Re: [PATCH v8 1/5] RDMA/umem: Support importing dma-buf as user memory region

[Daniel Vetter <daniel@ffwll.ch>]

On Tue, Nov 10, 2020 at 10:27:57AM -0400, Jason Gunthorpe wrote:
> On Tue, Nov 10, 2020 at 03:14:45PM +0100, Daniel Vetter wrote:
> > On Fri, Nov 06, 2020 at 12:39:53PM -0400, Jason Gunthorpe wrote:
> > > On Fri, Nov 06, 2020 at 04:34:07PM +0000, Xiong, Jianxin wrote:
> > > 
> > > > > The user could specify a length that is beyond the dma buf, can
> > > > > the dma buf length be checked during get?
> > > > 
> > > > In order to check the length, the buffer needs to be mapped. That can be done.
> > > 
> > > Do DMA bufs even have definitate immutable lengths? Going to be a
> > > probelm if they can shrink
> > 
> > Yup. Unfortunately that's not document in the structures themselves,
> > there's just some random comments sprinkled all over that dma-buf size is
> > invariant, e.g. see the @mmap kerneldoc in dma_buf_ops:
> > 
> > https://dri.freedesktop.org/docs/drm/driver-api/dma-buf.html?highlight=dma_buf_ops#c.dma_buf_ops
> > 
> > "Because dma-buf buffers have invariant size over their lifetime, ..."
> > 
> > Jianxin, can you pls do a kerneldoc patch which updates the comment for
> > dma_buf.size and dma_buf_export_info.size?
> 
> So we can check the size without doing an attachment?

Yeah size should be invariant over the lifetime of the dma_buf (it's also
needed for dma_buf_vmap kernel cpu access and dma_buf_mmap userspace mmap
forwarding). No lock or attachment needed. But like I said, would be good
to have the kerneldoc patch to make this clear.

The history here is that the X shm extension suffered badly from
resizeable storage object exploits of the X server, this is why both
dma-buf (and also drm_gem_buffer_object before that generalization) and
memfd are size sealed.
-Daniel

> That means the flow should be put back to how it was a series or two
> ago where the SGL is only attached during page fault with the entire
> programming sequence under the resv lock
> 
> Jason

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch