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

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Message-ID: <20200914140632.GD1221970@ziepe.ca>
Date:   Mon, 14 Sep 2020 11:06:32 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Christian König <ckoenig.leichtzumerken@...il.com>
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()

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

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

> > 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?

Jason