On 4/14/22 2:46 PM, Tyler Fanelli wrote:
On 4/11/22 10:57 AM, Cole Robinson wrote:
On 3/23/22 3:36 PM, Tyler Fanelli wrote:
This an RFC discussing a new API, virDomainGetSevAttestationReport (along with a virsh command "domgetsevreport"), with initial QEMU support via the "query-sev-attestation-report" QAPI mechanism. "query-sev-attestation-report" is supplied a base64-encoded 16 byte "mnonce" string as input, with a purpose of being embedded into the attestation report to provide protection.
My main point of concern is the design/communication of the virTypedParameterPtr exchanged between the client and libvirtd and how they interact together, as I have seen no other API follow the method I used. Namely, the same virTypedParameterPtr is used for both input _AND_ output. The same virTypedParameterPtr containing the original mnonce string inputted to the API is also used to contain the attestation report upon being returned from the API.
This contrasts with much of the APIs I've noticed, which use a virTypedParameterPtr for either input or output, but not both.
This patch is not final, as I still would like some human-readable outputting and storage of the attestation report.
Looking for thoughts on the design of this API, as well as suggested improvements. The proposed API name contains Sev, when all the existing domain APIs have generic names: virDomainGetLaunchSecurityInfo, virDomainSetLaunchSecurityState.
I was thinking it would be nice to avoid that Sev specific bit. So I looked at upcoming SNP and TDX qemu patches to see if they add any qmp commands that take input and return a lot of output. Then maybe it would make sense to name this something like virDomainGetLaunchSecurityInfoWithParams which we could use more in the future.
qemu SNP patches: https://github.com/AMDESE/qemu/tree/snp-v3 - Only extend existing query-sev and query-sev-capabilities
qemu TDX patches: https://github.com/intel/qemu-tdx - Adds query-tdx and query-tdx-capabilities, both with no input
So, that doesn't help.
What about adding the attestation report to virDomainGetLaunchSecurityInfo? If that route is taken, there would probably need to be some extra logic added to decipher getting launch security info on SEV vs. SGX/TDX, right?
The problem is virDomainGetLaunchSecurityInfo doesn't have any way to pass in the nonce, so we can't reuse that API as is.
After that, my question is, what query-sev-attestation-report adds. The kernel commit explains what it is: https://github.com/torvalds/linux/commit/2c07ded06427dd3339278487a1413d5e478...
The SEV FW version >= 0.23 added a new command that can be used to query the attestation report containing the SHA-256 digest of the guest memory encrypted through the KVM_SEV_LAUNCH_UPDATE_{DATA, VMSA} commands and sign the report with the Platform Endorsement Key (PEK). See the SEV FW API spec section 6.8 for more details. Note there already exist a command (KVM_SEV_LAUNCH_MEASURE) that can be used to get the SHA-256 digest. The main difference between the KVM_SEV_LAUNCH_MEASURE and KVM_SEV_ATTESTATION_REPORT is that the latter can be called while the guest is running and the measurement value is signed with PEK. So it is LAUNCH_MEASURE, available at VM runtime, signed with an extra key.
And with a nonce passed in, I forgot to mention that. That's another bit I'm not sure what it adds over the traditional measurement
qemu caches the LAUNCH_MEASURE value at VM startup and lets us query it with qmp any time, so I don't think runtime access matters.
I'm surprised by this. I was under the impression that LAUNCH_MEASURE always had to be called when a VM is paused.
Maybe the extra key signing is a security fix or something. I haven't figured it out.
Signing with the PEK also allows a user to verify the root of trust between the owner and the platform.
But I don't understand what that means in practice. I figured key management via the session blob already took care of this, but I haven't tried to wrap my head around the details.
But as is it's not clear what this buys us over the launch measurement we already report with virDomainGetLaunchSecurityInfo
If we figure out what the point of this is, IMO we can more easily reason about whether it makes sense to add a Sev specific libvirt API, and whether we need virTypedParams for both input and output. For example if the API really is specific to this one and only KVM ioctl/QMP command, we could hardcode the parameters and skip the virTypedParams question entirely.
Interesting, although wouldn't hardcoding an nonce basically render it useless? User-specified nonce would allow a user to verify that their call was propagated to firmware at that instance. If they can't supply the nonce, they can't verify it's an attestation report from that specific call.
Sorry if I was unclear, I didn't mean hardcoding a specific nonce value. I meant that, if we decide we there's not much value in making this API generic, then we can strip out the virTypedParams usage and make the signature closer to: int virDomainGetSevAttestationReport(virDomainPtr domain, const char *mnonce, char *report); Thanks, Cole