linux-kernel - Re: [PATCH v1] s390/virtio_ccw: don't allocate/assign airqs for non-existing queues

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <87h6309k42.fsf@redhat.com>
Date: Mon, 07 Apr 2025 15:28:13 +0200
From: Cornelia Huck <cohuck@...hat.com>
To: David Hildenbrand <david@...hat.com>, Halil Pasic <pasic@...ux.ibm.com>,
 "Michael S. Tsirkin" <mst@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
 virtualization@...ts.linux.dev, kvm@...r.kernel.org, Chandra Merla
 <cmerla@...hat.com>, Stable@...r.kernel.org, Thomas Huth
 <thuth@...hat.com>, Eric Farman <farman@...ux.ibm.com>, Heiko Carstens
 <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>, Alexander Gordeev
 <agordeev@...ux.ibm.com>, Christian Borntraeger
 <borntraeger@...ux.ibm.com>, Sven Schnelle <svens@...ux.ibm.com>, Wei Wang
 <wei.w.wang@...el.com>
Subject: Re: [PATCH v1] s390/virtio_ccw: don't allocate/assign airqs for
 non-existing queues

On Mon, Apr 07 2025, David Hildenbrand <david@...hat.com> wrote:

> On 07.04.25 15:12, Halil Pasic wrote:
>> On Mon, 7 Apr 2025 04:34:29 -0400
>> "Michael S. Tsirkin" <mst@...hat.com> wrote:
>> 
>>> On Mon, Apr 07, 2025 at 10:17:10AM +0200, David Hildenbrand wrote:
>>>> On 07.04.25 09:52, Michael S. Tsirkin wrote:
>>>>> On Fri, Apr 04, 2025 at 05:39:10PM +0200, Halil Pasic wrote:
>>>>>>>
>>>>>>> Not perfect, but AFAIKS, not horrible.
>>>>>>
>>>>>> It is like it is. QEMU does queue exist if the corresponding feature
>>>>>> is offered by the device, and that is what we have to live with.
>>>>>
>>>>> I don't think we can live with this properly though.
>>>>> It means a guest that does not know about some features
>>>>> does not know where to find things.
>>>>
>>>> Please describe a real scenario, I'm missing the point.
>>>
>>>
>>> OK so.
>>>
>>> Device has VIRTIO_BALLOON_F_FREE_PAGE_HINT and VIRTIO_BALLOON_F_REPORTING
>>> Driver only knows about VIRTIO_BALLOON_F_REPORTING so
>>> it does not know what does VIRTIO_BALLOON_F_FREE_PAGE_HINT do.
>>> How does it know which vq to use for reporting?
>>> It will try to use the free page hint one.
>> 
>> First, sorry for not catching up again with the discussion earlier.
>> 
>> I think David's point is based on the assumption that by the time feature
>> with the feature bit N+1 is specified and allocates a queue Q, all
>> queues with indexes smaller than Q are allocated and possibly associated
>> with features that were previously specified (and probably have feature
>> bits smaller than N+1).
>> 
>> I.e. that we can mandate, even if you don't want to care about other
>> optional features, you have to, because we say so, for the matter of
>> virtqueue existence. And anything in the future, you don't have to care
>> about because the queue index associated with future features is larger
>> than Q, so it does not affect our position.
>> 
>> I think that argument can fall a part if:
>> * future features reference optional queues defined in the past
>> * somebody managed to introduce a limbo where a feature is reserved, and
>>    they can not decide if they want a queue or not, or make the existence
>>    of the queue depend on something else than a feature bit.
>
> Staring at the cross-vmm, including the adding+removing of features and 
> queues that are not in the spec, I am wondering if (in a world with 
> fixed virtqueues)
>
> 1) Feature bits must be reserved before used.
>
> 2) Queue indices must be reserved before used.
>
> It all smells like a problem similar to device IDs ...

Indeed, we need a rule "reserve a feature bit/queue index before using
it, even if you do not plan to spec it properly".