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

Hi,

On Mon, Oct 2, 2023 at 5:21 PM Chen-Yu Tsai wenst@chromium.org 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@vger.kernel.org Signed-off-by: Chen-Yu Tsai wenst@chromium.org

---

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