CVE-2021-46978

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 eVMCS mapped so nested_sync_vmcs12_to_shadow() reflects
changes in VMCS12 to eVMCS. As a band-aid, restore nested_get_evmcs_page()
when clearing KVM_REQ_GET_NESTED_STATE_PAGES in nested_vmx_vmexit(). The
‘fix’ is far from being ideal as we can’t easily propagate possible
failures and even if we could, this is most likely already too late to do
so. The whole ‘KVM_REQ_GET_NESTED_STATE_PAGES’ idea for mapping eVMCS after
migration seems to be fragile as we diverge too much from the ‘native’ path
when vmptr loading happens on vmx_set_nested_state().