Re: [PATCH 8/9] drm/xen-front: Implement GEM operations

[Boris Ostrovsky]

On 02/27/2018 01:52 AM, Oleksandr Andrushchenko wrote:
> On 02/27/2018 01:47 AM, Boris Ostrovsky wrote:
>> On 02/23/2018 10:35 AM, Oleksandr Andrushchenko wrote:
>>> On 02/23/2018 05:26 PM, Boris Ostrovsky wrote:
>>>> On 02/21/2018 03:03 AM, Oleksandr Andrushchenko wrote:
>>
>>>>> +ÂÂÂÂÂÂÂ ret = gem_alloc_pages_array(xen_obj, size);
>>>>> +ÂÂÂÂÂÂÂ if (ret < 0) {
>>>>> +ÂÂÂÂÂÂÂÂÂÂÂ gem_free_pages_array(xen_obj);
>>>>> +ÂÂÂÂÂÂÂÂÂÂÂ goto fail;
>>>>> +ÂÂÂÂÂÂÂ }
>>>>> +
>>>>> +ÂÂÂÂÂÂÂ ret = alloc_xenballooned_pages(xen_obj->num_pages,
>>>>> +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ xen_obj->pages);
>>>> Why are you allocating balloon pages?
>>> in this use-case we map pages provided by the backend
>>> (yes, I know this can be a problem from both security
>>> POV and that DomU can die holding pages of Dom0 forever:
>>> but still it is a configuration option, so user decides
>>> if her use-case needs this and takes responsibility for
>>> such a decision).
>>
>> Perhaps I am missing something here but when you say "I know this can be
>> a problem from both security POV ..." then there is something wrong with
>> your solution.
> well, in this scenario there are actually 2 concerns:
> 1. If DomU dies the pages/grants from Dom0/DomD cannot be
> reclaimed back
> 2. Misbehaving guest may send too many requests to the
> backend exhausting grant references and memory of Dom0/DomD
> (this is the only concern from security POV). Please see [1]
>
> But, we are focusing on embedded use-cases,
> so those systems we use are not that "dynamic" with respect to 2).
> Namely: we have fixed number of domains and their functionality
> is well known, so we can do rather precise assumption on resource
> usage. This is why I try to warn on such a use-case and rely on
> the end user who understands the caveats


How will dom0/backend know whether or not to trust the front end (and
thus whether or not to provide provide pages to it)? Will there be
something in xenstore, for example, to indicate such trusted frontends?

-boris


>
> I'll probably add more precise description of this use-case
> clarifying what is that security POV, so there is no confusion
>
> Hope this explanation answers your questions
>> -boris
>>
>>> Please see description of the buffering modes in xen_drm_front.h
>>> specifically for backend allocated buffers:
>>> Â
>>> *******************************************************************************
>>>
>>> Â * 2. Buffers allocated by the backend
>>> Â
>>> *******************************************************************************
>>>
>>> Â *
>>> Â * This mode of operation is run-time configured via guest domain
>>> configuration
>>> Â * through XenStore entries.
>>> Â *
>>> Â * For systems which do not provide IOMMU support, but having specific
>>> Â * requirements for display buffers it is possible to allocate such
>>> buffers
>>> Â * at backend side and share those with the frontend.
>>> Â * For example, if host domain is 1:1 mapped and has DRM/GPU hardware
>>> expecting
>>> Â * physically contiguous memory, this allows implementing zero-copying
>>> Â * use-cases.
>>>
>>>> -boris
>>>>
>>>>> +ÂÂÂÂÂÂÂ if (ret < 0) {
>>>>> +ÂÂÂÂÂÂÂÂÂÂÂ DRM_ERROR("Cannot allocate %zu ballooned pages: %d\n",
>>>>> +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ xen_obj->num_pages, ret);
>>>>> +ÂÂÂÂÂÂÂÂÂÂÂ goto fail;
>>>>> +ÂÂÂÂÂÂÂ }
>>>>> +
>>>>> +ÂÂÂÂÂÂÂ return xen_obj;
>>>>> +ÂÂÂ }
>>>>> +ÂÂÂ /*
>>>>> +ÂÂÂÂ * need to allocate backing pages now, so we can share those
>>>>> +ÂÂÂÂ * with the backend
>>>>> +ÂÂÂÂ */
>>>>> +ÂÂÂ xen_obj->num_pages = DIV_ROUND_UP(size, PAGE_SIZE);
>>>>> +ÂÂÂ xen_obj->pages = drm_gem_get_pages(&xen_obj->base);
>>>>> +ÂÂÂ if (IS_ERR_OR_NULL(xen_obj->pages)) {
>>>>> +ÂÂÂÂÂÂÂ ret = PTR_ERR(xen_obj->pages);
>>>>> +ÂÂÂÂÂÂÂ xen_obj->pages = NULL;
>>>>> +ÂÂÂÂÂÂÂ goto fail;
>>>>> +ÂÂÂ }
>>>>> +
>>>>> +ÂÂÂ return xen_obj;
>>>>> +
>>>>> +fail:
>>>>> +ÂÂÂ DRM_ERROR("Failed to allocate buffer with size %zu\n", size);
>>>>> +ÂÂÂ return ERR_PTR(ret);
>>>>> +}
>>>>> +
>>>>>
> Thank you,
> Oleksandr
>
> [1]
> https://lists.xenproject.org/archives/html/xen-devel/2017-07/msg03100.html
>