From 48bb9bb210c6c2f185d891e3e7a401d849409f84 Mon Sep 17 00:00:00 2001
From: Paolo Bonzini <pbonzini@redhat.com>
Date: Tue, 10 Nov 2015 13:22:53 +0100
Subject: [PATCH 2/2] KVM: svm: unconditionally intercept #DB

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@suse.com>
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 arch/x86/kvm/svm.c | 14 +++-----------
 1 file changed, 3 insertions(+), 11 deletions(-)

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 7203b3cc71b5..184e50b3c35a 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -1111,6 +1111,7 @@ static void init_vmcb(struct vcpu_svm *svm)
 	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);
@@ -1645,20 +1646,13 @@ static void svm_set_segment(struct kvm_vcpu *vcpu,
 	mark_dirty(svm->vmcb, VMCB_SEG);
 }
 
-static void update_db_bp_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
@@ -1764,7 +1758,6 @@ static int db_interception(struct vcpu_svm *svm)
 		if (!(svm->vcpu.guest_debug & KVM_GUESTDBG_SINGLESTEP))
 			svm->vmcb->save.rflags &=
 				~(X86_EFLAGS_TF | X86_EFLAGS_RF);
-		update_db_bp_intercept(&svm->vcpu);
 	}
 
 	if (svm->vcpu.guest_debug &
@@ -3753,7 +3746,6 @@ static void enable_nmi_window(struct kvm_vcpu *vcpu)
 	 */
 	svm->nmi_singlestep = true;
 	svm->vmcb->save.rflags |= (X86_EFLAGS_TF | X86_EFLAGS_RF);
-	update_db_bp_intercept(vcpu);
 }
 
 static int svm_set_tss_addr(struct kvm *kvm, unsigned int addr)
@@ -4379,7 +4371,7 @@ static struct kvm_x86_ops svm_x86_ops = {
 	.vcpu_load = svm_vcpu_load,
 	.vcpu_put = svm_vcpu_put,
 
-	.update_db_bp_intercept = update_db_bp_intercept,
+	.update_db_bp_intercept = update_bp_intercept,
 	.get_msr = svm_get_msr,
 	.set_msr = svm_set_msr,
 	.get_segment_base = svm_get_segment_base,
-- 
2.4.3