Linux kernel (arch/x86/kvm/svm): The SEV debug-encrypt path bounds each iteration by the source page offset but not the

Impact

The SEV debug-encrypt path bounds each iteration by the source page offset but not the destination offset, so the PSP and a following memcpy write past the end of a single-page kernel buffer - KASAN reports a 4095-byte out-of-bounds write. That is host kernel heap corruption, i.e. root on the node, driven by whoever drives the SEV VM.

Who can reach it

Reachable by any process that can open /dev/kvm and create an SEV VM, via KVM_MEMORY_ENCRYPT_OP with a debug-encrypt request whose destination offset exceeds the source offset. It is not the guest that triggers it - it is the process owning the VM, so this matters wherever tenants hold /dev/kvm (nested virtualization exposed, or bare-metal tenants). AMD SEV hosts running kvm_amd only.

What to do

Update to a kernel with the referenced stable commits. Interim: do not expose /dev/kvm to tenants, and disable SEV on nodes where you cannot patch promptly (kvm_amd sev=0).

References