KVM: VMX: Fix DR6 update on #DB exception

According to the SDM, only bits 0-3 of DR6 "may" be cleared by "certain"
debug exception. So do update them on #DB exception in KVM, but leave
the rest alone, only setting BD and BS in addition to already set bits
in DR6. This also aligns us with kvm_vcpu_check_singlestep.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 2 insertions, 1 deletions

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 79b360e4fed1..c8eb27ff1a2d 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -4869,7 +4869,8 @@ static int handle_exception(struct kvm_vcpu *vcpu)
                 dr6 = vmcs_readl(EXIT_QUALIFICATION);
                 if (!(vcpu->guest_debug &
                       (KVM_GUESTDBG_SINGLESTEP | KVM_GUESTDBG_USE_HW_BP))) {
-                        vcpu->arch.dr6 = dr6 | DR6_FIXED_1;
+                        vcpu->arch.dr6 &= ~15;
+                        vcpu->arch.dr6 |= dr6;
                         kvm_queue_exception(vcpu, DB_VECTOR);
                         return 1;
                 }