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

[Daniel Vetter]

On Mon, May 11, 2020 at 5:24 PM Rob Clark <robdclark at gmail.com> wrote:
>
> On Mon, May 11, 2020 at 2:36 AM Daniel Vetter <daniel.vetter at 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.

Yeah if that's possible and won't blow up (I can't test) I think it'd
be best. Something like
if (bo->import_attach) return NULL; should do the trick I think.
Should I type that up as v2 of this?
-Daniel

>
> 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 at intel.com>
> > Cc: Rob Clark <robdclark at gmail.com>
> > Cc: Sean Paul <sean at poorly.run>
> > Cc: linux-arm-msm at vger.kernel.org
> > Cc: freedreno at 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
> >



-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch