linux-kernel - Re: Changing vma->vm_file in dma_buf

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Date:   Mon, 14 Sep 2020 20:26:47 +0200
From:   Christian König <ckoenig.leichtzumerken@...il.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     akpm@...ux-foundation.org, sumit.semwal@...aro.org,
        linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linaro-mm-sig@...ts.linaro.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Daniel Vetter <daniel.vetter@...ll.ch>
Subject: Re: Changing vma->vm_file in dma_buf_mmap()

Am 14.09.20 um 16:06 schrieb Jason Gunthorpe:
> On Mon, Sep 14, 2020 at 03:30:47PM +0200, Christian König wrote:
>> Am 14.09.20 um 15:29 schrieb Christian König:
>>> Hi Andrew,
>>>
>>> I'm the new DMA-buf maintainer and Daniel and others came up with
>>> patches extending the use of the dma_buf_mmap() function.
>>>
>>> Now this function is doing something a bit odd by changing the
>>> vma->vm_file while installing a VMA in the mmap() system call
> It doesn't look obviously safe as mmap_region() has an interesting mix
> of file and vma->file
>
> Eg it calls mapping_unmap_writable() using both routes

Thanks for the hint, going to take a look at that code tomorrow.

> What about security? Is it OK that some other random file, maybe in
> another process, is being linked to this mmap?

Good question, I have no idea. That's why I send out this mail.

>>> The background here is that DMA-buf allows device drivers to
>>> export buffer which are then imported into another device
>>> driver. The mmap() handler of the importing device driver then
>>> find that the pgoff belongs to the exporting device and so
>>> redirects the mmap() call there.
> So the pgoff is some virtualized thing?

Yes, absolutely.

Christian.

>
> Jason