linux-kernel - Re: [PATCH v2 7/9] xen/gntdev: Implement dma-buf export functionality

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Date:   Thu, 7 Jun 2018 11:44:03 +0300
From:   Oleksandr Andrushchenko <andr2000@...il.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linux-media@...r.kernel.org,
        jgross@...e.com, konrad.wilk@...cle.com
Cc:     daniel.vetter@...el.com, dongwon.kim@...el.com,
        matthew.d.roper@...el.com,
        Oleksandr Andrushchenko <oleksandr_andrushchenko@...m.com>
Subject: Re: [PATCH v2 7/9] xen/gntdev: Implement dma-buf export functionality

On 06/07/2018 12:48 AM, Boris Ostrovsky wrote:
> On 06/06/2018 08:10 AM, Oleksandr Andrushchenko wrote:
>> On 06/05/2018 01:07 AM, Boris Ostrovsky wrote:
>>> On 06/01/2018 07:41 AM, Oleksandr Andrushchenko wrote:
>
>>> +
>>> +static struct sg_table *
>>> +dmabuf_exp_ops_map_dma_buf(struct dma_buf_attachment *attach,
>>> +               enum dma_data_direction dir)
>>> +{
>>> +    struct gntdev_dmabuf_attachment *gntdev_dmabuf_attach =
>>> attach->priv;
>>> +    struct gntdev_dmabuf *gntdev_dmabuf = attach->dmabuf->priv;
>>> +    struct sg_table *sgt;
>>> +
>>> +    pr_debug("Mapping %d pages for dev %p\n", gntdev_dmabuf->nr_pages,
>>> +         attach->dev);
>>> +
>>> +    if (WARN_ON(dir == DMA_NONE || !gntdev_dmabuf_attach))
>>>
>>> WARN_ON_ONCE. Here and elsewhere.
>> Why? The UAPI may be used by different applications, thus we might
>> lose warnings for some of them. Having WARN_ON will show problems
>> for multiple users, not for the first one.
>> Does this make sense to still use WARN_ON?
>
> Just as with pr_err call somewhere else the concern here is that
> userland (which I think is where this is eventually called from?) may
> intentionally trigger the error, flooding the log.
>
> And even this is not directly called from userland there is still a
> possibility of triggering this error multiple times.
Ok, will use WARN_ON_ONCE
>
>>>> +
>>>> +    if (use_ptemod) {
>>>> +        pr_err("Cannot provide dma-buf: use_ptemode %d\n",
>>>> +               use_ptemod);
>>> No pr_err here please. This can potentially become a DoS vector as it
>>> comes directly from ioctl.
>>>
>>> I would, in fact, revisit other uses of pr_err in this file.
>> Sure, all of pr_err can actually be pr_debug...
> I'd check even further and see if any prink is needed. I think I saw a
> couple that were not especially useful.
All those were useful while debugging the code and use-cases,
so I would prefer to have them all still available, but as pr_debug
instead of pr_err
If hyper_dmabuf will use this Xen dma-buf solution then I believe
those will help as well
>
>>>> +        return -EINVAL;
>>>> +    }
>>>> +
>>>> +    map = dmabuf_exp_alloc_backing_storage(priv, flags, count);
>>> @count comes from userspace. dmabuf_exp_alloc_backing_storage only
>>> checks for it to be >0. Should it be checked for some sane max value?
>> This is not easy as it is hard to tell what could be that
>> max value. For DMA buffers if count is too big then allocation
>> will fail, so need to check for max here  (dma_alloc_{xxx} will
>> filter out too big allocations).
> OK, that may be sufficient. BTW, I believe there were other loops with
> @count being the control variable. Please see if a user can pass a bogus
> value.
Will check for op.count in IOCTLs
>> For Xen balloon allocations I cannot tell what could be that
>> max value neither. Tough question how to limit.
> I think in balloon there is also a guarantee (of sorts) that something
> prior to a loop will fail.
>
>
> -boris
Thank you,
Oleksandr