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Message-ID: <fdc246db-e6cd-ff31-365f-0d2e0690364d@collabora.com>
Date:   Thu, 23 Nov 2023 15:36:51 +0300
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Subject: Re: [PATCH v18 22/26] drm/shmem-helper: Don't free refcounted GEM
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To:     Boris Brezillon <boris.brezillon@collabora.com>
Cc:     David Airlie <airlied@gmail.com>,
        Gerd Hoffmann <kraxel@redhat.com>,
        Gurchetan Singh <gurchetansingh@chromium.org>,
        Chia-I Wu <olvaffe@gmail.com>, Daniel Vetter <daniel@ffwll.ch>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <mripard@kernel.org>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>,
        Qiang Yu <yuq825@gmail.com>,
        Steven Price <steven.price@arm.com>,
        Emma Anholt <emma@anholt.net>, Melissa Wen <mwen@igalia.com>,
        dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
        kernel@collabora.com, virtualization@lists.linux-foundation.org
References: <20231029230205.93277-1-dmitry.osipenko@collabora.com>
 <20231029230205.93277-23-dmitry.osipenko@collabora.com>
 <20231113105438.60896fdf@collabora.com>
 <2faf1111-9646-21e3-feb3-f59c955bc719@collabora.com>
 <20231123100829.2e4addd0@collabora.com>
From:   Dmitry Osipenko <dmitry.osipenko@collabora.com>
In-Reply-To: <20231123100829.2e4addd0@collabora.com>
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On 11/23/23 12:08, Boris Brezillon wrote:
> On Thu, 23 Nov 2023 01:30:24 +0300
> Dmitry Osipenko <dmitry.osipenko@collabora.com> wrote:
> 
>> On 11/13/23 12:54, Boris Brezillon wrote:
>>> On Mon, 30 Oct 2023 02:02:01 +0300
>>> Dmitry Osipenko <dmitry.osipenko@collabora.com> wrote:
>>>   
>>>> Don't free refcounted shmem object to prevent use-after-free bug that
>>>> is worse than a memory leak.
>>>>
>>>> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
>>>> ---
>>>>  drivers/gpu/drm/drm_gem_shmem_helper.c | 7 ++++---
>>>>  1 file changed, 4 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
>>>> index 6dd087f19ea3..4253c367dc07 100644
>>>> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
>>>> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
>>>> @@ -203,9 +203,10 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object *shmem)
>>>>  	if (obj->import_attach)
>>>>  		drm_prime_gem_destroy(obj, shmem->sgt);
>>>>  
>>>> -	drm_WARN_ON(obj->dev, refcount_read(&shmem->vmap_use_count));
>>>> -	drm_WARN_ON(obj->dev, refcount_read(&shmem->pages_use_count));
>>>> -	drm_WARN_ON(obj->dev, refcount_read(&shmem->pages_pin_count));
>>>> +	if (drm_WARN_ON(obj->dev, refcount_read(&shmem->vmap_use_count)) ||
>>>> +	    drm_WARN_ON(obj->dev, refcount_read(&shmem->pages_use_count)) ||
>>>> +	    drm_WARN_ON(obj->dev, refcount_read(&shmem->pages_pin_count)))
>>>> +		return;  
>>>
>>> I guess you're worried about ->sgt being referenced by the driver after
>>> the GEM is destroyed. If we assume drivers don't cache the sgt and
>>> always call get_pages_sgt() when they need it that shouldn't be an
>>> issue. What we really don't want to release is the pages themselves,
>>> but the GPU MMU might still have active mappings pointing to these
>>> pages.
>>>
>>> In any case, I'm not against leaking the GEM object when any of these
>>> counters are not zero, but can we at least have a comment in the
>>> code explaining why we're doing that, so people don't have to go look
>>> at the git history to figure it out.  
>>
>> This patch is a minor improvement, it doesn't address any specific
>> issue. This should be a common pattern in kernel. If you're giving a
>> warning and know about the inevitable catastrophe, then avoid it if you can.
> 
> Sure, I'm just asking that we add a comment to explain why we leak
> memory here. Is that too much to ask?

Will add the comment. The reason why I added this patch was unrelated to
the sgt, that's what I'm talking about.

-- 
Best regards,
Dmitry