Re: [PATCH v18 22/26] drm/shmem-helper: Don't free refcounted GEM

[Dmitry Osipenko]

On 11/23/23 12:08, Boris Brezillon wrote:
> On Thu, 23 Nov 2023 01:30:24 +0300
> Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx> wrote:
> 
>> On 11/13/23 12:54, Boris Brezillon wrote:
>>> On Mon, 30 Oct 2023 02:02:01 +0300
>>> Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
>>>> ---
>>>>  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