Linux kernel (arch/x86/kvm/svm): KVM computed the usable size of the guest-provided GHCB scratch area wrongly, so a

Impact

KVM computed the usable size of the guest-provided GHCB scratch area wrongly, so a guest that points its scratch pointer partway into the shared buffer gets KVM to read and write past the end of it. That is a guest-controlled host-kernel buffer overflow out of an encrypted VM - the escape primitive the whole SEV design is meant to prevent.

Who can reach it

A malicious SEV-ES / SEV-SNP guest sets an in-GHCB scratch pointer at an offset inside the shared buffer and then issues an operation (notably a Page State Change request) whose payload runs past the real end of the area. Reachable purely from guest ring 0 on any node running AMD confidential VMs; no host access needed.

What to do

Update to a kernel containing the referenced stable commits (no fixed version string was published with the record - match the commit hash). Interim: drain SEV-ES/SNP tenants off unpatched nodes, or disable SEV-ES/SNP guest types in the scheduler until patched.

References