Changing vma->vm_file in dma_buf_mmap()

[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

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