linux-kernel - Re: [intel-sgx-kernel-dev] [PATCH v11 13/13] intel

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Message-ID: <20180621152903.GB1324@hmswarspite.think-freely.org>
Date:   Thu, 21 Jun 2018 11:29:03 -0400
From:   Neil Horman <nhorman@...hat.com>
To:     Nathaniel McCallum <npmccallum@...hat.com>
Cc:     sean.j.christopherson@...el.com, jethro@...tanix.com,
        luto@...nel.org, jarkko.sakkinen@...ux.intel.com, x86@...nel.org,
        platform-driver-x86@...r.kernel.org, linux-kernel@...r.kernel.org,
        mingo@...hat.com, intel-sgx-kernel-dev@...ts.01.org, hpa@...or.com,
        dvhart@...radead.org, tglx@...utronix.de, andy@...radead.org,
        Peter Jones <pjones@...hat.com>
Subject: Re: [intel-sgx-kernel-dev] [PATCH v11 13/13] intel_sgx: in-kernel
 launch enclave

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@...el.com> 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
> > >
> >
> >