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

[Dmitry Osipenko]

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.

Actually, there are other similar cases in drm-shmem that can be improved.

-- 
Best regards,
Dmitry