linux-kernel - Re: [PATCH] s390/vfio-ap: Clean up vfio_ap resources when KVM pointer invalidated

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Message-ID: <20201207162411.050c6cea.pasic@linux.ibm.com>
Date:   Mon, 7 Dec 2020 16:24:11 +0100
From:   Halil Pasic <pasic@...ux.ibm.com>
To:     Tony Krowiak <akrowiak@...ux.ibm.com>
Cc:     linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, borntraeger@...ibm.com, cohuck@...hat.com,
        alex.williamson@...hat.com, kwankhede@...dia.com, david@...hat.com
Subject: Re: [PATCH] s390/vfio-ap: Clean up vfio_ap resources when KVM
 pointer invalidated

On Fri, 4 Dec 2020 11:48:24 -0500
Tony Krowiak <akrowiak@...ux.ibm.com> wrote:

> On 12/3/20 12:55 PM, Halil Pasic wrote:
> > On Wed,  2 Dec 2020 18:41:01 -0500
> > Tony Krowiak <akrowiak@...ux.ibm.com> wrote:
> >  
> >> The vfio_ap device driver registers a group notifier with VFIO when the
> >> file descriptor for a VFIO mediated device for a KVM guest is opened to
> >> receive notification that the KVM pointer is set (VFIO_GROUP_NOTIFY_SET_KVM
> >> event). When the KVM pointer is set, the vfio_ap driver stashes the pointer
> >> and calls the kvm_get_kvm() function to increment its reference counter.
> >> When the notifier is called to make notification that the KVM pointer has
> >> been set to NULL, the driver should clean up any resources associated with
> >> the KVM pointer and decrement its reference counter. The current
> >> implementation does not take care of this clean up.
> >>
> >> Signed-off-by: Tony Krowiak <akrowiak@...ux.ibm.com>  
> > Do we need a Fixes tag? Do we need this backported? In my opinion
> > this is necessary since the interrupt patches.  
> 
> I'll put in a fixes tag:
> Fixes: 258287c994de (s390: vfio-ap: implement mediated device open callback)
> 
> Yes, this should probably be backported.

I changed my mind regarding the severity of this issue. I was paranoid
about post-mortem interrupts, and resulting notifier byte updates by the
machine. What I overlooked is that the pin is going to prevent the memory
form getting repurposed. I.e. if we have something like vmalloc(),
vfio_pin(notifier_page), vfree(), I believe the notifier_page is not free
(available for allocation). So the worst case scenario is IMHO a resource
leak and not corruption. So I'm not sure this must be backported.
Opinions?

Regards,
Halil