Re: [PATCH] drm/mediatek: Correctly free sg_table in gem prime vmap

[Chen-Yu Tsai]

On Tue, Oct 3, 2023 at 11:14 PM Fei Shao <fshao@xxxxxxxxxxxx> wrote:
>
> Hi,
>
> On Mon, Oct 2, 2023 at 5:21 PM Chen-Yu Tsai <wenst@xxxxxxxxxxxx> wrote:
> >
> > The MediaTek DRM driver implements GEM PRIME vmap by fetching the
> > sg_table for the object, iterating through the pages, and then
> > vmapping them. In essence, unlike the GEM DMA helpers which vmap
> > when the object is first created or imported, the MediaTek version
> > does it on request.
> >
> > Unfortunately, the code never correctly frees the sg_table contents.
> > This results in a kernel memory leak. On a Hayato device with a text
> > console on the internal display, this results in the system running
> > out of memory in a few days from all the console screen cursor updates.
> >
> > Add sg_free_table() to correctly free the contents of the sg_table. This
> > was missing despite explicitly required by mtk_gem_prime_get_sg_table().
> >
> > Fixes: 3df64d7b0a4f ("drm/mediatek: Implement gem prime vmap/vunmap function")
> > Cc: <stable@xxxxxxxxxxxxxxx>
> > Signed-off-by: Chen-Yu Tsai <wenst@xxxxxxxxxxxx>
> > ---
> > Please merge for v6.6 fixes.
> >
> > Also, I was wondering why the MediaTek DRM driver implements a lot of
> > the GEM functionality itself, instead of using the GEM DMA helpers.
> > From what I could tell, the code closely follows the DMA helpers, except
> > that it vmaps the buffers only upon request.
> >
> >
> >  drivers/gpu/drm/mediatek/mtk_drm_gem.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/drivers/gpu/drm/mediatek/mtk_drm_gem.c b/drivers/gpu/drm/mediatek/mtk_drm_gem.c
> > index 9f364df52478..297ee090e02e 100644
> > --- a/drivers/gpu/drm/mediatek/mtk_drm_gem.c
> > +++ b/drivers/gpu/drm/mediatek/mtk_drm_gem.c
> > @@ -239,6 +239,7 @@ int mtk_drm_gem_prime_vmap(struct drm_gem_object *obj, struct iosys_map *map)
> >         npages = obj->size >> PAGE_SHIFT;
> >         mtk_gem->pages = kcalloc(npages, sizeof(*mtk_gem->pages), GFP_KERNEL);
> >         if (!mtk_gem->pages) {
> > +               sg_free_table(sgt);
> >                 kfree(sgt);
> >                 return -ENOMEM;
> >         }
> > @@ -248,11 +249,13 @@ int mtk_drm_gem_prime_vmap(struct drm_gem_object *obj, struct iosys_map *map)
> >         mtk_gem->kvaddr = vmap(mtk_gem->pages, npages, VM_MAP,
> >                                pgprot_writecombine(PAGE_KERNEL));
> >         if (!mtk_gem->kvaddr) {
> > +               sg_free_table(sgt);
> >                 kfree(sgt);
> >                 kfree(mtk_gem->pages);
> >                 return -ENOMEM;
> >         }
> >  out:
> > +       sg_free_table(sgt);
>
> I think this will cause invalid access from the "goto out" path -
> sg_free_table() accesses the provided sg table pointer, but it doesn't
> handle NULL pointers like kfree() does.

You're right. I'll send a new version fixing this.