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

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Message-ID: <2faf1111-9646-21e3-feb3-f59c955bc719@collabora.com>
Date:   Thu, 23 Nov 2023 01:30:24 +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 v18 22/26] drm/shmem-helper: Don't free refcounted GEM

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

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

-- 
Best regards,
Dmitry