linux-kernel - Re: [PATCH v16 15/20] drm/shmem-helper: Add memory shrinker

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Date:   Thu, 14 Sep 2023 16:58:24 +0300
From:   Dmitry Osipenko <dmitry.osipenko@...labora.com>
To:     Boris Brezillon <boris.brezillon@...labora.com>
Cc:     David Airlie <airlied@...il.com>,
        Gerd Hoffmann <kraxel@...hat.com>,
        Gurchetan Singh <gurchetansingh@...omium.org>,
        Chia-I Wu <olvaffe@...il.com>, Daniel Vetter <daniel@...ll.ch>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Christian König <christian.koenig@....com>,
        Qiang Yu <yuq825@...il.com>,
        Steven Price <steven.price@....com>,
        Emma Anholt <emma@...olt.net>, Melissa Wen <mwen@...lia.com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        kernel@...labora.com, virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v16 15/20] drm/shmem-helper: Add memory shrinker

On 9/14/23 16:27, Boris Brezillon wrote:
...
> If you added this pages_use_count > 0 check to deal with the
> 'free-partially-imported-GEM' case, I keep thinking this is not
> the right fix. You should just assume that obj->import_attach == NULL
> means not-a-prime-buffer, and then make sure
> partially-initialized-prime-GEMs have import_attach assigned (see the
> oneliner I suggested in my review of
> `[PATCH v15 01/23] drm/shmem-helper: Fix UAF in error path when
> freeing SGT of imported GEM`).

Yes, I added it to deal with the partially imported GEM. The
obj->import_attach can't be set until dma-buf is fully imported as it
also will cause trouble for the error code path, now dma-buf will be
freed two times.

>> 			dma_unmap_sgtable(obj->dev->dev, shmem->sgt,
>> 					  DMA_BIDIRECTIONAL, 0);
>> 			sg_free_table(shmem->sgt);
>> 			kfree(shmem->sgt);
>>
>> 			__drm_gem_shmem_put_pages(shmem);
> You need to decrement pages_use_count:
> 
> 			/* shmem->pages_use_count should be 1 when ->sgt != NULL and
> 			 * zero otherwise. If some users still hold a pages reference
> 			 * that's a bug, and we intentionally leak the pages so they
> 			 * can't be re-allocated to someone else while the GPU/CPU
> 			 * still have access to it.
> 		 	 */
> 			if (refcount_dec_and_test(&shmem->pages_use_count))
> 				__drm_gem_shmem_put_pages(shmem);
> 

The put_pages() itself decrements the refcnt.

I'm going back to deferring all this questionable changes for the later
times. It is not essential problem for this patchset.

-- 
Best regards,
Dmitry