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SubjectRe: [PATCH 2/6] KVM: nSVM: call nested_svm_load_cr3 on nested state load
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On 01/04/21 16:18, 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 the 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@redhat.com>
> Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
> ---
> arch/x86/kvm/svm/nested.c | 40 +++++++++++++++++++++------------------
> 1 file changed, 22 insertions(+), 18 deletions(-)
>
> diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> index 8523f60adb92..ac5e3e17bda4 100644
> --- a/arch/x86/kvm/svm/nested.c
> +++ b/arch/x86/kvm/svm/nested.c
> @@ -215,24 +215,6 @@ static bool nested_svm_vmrun_msrpm(struct vcpu_svm *svm)
> return true;
> }
>
> -static bool svm_get_nested_state_pages(struct kvm_vcpu *vcpu)
> -{
> - struct vcpu_svm *svm = to_svm(vcpu);
> -
> - if (WARN_ON(!is_guest_mode(vcpu)))
> - return true;
> -
> - if (!nested_svm_vmrun_msrpm(svm)) {
> - vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
> - vcpu->run->internal.suberror =
> - KVM_INTERNAL_ERROR_EMULATION;
> - vcpu->run->internal.ndata = 0;
> - return false;
> - }
> -
> - return true;
> -}
> -
> static bool nested_vmcb_check_controls(struct vmcb_control_area *control)
> {
> if (CC(!vmcb_is_intercept(control, INTERCEPT_VMRUN)))
> @@ -1312,6 +1294,28 @@ static int svm_set_nested_state(struct kvm_vcpu *vcpu,
> return ret;
> }
>
> +static bool svm_get_nested_state_pages(struct kvm_vcpu *vcpu)
> +{
> + struct vcpu_svm *svm = to_svm(vcpu);
> +
> + if (WARN_ON(!is_guest_mode(vcpu)))
> + return true;
> +
> + if (nested_svm_load_cr3(&svm->vcpu, vcpu->arch.cr3,
> + nested_npt_enabled(svm)))
> + return false;
> +
> + if (!nested_svm_vmrun_msrpm(svm)) {
> + vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
> + vcpu->run->internal.suberror =
> + KVM_INTERNAL_ERROR_EMULATION;
> + vcpu->run->internal.ndata = 0;
> + return false;
> + }
> +
> + return true;
> +}
> +
> struct kvm_x86_nested_ops svm_nested_ops = {
> .check_events = svm_check_nested_events,
> .triple_fault = nested_svm_triple_fault,
>

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>

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