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

[Jarkko Sakkinen]

On Mon, Jun 29, 2020 at 03:04:00PM -0700, Sean Christopherson wrote:
> 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

Section 3 in [*] is what describes the infrastructure. DCAP is only a
component in the whole attestation infrastructure.

[*] https://software.intel.com/sites/default/files/managed/f1/b8/intel-sgx-support-for-third-party-attestation.pdf

/Jarkko