Linux kernel (arch/x86/kvm/mmu): If reclaiming shadow pages invalidates the root a fault is being serviced against, KVM

Impact

If reclaiming shadow pages invalidates the root a fault is being serviced against, KVM maps into that invalid root and creates child shadow pages that inherit the invalid role, putting invalid pages on the active MMU list. That breaks the invariant the zapping code relies on, leaving live shadow-page-table state pointing at pages KVM believes are gone - the classic setup for a host-side use-after-free reachable from guest page faults.

Who can reach it

Reachable from an ordinary guest: fault in enough memory to push the shadow MMU into reclaiming pages while a root is being used, on any node where the shadow MMU is active (nested guests, or guests running without TDP). No host access required.

What to do

Update to a kernel with the referenced stable commits. Interim: keep TDP MMU enabled and avoid exposing nested virtualization to tenants on unpatched nodes; consider raising kvm.mmu_shadow_page limits so reclaim is not hit under normal tenant load.

References