Re: [PATCH v3 5/8] nSVM: use svm->nested.save to load vmcb12 registers and avoid TOC/TOU races

[Maxim Levitsky <mlevitsk@redhat.com>]

On Mon, 2021-10-11 at 10:36 -0400, Emanuele Giuseppe Esposito wrote:
> Use the already checked svm->nested.save cached fields
> (EFER, CR0, CR4, ...) instead of vmcb12's in
> nested_vmcb02_prepare_save().
> This prevents from creating TOC/TOU races, since the
> guest could modify the vmcb12 fields.
> 
> This also avoids the need of force-setting EFER_SVME in
> nested_vmcb02_prepare_save.
> 
> Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
> ---
>  arch/x86/kvm/svm/nested.c | 25 +++++++------------------
>  1 file changed, 7 insertions(+), 18 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> index d07cd4b88acd..e08f2c31beae 100644
> --- a/arch/x86/kvm/svm/nested.c
> +++ b/arch/x86/kvm/svm/nested.c
> @@ -234,13 +234,7 @@ static bool nested_vmcb_valid_sregs(struct kvm_vcpu *vcpu)
>  {
>  	struct vcpu_svm *svm = to_svm(vcpu);
>  	struct vmcb_save_area_cached *save = &svm->nested.save;
> -	/*
> -	 * FIXME: these should be done after copying the fields,
> -	 * to avoid TOC/TOU races.  For these save area checks
> -	 * the possible damage is limited since kvm_set_cr0 and
> -	 * kvm_set_cr4 handle failure; EFER_SVME is an exception
> -	 * so it is force-set later in nested_prepare_vmcb_save.
> -	 */
> +
>  	if (CC(!(save->efer & EFER_SVME)))
>  		return false;
>  
> @@ -476,15 +470,10 @@ static void nested_vmcb02_prepare_save(struct vcpu_svm *svm, struct vmcb *vmcb12
>  
>  	kvm_set_rflags(&svm->vcpu, vmcb12->save.rflags | X86_EFLAGS_FIXED);
>  
> -	/*
> -	 * Force-set EFER_SVME even though it is checked earlier on the
> -	 * VMCB12, because the guest can flip the bit between the check
> -	 * and now.  Clearing EFER_SVME would call svm_free_nested.
> -	 */
> -	svm_set_efer(&svm->vcpu, vmcb12->save.efer | EFER_SVME);
> +	svm_set_efer(&svm->vcpu, svm->nested.save.efer);
>  
> -	svm_set_cr0(&svm->vcpu, vmcb12->save.cr0);
> -	svm_set_cr4(&svm->vcpu, vmcb12->save.cr4);
> +	svm_set_cr0(&svm->vcpu, svm->nested.save.cr0);
> +	svm_set_cr4(&svm->vcpu, svm->nested.save.cr4);
>  
>  	svm->vcpu.arch.cr2 = vmcb12->save.cr2;
>  
> @@ -499,8 +488,8 @@ static void nested_vmcb02_prepare_save(struct vcpu_svm *svm, struct vmcb *vmcb12
>  
>  	/* These bits will be set properly on the first execution when new_vmc12 is true */
>  	if (unlikely(new_vmcb12 || vmcb_is_dirty(vmcb12, VMCB_DR))) {
> -		svm->vmcb->save.dr7 = vmcb12->save.dr7 | DR7_FIXED_1;
> -		svm->vcpu.arch.dr6  = vmcb12->save.dr6 | DR6_ACTIVE_LOW;
> +		svm->vmcb->save.dr7 = svm->nested.save.dr7 | DR7_FIXED_1;
> +		svm->vcpu.arch.dr6  = svm->nested.save.dr6 | DR6_ACTIVE_LOW;
>  		vmcb_mark_dirty(svm->vmcb, VMCB_DR);
>  	}
>  }
> @@ -609,7 +598,7 @@ int enter_svm_guest_mode(struct kvm_vcpu *vcpu, u64 vmcb12_gpa,
>  	nested_vmcb02_prepare_control(svm);
>  	nested_vmcb02_prepare_save(svm, vmcb12);
>  
> -	ret = nested_svm_load_cr3(&svm->vcpu, vmcb12->save.cr3,
> +	ret = nested_svm_load_cr3(&svm->vcpu, svm->nested.save.cr3,
>  				  nested_npt_enabled(svm), true);
>  	if (ret)
>  		return ret;

Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>

Best regards,
	Maxim Levitsky