Linux KVM - GHCB v2+ scratch area location enforcement: MULTI-TENANT ISOLATION: KVM did not require the GHCB software

Impact

MULTI-TENANT ISOLATION: KVM did not require the GHCB software scratch area to live inside the GHCB's own shared buffer when GHCB v2+ is in use, as the spec demands. A guest can therefore point the scratch area at memory outside the shared region and get the host to read or write there on its behalf - a confused-deputy path from a confidential guest into host memory. Guest-to-host escape shape, and the CVSS 8.8 reflects it.

Who can reach it

From inside an SEV-ES/SNP guest via the GHCB protocol - tenant-reachable with no host privilege.

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. Top-of-queue for any node hosting tenant-supplied confidential VMs.

References