Linux kernel (arch/x86/kvm/svm): A confidential guest can hand KVM a port-I/O request with length or count zero

Impact

A confidential guest can hand KVM a port-I/O request with length or count zero, underflowing the size arithmetic that sizes the GHCB scratch area and pushing host-kernel accesses past the end of that buffer. The SEV-ES/SNP boundary that is supposed to keep an encrypted tenant away from host memory is what fails here.

Who can reach it

Driven entirely from inside a running SEV-ES / SEV-SNP guest: the guest issues a VMGEXIT port-I/O exit through its own GHCB with len/count of 0. Needs no host account and no VMM cooperation. Only applies to nodes actually running AMD SEV-ES/SNP guests under kvm_amd; a container tenant with no VM of its own cannot reach it.

What to do

Boot a kernel carrying the referenced stable commits - the kernel CNA published no fixed release string for this record, so match on the commit in your distro's changelog. Interim control: stop scheduling SEV-ES/SNP guests on unpatched nodes, or run those tenants as ordinary (non-confidential) VMs until the fix lands.

References