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>

1 files changed, 3 insertions, 11 deletions

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 183926483c3a..1cc1ffca0d8c 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -1020,6 +1020,7 @@ 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);
@@ -1554,20 +1555,13 @@ 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
@@ -1673,7 +1667,6 @@ 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 &
@@ -3661,7 +3654,6 @@ 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)
@@ -4287,7 +4279,7 @@ 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,