KVM: svm: unconditionally intercept #DB

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@suse.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
[bwh: Backported to 3.2, with thanks to Paolo:
 - update_db_bp_intercept() was called update_db_intercept()
 - The remaining call is in svm_guest_debug() rather than through svm_x86_ops]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 0de6783..ff3535a 100644 (file)
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -1015,6 +1015,7 @@ 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);
@@ -1550,20 +1551,13 @@ static void svm_set_segment(struct kvm_vcpu *vcpu,
        mark_dirty(svm->vmcb, VMCB_SEG);
 }
 
-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
@@ -1581,7 +1575,7 @@ static void svm_guest_debug(struct kvm_vcpu *vcpu, struct kvm_guest_debug *dbg)
 
        mark_dirty(svm->vmcb, VMCB_DR);
 
-       update_db_intercept(vcpu);
+       update_bp_intercept(vcpu);
 }
 
 static void new_asid(struct vcpu_svm *svm, struct svm_cpu_data *sd)
@@ -1655,7 +1649,6 @@ 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_intercept(&svm->vcpu);
        }
 
        if (svm->vcpu.guest_debug &
@@ -3557,7 +3550,6 @@ 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_intercept(vcpu);
 }
 
 static int svm_set_tss_addr(struct kvm *kvm, unsigned int addr)