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Date:   Thu, 23 Nov 2023 10:08:29 +0100
From:   Boris Brezillon <boris.brezillon@collabora.com>
To:     Dmitry Osipenko <dmitry.osipenko@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>,
        Christian =?UTF-8?B?S8O2bmln?= <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
Subject: Re: [PATCH v18 22/26] drm/shmem-helper: Don't free refcounted GEM
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In-Reply-To: <2faf1111-9646-21e3-feb3-f59c955bc719@collabora.com>
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        <20231029230205.93277-23-dmitry.osipenko@collabora.com>
        <20231113105438.60896fdf@collabora.com>
        <2faf1111-9646-21e3-feb3-f59c955bc719@collabora.com>
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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?