[PATCH 3.2 41/52] KVM: svm: unconditionally intercept #DB

[Ben Hutchings]

3.2.74-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Paolo Bonzini <pbonzini@xxxxxxxxxx>

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@xxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
[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]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 arch/x86/kvm/svm.c | 14 +++-----------
 1 file changed, 3 insertions(+), 11 deletions(-)

--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -1015,6 +1015,7 @@ static void init_vmcb(struct vcpu_svm *s
 	set_exception_intercept(svm, UD_VECTOR);
 	set_exception_intercept(svm, MC_VECTOR);
 	set_exception_intercept(svm, AC_VECTOR);
+	set_exception_intercept(svm, DB_VECTOR);
 
 	set_intercept(svm, INTERCEPT_INTR);
 	set_intercept(svm, INTERCEPT_NMI);
@@ -1550,26 +1551,6 @@ static void svm_set_segment(struct kvm_v
 	mark_dirty(svm->vmcb, VMCB_SEG);
 }
 
-static void update_db_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;
-}
-
 static void svm_guest_debug(struct kvm_vcpu *vcpu, struct kvm_guest_debug *dbg)
 {
 	struct vcpu_svm *svm = to_svm(vcpu);
@@ -1580,8 +1561,6 @@ static void svm_guest_debug(struct kvm_v
 		svm->vmcb->save.dr7 = vcpu->arch.dr7;
 
 	mark_dirty(svm->vmcb, VMCB_DR);
-
-	update_db_intercept(vcpu);
 }
 
 static void new_asid(struct vcpu_svm *svm, struct svm_cpu_data *sd)
@@ -1655,7 +1634,6 @@ static int db_interception(struct vcpu_s
 		if (!(svm->vcpu.guest_debug & KVM_GUESTDBG_SINGLESTEP))
 			svm->vmcb->save.rflags &=
 				~(X86_EFLAGS_TF | X86_EFLAGS_RF);
-		update_db_intercept(&svm->vcpu);
 	}
 
 	if (svm->vcpu.guest_debug &
@@ -3557,7 +3535,6 @@ static void enable_nmi_window(struct kvm
 	 */
 	svm->nmi_singlestep = true;
 	svm->vmcb->save.rflags |= (X86_EFLAGS_TF | X86_EFLAGS_RF);
-	update_db_intercept(vcpu);
 }
 
 static int svm_set_tss_addr(struct kvm *kvm, unsigned int addr)

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