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

[Sean Christopherson]

On Mon, Jun 29, 2020 at 06:02:42PM +0200, Borislav Petkov wrote:
> On Thu, Jun 18, 2020 at 01:08:34AM +0300, Jarkko Sakkinen wrote:
> > Provisioning Certification Enclave (PCE), the root of trust for other
> > enclaves, generates a signing key from a fused key called Provisioning
> > Certification Key. PCE can then use this key to certify an attestation key
> > of a QE, e.g. we get the chain of trust down to the hardware if the Intel
> 
> What's a QE?
> 
> I don't see this acronym resolved anywhere in the whole patchset.

Quoting Enclave.

> > signed PCE is used.
> > 
> > To use the needed keys, ATTRIBUTE.PROVISIONKEY is required but should be
> > only allowed for those who actually need it so that only the trusted
> > parties can certify QE's.
> > 
> > Obviously the attestation service should know the public key of the used
> > PCE and that way detect illegit attestation, but whitelisting the legit
> > users still adds an additional layer of defence.
> > 
> > Add new device file called /dev/sgx/provision. The sole purpose of this
> > file is to provide file descriptors that act as privilege tokens to allow
> > to build enclaves with ATTRIBUTE.PROVISIONKEY set. A new ioctl called
> > SGX_IOC_ENCLAVE_SET_ATTRIBUTE is used to assign this token to an enclave.
> 
> So I'm sure I'm missing something here: what controls which
> enclave can open /dev/sgx/provision and thus pass the FD to
> SGX_IOC_ENCLAVE_SET_ATTRIBUTE?

/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.

> And in general, how does that whole flow look like: what calls
> SGX_IOC_ENCLAVE_SET_ATTRIBUTE when?

The basic gist is that the host process of an enclave that needs/wants access
to the PROVISION_KEY will invoke SGX_IOC_ENCLAVE_SET_ATTRIBUTE when building
the enclave.  Any enclave can request access to PROVISION_KEY, but practically
speaking only the PCE and QE (or their non-Intel equivalents) actually need
access to the key.  KVM (future series) will also respect /dev/sgx/provision,
i.e. require a similar ioctl() to expose the PROVISION_KEY to a guest.

E.g. for my own personal testing, I never do anything attestation related, so
none of the enclaves I run request PROVISION_KEY, but I do expose it to VMs to
test the KVM paths.

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.

If you want super gory details, Intel's whitepaper on attestation in cloud
environments is a good starting point[*], but I don't recommended doing much
more than skimming unless you really like attestation stuff or are
masochistic, which IMO amount to the same thing :-)

[*] https://download.01.org/intel-sgx/dcap-1.0/docs/SGX_ECDSA_QuoteGenReference_DCAP_API_Linux_1.0.pdf