Commit 355d0473 authored by Emanuele Giuseppe Esposito's avatar Emanuele Giuseppe Esposito Committed by Paolo Bonzini
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KVM: nSVM: use svm->nested.save to load vmcb12 registers and avoid TOC/TOU races 



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: default avatarEmanuele Giuseppe Esposito <eesposit@redhat.com>
Reviewed-by: default avatarMaxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20211103140527.752797-6-eesposit@redhat.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent b7a3d8b6

arch/x86/kvm/svm/nested.c

+6 −18
Original line number Diff line number Diff line
@@ -248,13 +248,6 @@ static bool nested_vmcb_check_controls(struct kvm_vcpu *vcpu, 
static bool __nested_vmcb_check_save(struct kvm_vcpu *vcpu,

				     struct vmcb_save_area_cached *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;
@@ -511,15 +504,10 @@ 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;
@@ -534,8 +522,8 @@ 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);

	}

}
@@ -649,7 +637,7 @@ 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), from_vmrun);

	if (ret)

		return ret;