On Fri, Apr 26, 2024, Michael Roth wrote: > On Wed, Apr 24, 2024 at 05:15:40PM -0700, Sean Christopherson wrote: > > On Sun, Apr 21, 2024, Michael Roth wrote: > > > These commands can be used to pause servicing of guest attestation > > > requests. This useful when updating the reported TCB or signing key with > > > commands such as SNP_SET_CONFIG/SNP_COMMIT/SNP_VLEK_LOAD, since they may > > > in turn require updates to userspace-supplied certificates, and if an > > > attestation request happens to be in-flight at the time those updates > > > are occurring there is potential for a guest to receive a certificate > > > blob that is out of sync with the effective signing key for the > > > attestation report. > > > > > > These interfaces also provide some versatility with how similar > > > firmware/certificate update activities can be handled in the future. > > > > Wait, IIUC, this is using the kernel to get two userspace components to not > > stomp over each other. Why is this the kernel's problem to solve? > > It's not that they are stepping on each other, but that kernel and > userspace need to coordinate on updating 2 components whose updates need > to be atomic from a guest perspective. Take an update to VLEK key for > instance: > > 1) management gets a new VLEK endorsement key from KDS along with What is "management"? I assume its some userspace daemon? > associated certificate chain > 2) management uses SNP_VLEK_LOAD to update key > 3) management updates the certs at the path VMM will grab them > from when the EXT_GUEST_REQUEST userspace exit is issued > > If an attestation request comes in after 2), but before 3), then the > guest sees an attestation report signed with the new key, but still > gets the old certificate. > > If you reverse the ordering: > > 1) management gets a new VLEK endorsement key from KDS along with > associated certificate chain > 2) management updates the certs at the path VMM will grab them > from when the EXT_GUEST_REQUEST userspace exit is issued > 3) management uses SNP_VLEK_LOAD to update key > > then an attestation request between 2) and 3) will result in the guest > getting the new cert, but getting an attestation report signed with an old > endorsement key. > > Providing a way to pause guest attestation requests prior to 2), and > resume after 3), provides a straightforward way to make those updates > atomic to the guest. Assuming "management" is a userspace component, I still don't see why this requires kernel involvement. "management" can tell VMMs to pause attestation without having to bounce through the kernel. It doesn't even require a push model, e.g. wrap/redirect the certs with a file that has a "pause" flag and a sequence counter.
