linux-kernel - Re: [PATCH v33 12/21] x86/sgx: Allow a limited use of ATTRIBUTE.PROVISIONKEY for attestation

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Date:   Tue, 30 Jun 2020 07:20:28 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>, x86@...nel.org,
        linux-sgx@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        Jethro Beekman <jethro@...tanix.com>,
        Andy Lutomirski <luto@...nel.org>, akpm@...ux-foundation.org,
        andriy.shevchenko@...ux.intel.com, asapek@...gle.com,
        cedric.xing@...el.com, chenalexchen@...gle.com,
        conradparker@...gle.com, cyhanish@...gle.com,
        dave.hansen@...el.com, haitao.huang@...el.com,
        josh@...htriplett.org, kai.huang@...el.com, kai.svahn@...el.com,
        kmoy@...gle.com, ludloff@...gle.com, nhorman@...hat.com,
        npmccallum@...hat.com, puiterwijk@...hat.com, rientjes@...gle.com,
        tglx@...utronix.de, yaozhangx@...gle.com
Subject: Re: [PATCH v33 12/21] x86/sgx: Allow a limited use of
 ATTRIBUTE.PROVISIONKEY for attestation

On Tue, Jun 30, 2020 at 10:49:56AM +0200, Borislav Petkov wrote:
> On Mon, Jun 29, 2020 at 03:04:00PM -0700, Sean Christopherson wrote:
> > /dev/sgx/provision is root-only by default, the expectation is that the admin
> > will configure the system to grant only specific enclaves access to the
> > PROVISION_KEY.
> 
> Uuh, I don't like "the expectation is" - the reality happens to turn
> differently, more often than not.

Would it help if I worded it as "only root should ever be able to run an
enclave with access to PROVISION_KEY"?  We obviously can't control what
admins actually do, hence my wording of it as the expected behavior.

> > In this series, access is fairly binary, i.e. there's no additional kernel
> > infrastructure to help userspace make per-enclave decisions.  There have been
> > more than a few proposals on how to extend the kernel to help provide better
> > granularity, e.g. LSM hooks, but it was generally agreed to punt that stuff
> > to post-upstreaming to keep things "simple" once we went far enough down
> > various paths to ensure we weren't painting ourselves into a corner.
> 
> So this all sounds to me like we should not upstream /dev/sgx/provision
> now but delay it until the infrastructure for that has been made more
> concrete. We can always add it then. Changing it after the fact -
> if we have to and for whatever reason - would be a lot harder for a
> user-visible interface which someone has started using already.

The userspace and attestation infrastructure is very concrete, i.e. the
need for userspace to be able to access PROVISION_KEY is there, as is the
desire to be able to restrict access to PROVISION_KEY, e.g. I believe Andy
Lutomirski originally requested the ability to restrict access.

The additional infrastructure for per-enclave decisions is somewhat
orthogonal to the PROVISION_KEY, e.g. they won't necessarily be employed
by everyone running enclaves, and environments that do have per-enclave
policies would still likely want the extra layer of restriction for
PROVISION_KEY.  I only brought the additional policy crud to call out that
we've done enough path-finding on additional restrictions to have strong
confidence that adding /dev/sgx/provision won't prevent us from adding more
fine grained control in the future.

> So I'd leave  that out from the initial patchset.