Re: [PATCH] KVM: nSVM: call nested_svm_load_cr3 on nested state load

[Paolo Bonzini]

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