linux-kernel - Re: [PATCH 3.2 41/52] KVM: svm: unconditionally intercept #DB

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Date:	Wed, 25 Nov 2015 12:31:33 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Ben Hutchings <ben@...adent.org.uk>, linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Cc:	akpm@...ux-foundation.org, Jan Beulich <jbeulich@...e.com>
Subject: Re: [PATCH 3.2 41/52] KVM: svm: unconditionally intercept #DB



On 24/11/2015 23:33, Ben Hutchings wrote:
> 3.2.74-rc1 review patch.  If anyone has any objections, please let me know.
> 
> ------------------
> 
> From: Paolo Bonzini <pbonzini@...hat.com>
> 
> commit cbdb967af3d54993f5814f1cee0ed311a055377d upstream.
> 
> This is needed to avoid the possibility that the guest triggers
> an infinite stream of #DB exceptions (CVE-2015-8104).
> 
> VMX is not affected: because it does not save DR6 in the VMCS,
> it already intercepts #DB unconditionally.
> 
> Reported-by: Jan Beulich <jbeulich@...e.com>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> [bwh: Backported to 3.2: #DB and #BP did not share a function, and there is
>  no operation pointer referring to it, so remove update_db_intercept()
>  entirely]

This is wrong, you still need to check the BP intercept in the
(incorrectly named as of 3.2) update_db_intercept function.

Something like:

-static void update_db_intercept(struct kvm_vcpu *vcpu)
+static void update_bp_intercept(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_svm *svm = to_svm(vcpu);

-	clr_exception_intercept(svm, DB_VECTOR);
 	clr_exception_intercept(svm, BP_VECTOR);
-
-	if (svm->nmi_singlestep)
-		set_exception_intercept(svm, DB_VECTOR);
-
 	if (vcpu->guest_debug & KVM_GUESTDBG_ENABLE) {
-		if (vcpu->guest_debug &
-		    (KVM_GUESTDBG_SINGLESTEP | KVM_GUESTDBG_USE_HW_BP))
-			set_exception_intercept(svm, DB_VECTOR);
 		if (vcpu->guest_debug & KVM_GUESTDBG_USE_SW_BP)
 			set_exception_intercept(svm, BP_VECTOR);
 	} else
		vcpu->guest_debug = 0;
 }


Then the calls in db_interception and enable_nmi_window can be removed,
but the one in svm_guest_debug is important.

Paolo
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