Re: adding 'official' way to dump SEV VMSA

[Dr. David Alan Gilbert]

* Dov Murik (dovmurik@linux.ibm.com) wrote:
> Hi Cole,
> 
> On 13/04/2022 16:36, Cole Robinson wrote:
> > Hi all,
> > 
> > SEV-ES and SEV-SNP attestation require a copy of the initial VMSA to
> > validate the launch measurement. For developers dipping their toe into
> > SEV-* work, the easiest way to get sample VMSA data for their machine is
> > to grab it from a running VM.
> > 
> > There's two techniques I've seen for that: patch some printing into
> > kernel __sev_launch_update_vmsa, or use systemtap like danpb's script
> > here: https://gitlab.com/berrange/libvirt/-/blob/lgtm/scripts/sev-vmsa.stp
> > 
> > Seems like this could be friendlier though. I'd like to work on this if
> > others agree.
> > 
> > Some ideas I've seen mentioned in passing:
> > 
> > - debugfs entry in /sys/kernel/debug/kvm/.../vcpuX/
> > - new KVM ioctl
> > - something with tracepoints
> > - some kind of dump in dmesg that doesn't require a patch
> > 
> > Thoughts?
> 
> 
> Brijesh suggested to me to construct the VMSA without getting any info from
> the host (except number of vcpus), because the initial state of the vcpus
> is standard and known if you use QEMU and OVMF (but that's open for 
> discussion).
> 
> I took his approach (thanks Brijesh!) and now it's how we calculate expected
> SNP measurements in sev-snp-measure [1].  The relevant part for VMSA 
> construction
> is in [2].
> 
> I plan to add SEV-ES and SEV measurements calculation to this 
> library/program as well.

Everyone seems to be writing one; you, Dan etc!

I think I agree the right way is to build it programmatically rather
than taking a copy from the kernel;  it's fairly simple, although the
scripts get increasingly hairy as you deal with more and more VMM's and
firmwares.

I think I'd like to see a new ioctl to read the initial VMSA, primarily
as a way of debugging so you can see what VMSA you have when something
goes wrong.

Dave

> 
> [1] https://github.com/IBM/sev-snp-measure
> [2] https://github.com/IBM/sev-snp-measure/blob/main/sevsnpmeasure/vmsa.py
> 
> -Dov
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK