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

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Date:   Mon, 7 Dec 2020 16:42:05 +0100
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Halil Pasic <pasic@...ux.ibm.com>,
        Tony Krowiak <akrowiak@...ux.ibm.com>
Cc:     linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, 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 07.12.20 16:24, Halil Pasic wrote:
> 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?

Resource leaks qualify for backport and cc stable, but it is not a security
issue so this has no urgency and CVE and these kind of things.

So lets finish this without hurry, add cc stable and then look for necessary
distro backports.