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

[Rob Clark <robdclark@gmail.com>]

On Thu, May 14, 2020 at 1:11 PM 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, we 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.
>
> But even if that WARN_ON is wrong, cleaning up a vmap() done by msm by
> calling dma_buf_vunmap is the wrong thing to do.
>
> v2: Rob said in review that we do indeed have a gap in get_vaddr() that
> needs to be plugged. But the users I've found aren't legit users on
> imported dma-buf, so we can just reject that.
>
> 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

Reviewed-by: Rob Clark <robdclark@gmail.com>

> ---
>  drivers/gpu/drm/msm/msm_gem.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/msm/msm_gem.c b/drivers/gpu/drm/msm/msm_gem.c
> index 5a6a79fbc9d6..e70abd1cde43 100644
> --- a/drivers/gpu/drm/msm/msm_gem.c
> +++ b/drivers/gpu/drm/msm/msm_gem.c
> @@ -554,6 +554,9 @@ static void *get_vaddr(struct drm_gem_object *obj, unsigned madv)
>         struct msm_gem_object *msm_obj = to_msm_bo(obj);
>         int ret = 0;
>
> +       if (obj->import_attach)
> +               return ERR_PTR(-ENODEV);
> +
>         mutex_lock(&msm_obj->lock);
>
>         if (WARN_ON(msm_obj->madv > madv)) {
> @@ -907,8 +910,7 @@ 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
>