linux-kernel - Re: [PATCH V2 5/7] vhost-vdpa: VHOST_IOTLB

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Message-ID: <53630267-d22f-45ab-8919-3103739e360c@oracle.com>
Date: Thu, 18 Jul 2024 16:19:12 -0400
From: Steven Sistare <steven.sistare@...cle.com>
To: "Michael S. Tsirkin" <mst@...hat.com>, Jason Wang <jasowang@...hat.com>
Cc: virtualization@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        Si-Wei Liu <si-wei.liu@...cle.com>,
        Eugenio Perez Martin <eperezma@...hat.com>,
        Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
        Dragos Tatulea
 <dtatulea@...dia.com>,
        Alex Williamson <alex.williamson@...hat.com>
Subject: Re: [PATCH V2 5/7] vhost-vdpa: VHOST_IOTLB_REMAP

On 7/18/2024 3:39 PM, Michael S. Tsirkin wrote:
> On Thu, Jul 18, 2024 at 08:45:31AM +0800, Jason Wang wrote:
>>>> For example:
>>>>
>>>> 1) old owner pass fd to new owner which is another process
>>>> 2) the new owner do VHOST_NEW_OWNER
>>>> 3) new owner doesn't do remap correctly
>>>>
>>>> There's no way for the old owner to remove/unpin the mappings as we
>>>> have the owner check in IOTLB_UPDATE. Looks like a potential way for
>>>> DOS.
>>>
>>> This is a bug in the second cooperating process, not a DOS.  The application
>>> must fix it.  Sometimes you cannot recover from an application bug at run time.
>>>
>>> BTW, at one time vfio enforced the concept of an owner, but Alex deleted it.
>>> It adds no value, because possession of the fd is the key.
>>>     ffed0518d871 ("vfio: remove useless judgement")
>>
>> This seems to be a great relaxation of the ownership check. I would
>> like to hear from Michael first.
>>
>> Thanks
> 
> It could be that the ownership model is too restrictive.
> But again, this is changing a security assumption.
> Looks like yes another reason to tie this to the switch to iommufd.

iommufd, like vfio, does not impose an ownership requirement.  If vdpa has a
stricter requirement, such as allowing the vhost-net sharing that Jason
described, then we need to surface that now, and extend it to allow change
of ownership for live update.

Is the vhost-net scenario currently used, or aspirational?
Copying from Jason's email:
   1) Two processes (A and B) share a part of the memory
   2) A is the owner of the vhost-net who is in charge of building memory
   mappings via IOTLB
   3) A passes vhost-net fd to process B

- Steve