Re: [PATCH v1 01/22] KVM: x86: race between KVM_SET_GSI_ROUTING and KVM_CREATE_IRQCHIP

[Peter Xu <peterx@redhat.com>]

On Wed, Mar 15, 2017 at 10:19:53AM +0100, David Hildenbrand wrote:
> Am 15.03.2017 um 07:24 schrieb Peter Xu:
> > On Tue, Mar 14, 2017 at 02:34:29PM +0100, David Hildenbrand wrote:
> >> Avoid races between KVM_SET_GSI_ROUTING and KVM_CREATE_IRQCHIP by taking
> >> the kvm->lock when setting up routes.
> >>
> >> If KVM_CREATE_IRQCHIP fails, KVM_SET_GSI_ROUTING could have already set
> >> up routes pointing at pic/ioapic, being silently removed already.
> >>
> >> Signed-off-by: David Hildenbrand <david@redhat.com>
> >> ---
> >>  virt/kvm/kvm_main.c | 3 +++
> >>  1 file changed, 3 insertions(+)
> >>
> >> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> >> index a17d787..ad0f8b2 100644
> >> --- a/virt/kvm/kvm_main.c
> >> +++ b/virt/kvm/kvm_main.c
> >> @@ -3079,8 +3079,11 @@ static long kvm_vm_ioctl(struct file *filp,
> >>  					   routing.nr * sizeof(*entries)))
> >>  				goto out_free_irq_routing;
> >>  		}
> >> +		/* avoid races with KVM_CREATE_IRQCHIP on x86 */
> >> +		mutex_lock(&kvm->lock);
> >>  		r = kvm_set_irq_routing(kvm, entries, routing.nr,
> >>  					routing.flags);
> >> +		mutex_unlock(&kvm->lock);
> >>  out_free_irq_routing:
> >>  		vfree(entries);
> >>  		break;
> >> -- 
> >> 2.9.3
> >>
> > 
> > Out of my curiousity: do we have a use case that these two operations
> > might collapse (KVM_CREATE_IRQCHIP and KVM_SET_GSI_ROUTING)? Or is
> > this patch only for the sake of security?
> 
> Just for the sake of security. I think, in general such calls are not
> supposed to be done in parallel. But of course, user space can.

I see.

> 
> > 
> > Another thing to mention is that, I guess adding this lock will
> > benefit KVM_CAP_SPLIT_IRQCHIP as well, maybe nice to mention it too in
> > the commit message. No worth a repost for this single reason though.
> > 
> 
> I added
> 
> "Also, as a side effect, this patch makes sure that KVM_SET_GSI_ROUTING
> and KVM_CAP_SPLIT_IRQCHIP cannot run in parallel."

Yeah it looks better. Thanks!

-- peterx