Re: [PATCH 1/9] drm/msm: Don't call dma_buf_vunmap without _vmap

[Rob Clark <robdclark@gmail.com>]

On Mon, May 11, 2020 at 2:36 AM Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
>
> I honestly don't exactly understand what's going on here, but the
> current code is wrong for sure: It calls dma_buf_vunmap without ever
> calling dma_buf_vmap.
>
> What I'm not sure about is whether the WARN_ON is correct:
> - msm imports dma-buf using drm_prime_sg_to_page_addr_arrays. Which is
>   a pretty neat layering violation of how you shouldn't peek behind
>   the curtain of the dma-buf exporter, but par for course. Note that
>   all the nice new helpers don't (and we should probably have a bit a
>   warning about this in the kerneldoc).
>
> - but then in the get_vaddr() in msm_gem.c, and that seems to happily
>   wrap a vmap() around any object with ->pages set (so including
>   imported dma-buf)
>
> - I'm not seeing any guarantees that userspace can't use an imported
>   dma-buf for e.g. MSM_SUBMIT_CMD_BUF in a5xx_submit_in_rb, so no
>   guarantees that an imported dma-buf won't end up with a ->vaddr set.

fwiw, a5xx_submit_in_rb() isn't a "normal" path (build-time disabled
by default, and restricted to sudo).. it really only exists to
simplify poking at fw.

There could be vmap's in the msm_gem_submit path, however.  If we
don't, we should probably just disallow using an imported dma-buf as
cmdstream.. I don't think there is any sane reason to permit that.  We
should probably also disallow get_vaddr() on imported buffers.

BR,
-R

>
> But even if that WARN_ON is wrong, cleaning up a vmap() done by msm by
> calling dma_buf_vmap is the wrong thing to do.
>
> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> Cc: Rob Clark <robdclark@gmail.com>
> Cc: Sean Paul <sean@poorly.run>
> Cc: linux-arm-msm@vger.kernel.org
> Cc: freedreno@lists.freedesktop.org
> ---
>  drivers/gpu/drm/msm/msm_gem.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/msm/msm_gem.c b/drivers/gpu/drm/msm/msm_gem.c
> index 5a6a79fbc9d6..3305a457960e 100644
> --- a/drivers/gpu/drm/msm/msm_gem.c
> +++ b/drivers/gpu/drm/msm/msm_gem.c
> @@ -907,8 +907,7 @@ static void free_object(struct msm_gem_object *msm_obj)
>         put_iova(obj);
>
>         if (obj->import_attach) {
> -               if (msm_obj->vaddr)
> -                       dma_buf_vunmap(obj->import_attach->dmabuf, msm_obj->vaddr);
> +               WARN_ON(msm_obj->vaddr);
>
>                 /* Don't drop the pages for imported dmabuf, as they are not
>                  * ours, just free the array we allocated:
> --
> 2.26.2
>
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