kvm: initialize all of the kvm_debugregs structure before sending it to userspace

When calling the KVM_GET_DEBUGREGS ioctl, on some configurations, there
might be some unitialized portions of the kvm_debugregs structure that
could be copied to userspace.  Prevent this as is done in the other kvm
ioctls, by setting the whole structure to 0 before copying anything into
it.

Bonus is that this reduces the lines of code as the explicit flag
setting and reserved space zeroing out can be removed.

Cc: Sean Christopherson <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: stable <[email protected]>
Reported-by: Xingyuan Mo <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Message-Id: <[email protected]>
Tested-by: Xingyuan Mo <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>

Author: gregkh

arch/x86/kvm/x86.c

@@ -5263,12 +5263,11 @@ static void kvm_vcpu_ioctl_x86_get_debugregs(struct kvm_vcpu *vcpu,
 {
 	unsigned long val;
 
+	memset(dbgregs, 0, sizeof(*dbgregs));
 	memcpy(dbgregs->db, vcpu->arch.db, sizeof(vcpu->arch.db));
 	kvm_get_dr(vcpu, 6, &val);
 	dbgregs->dr6 = val;
 	dbgregs->dr7 = vcpu->arch.dr7;
-	dbgregs->flags = 0;
-	memset(&dbgregs->reserved, 0, sizeof(dbgregs->reserved));
 }
 
 static int kvm_vcpu_ioctl_x86_set_debugregs(struct kvm_vcpu *vcpu,