From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paolo Bonzini Subject: Re: [PATCH] KVM: Disable irq while unregistering user notifier Date: Fri, 4 Nov 2016 10:48:03 +0100 Message-ID: <91cb7682-cf2a-b90c-b4c6-ea50f58acdc4@redhat.com> References: <1477087712-175064-1-git-send-email-ikalvarado@google.com> <61b5a6c3-9a30-feef-869b-bf9251454381@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Cc: Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , kvm@vger.kernel.org, =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= , David Matlack To: Ignacio Alvarado Return-path: Received: from mail-wm0-f67.google.com ([74.125.82.67]:35361 "EHLO mail-wm0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757853AbcKDJsI (ORCPT ); Fri, 4 Nov 2016 05:48:08 -0400 Received: by mail-wm0-f67.google.com with SMTP id 68so3026548wmz.2 for ; Fri, 04 Nov 2016 02:48:07 -0700 (PDT) In-Reply-To: Sender: kvm-owner@vger.kernel.org List-ID: On 03/11/2016 23:26, Ignacio Alvarado wrote: > The risk would still be present since fire_user_return_notifiers could > potentially run at any time and kvm_on_user return could be called > from drop_user_return_notifiers either from the kvm_put_kvm path or > the kvm_arch_exit path. So, do the unregistration locals->registered = false; user_return_notifier_unregister(urn); first in kvm_on_user_return? Nothing else needs to run with IRQs disabled, and doing it first prevents reentering the code from kvm_arch_hardware_disable. Paolo > Regards > > On Wed, Nov 2, 2016 at 10:25 AM, Paolo Bonzini wrote: >> >> >> On 22/10/2016 00:08, Ignacio Alvarado wrote: >>> Function user_notifier_unregister should be called only once for each >>> registered user notifier. >>> >>> Function kvm_arch_hardware_disable can be executed from an IPI context >>> which could cause a race condition with a VCPU returning to user mode >>> and attempting to unregister the notifier. >>> >>> Signed-off-by: Ignacio Alvarado >> >> Should drop_user_return_notifiers instead be moved to kvm_arch_exit? >> >> Thanks, >> >> Paolo >> >>> --- >>> arch/x86/kvm/x86.c | 10 ++++++++++ >>> 1 file changed, 10 insertions(+) >>> >>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c >>> index e375235..51f87f0 100644 >>> --- a/arch/x86/kvm/x86.c >>> +++ b/arch/x86/kvm/x86.c >>> @@ -210,7 +210,15 @@ static void kvm_on_user_return(struct user_return_notifier *urn) >>> struct kvm_shared_msrs *locals >>> = container_of(urn, struct kvm_shared_msrs, urn); >>> struct kvm_shared_msr_values *values; >>> + unsigned long flags; >>> >>> + /* >>> + * Disabling irqs at this point since the following code could be >>> + * interrupted and executed through kvm_arch_hardware_disable() >>> + */ >>> + local_irq_save(flags); >>> + if (!locals->registered) >>> + goto out; >>> for (slot = 0; slot < shared_msrs_global.nr; ++slot) { >>> values = &locals->values[slot]; >>> if (values->host != values->curr) { >>> @@ -220,6 +228,8 @@ static void kvm_on_user_return(struct user_return_notifier *urn) >>> } >>> locals->registered = false; >>> user_return_notifier_unregister(urn); >>> +out: >>> + local_irq_restore(flags); >>> } >>> >>> static void shared_msr_update(unsigned slot, u32 msr) >>> > > >