > > > > like this:
> > > > >
> > > > > MSR 0x3a 0x0000000000040005
> > > > > MSR 0x8c 0x20180620aaaaaaaa
> > > > > MSR 0x8d 0x20180620bbbbbbbb
> > > > > MSR 0x8e 0x20180620cccccccc
> > > > > MSR 0x8f 0x20180620dddddddd
> > > > >
> > > > > Since this is not production hardware, it could also be a CPU bug of course.
> > > > >
> > > > > If it is indeed possible to configure AND lock the MSR values to non-Intel
> > > > > values, I'm very much in favor of Nathaniels proposal to treat the launch
> > > > > enclave like any other firmware blob.
> > > >
> > > > It's not a CPU or documentation bug (though the latter is arguable).
> > > > SGX has an activation step that is triggered by doing a WRMSR(0x7a)
> > > > with bit 0 set.  Until SGX is activated, the SGX related bits in
> > > > IA32_FEATURE_CONTROL cannot be set, i.e. SGX can't be enabled.  But,
> > > > the LE hash MSRs are fully writable prior to activation, e.g. to
> > > > allow firmware to lock down the LE key with a non-Intel value.
> > > >
> > > > So yes, it's possible to lock the MSRs to a non-Intel value.  The
> > > > obvious caveat is that whatever blob is used to write the MSRs would
> > > > need be executed prior to activation.
> > >
> > > This implies that it should be possible to create MSR activation (and
> > > an embedded launch enclave?) entirely as a UEFI module. The kernel
> > > would still get to manage who has access to /dev/sgx and other
> > > important non-cryptographic policy details. Users would still be able
> > > to control the cryptographic policy details (via BIOS Secure Boot
> > > configuration that exists today). Distributions could still control
> > > cryptographic policy details via signing of the UEFI module with their
> > > own Secure Boot key (or using something like shim). The UEFI module
> > > (and possibly the external launch enclave) could be distributed via
> > > linux-firmware.
> > >
> > > Andy/Neil, does this work for you?
> > >
> > I need some time to digest it.  Who in your mind is writing the UEFI module.  Is
> > that the firmware vendor or IHV?
> >
> > Neil
> >
> > > > As for the SDM, it's a documentation... omission?  SGX activation
> > > > is intentionally omitted from the SDM.  The intended usage model is
> > > > that firmware will always do the activation (if it wants SGX enabled),
> > > > i.e. post-firmware software will only ever "see" SGX as disabled or
> > > > in the fully activated state, and so the SDM doesn't describe SGX
> > > > behavior prior to activation.  I believe the activation process, or
> > > > at least what is required from firmware, is documented in the BIOS
> > > > writer's guide.
> > > >
> > > > > Jethro Beekman | Fortanix
> > > > >
> > > >
> > > >