Code: static int db_interception(struct vcpu_svm *svm)
{
	struct kvm_run *kvm_run = svm->vcpu.run;

	if (!(svm->vcpu.guest_debug &
	      (KVM_GUESTDBG_SINGLESTEP | KVM_GUESTDBG_USE_HW_BP)) &&
		!svm->nmi_singlestep) {
		kvm_queue_exception(&svm->vcpu, DB_VECTOR);
		return 1;
	}

	if (svm->nmi_singlestep) {
		svm->nmi_singlestep = false;
 		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 &
	    (KVM_GUESTDBG_SINGLESTEP | KVM_GUESTDBG_USE_HW_BP)) {
		kvm_run->exit_reason = KVM_EXIT_DEBUG;
		kvm_run->debug.arch.pc =
			svm->vmcb->save.cs.base + svm->vmcb->save.rip;
		kvm_run->debug.arch.exception = DB_VECTOR;
		return 0;
	}

	return 1;
}

Vulnerability Type: DoS 
CWE ID: CWE-399
Summary: The KVM subsystem in the Linux kernel through 4.2.6, and Xen 4.3.x through 4.6.x, allows guest OS users to cause a denial of service (host OS panic or hang) by triggering many #DB (aka Debug) exceptions, related to svm.c.
Commit Message: 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>