On 10/02/21 16:59, Maxim Levitsky wrote:
While KVM's MMU should be fully reset by loading of nested CR0/CR3/CR4
by KVM_SET_SREGS, we are not in nested mode yet when we do it and therefore
only root_mmu is reset.
On regular nested entries we call nested_svm_load_cr3 which both updates the
guest's CR3 in the MMU when it is needed, and it also initializes
the mmu again which makes it initialize the walk_mmu as well when nested
paging is enabled in both host and guest.
Since we don't call nested_svm_load_cr3 on nested state load,
the walk_mmu can be left uninitialized, which can lead to a NULL pointer
dereference while accessing it if we happen to get a nested page fault
right after entering the nested guest first time after the migration and
we decide to emulate it, which leads to emulator trying to access
walk_mmu->gva_to_gpa which is NULL.
Therefore we should call this function on nested state load as well.
Suggested-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Signed-off-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
---
arch/x86/kvm/svm/nested.c | 8 ++++++++
1 file changed, 8 insertions(+)
diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index 519fe84f2100..c209f1232928 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -1282,6 +1282,14 @@ static int svm_set_nested_state(struct kvm_vcpu *vcpu,
nested_vmcb02_prepare_control(svm);
+ ret = nested_svm_load_cr3(&svm->vcpu, vcpu->arch.cr3,
+ nested_npt_enabled(svm));
+
+ if (ret) {
+ svm_leave_nested(svm);
+ goto out_free;
+ }
+
kvm_make_request(KVM_REQ_GET_NESTED_STATE_PAGES, vcpu);
ret = 0;
out_free:
I think you have to delay this to KVM_REQ_GET_NESTED_STATE_PAGES,
because the !nested_npt case can be accessing memory before the VM is
started (PDPTRs!).
In fact the same is true for VMX: this code
/* Shadow page tables on either EPT or shadow page tables. */
if (nested_vmx_load_cr3(vcpu, vmcs12->guest_cr3,
nested_cpu_has_ept(vmcs12),
entry_failure_code))
return -EINVAL;
must be moved from prepare_vmcs02 to both nested_vmx_enter_non_root_mode
and nested_get_vmcs12_pages.
Thanks,
Paolo
