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

[Fei Shao]

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.

Regards,
Fei


>         kfree(sgt);
>         iosys_map_set_vaddr(map, mtk_gem->kvaddr);
>
> --
> 2.42.0.582.g8ccd20d70d-goog
>
>