CVE-2021-46978

/// Technical description

In the Linux kernel, the following vulnerability has been resolved:

KVM: nVMX: Always make an attempt to map eVMCS after migration

When enlightened VMCS is in use and nested state is migrated with

vmx_get_nested_state()/vmx_set_nested_state() KVM can't map evmcs

page right away: evmcs gpa is not 'struct kvm_vmx_nested_state_hdr'

and we can't read it from VP assist page because userspace may decide

to restore HV_X64_MSR_VP_ASSIST_PAGE after restoring nested state

(and QEMU, for example, does exactly that). To make sure eVMCS is

mapped /vmx_set_nested_state() raises KVM_REQ_GET_NESTED_STATE_PAGES

request.

Commit f2c7ef3ba955 ("KVM: nSVM: cancel KVM_REQ_GET_NESTED_STATE_PAGES

on nested vmexit") added KVM_REQ_GET_NESTED_STATE_PAGES clearing to

nested_vmx_vmexit() to make sure MSR permission bitmap is not switched

when an immediate exit from L2 to L1 happens right after migration (caused

by a pending event, for example). Unfortunately, in the exact same

situation we still need to have eV

/// Remediation

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

/// References