Linux KVM - VMSA sync on an already-launched SEV vCPU: KVM allowed synchronising vCPU state into the VMSA

Impact

KVM allowed synchronising vCPU state into the VMSA after the VMSA had already been encrypted and the guest launched. Writing to an encrypted VMSA behind the guest's back corrupts the confidential vCPU state that attestation covered - the guest is no longer the thing that was measured, and the failure is silent rather than loud.

Who can reach it

Via the KVM ioctl surface, from the VMM process managing the guest.

What to do

Fixed in the Linux kernel - KVM/x86 SEV code or the ccp/PSP driver. Take the distro kernel update (RHEL/Rocky, Ubuntu, SLES) and **reboot the host**; SEV/SNP hypervisor paths cannot be live-patched in any meaningful way, and SNP platform init/shutdown is not safe to cycle under running guests. Drain confidential-VM tenants, reboot, then re-admit. No firmware, VBIOS or AGESA step needed, which makes this one of the cheaper classes of SEV fix to roll out.

References