linux-kernel - Re: [PATCH] drm/mediatek: fix device use-after-free on unbind

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Message-ID: <aP8wLMrFTsJI-Lr3@hovoldconsulting.com>
Date: Mon, 27 Oct 2025 09:41:16 +0100
From: Johan Hovold <johan@...nel.org>
To: Chun-Kuang Hu <chunkuang.hu@...nel.org>,
	Philipp Zabel <p.zabel@...gutronix.de>
Cc: David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
	Matthias Brugger <matthias.bgg@...il.com>,
	AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>,
	CK Hu <ck.hu@...iatek.com>, Ma Ke <make24@...as.ac.cn>,
	Sjoerd Simons <sjoerd@...labora.com>,
	dri-devel@...ts.freedesktop.org, linux-mediatek@...ts.infradead.org,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] drm/mediatek: fix device use-after-free on unbind

Hi Chun-Kuang,

On Mon, Oct 06, 2025 at 11:39:37AM +0200, Johan Hovold wrote:
> A recent change fixed device reference leaks when looking up drm
> platform device driver data during bind() but failed to remove a partial
> fix which had been added by commit 80805b62ea5b ("drm/mediatek: Fix
> kobject put for component sub-drivers").
> 
> This results in a reference imbalance on component bind() failures and
> on unbind() which could lead to a user-after-free.
> 
> Make sure to only drop the references after retrieving the driver data
> by effectively reverting the previous partial fix.
> 
> Note that holding a reference to a device does not prevent its driver
> data from going away so there is no point in keeping the reference.
> 
> Fixes: 1f403699c40f ("drm/mediatek: Fix device/node reference count leaks in mtk_drm_get_all_drm_priv")
> Reported-by: Sjoerd Simons <sjoerd@...labora.com>
> Link: https://lore.kernel.org/r/20251003-mtk-drm-refcount-v1-1-3b3f2813b0db@collabora.com
> Cc: stable@...r.kernel.org
> Cc: Ma Ke <make24@...as.ac.cn>
> Cc: AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>
> Signed-off-by: Johan Hovold <johan@...nel.org>

This patch fixes a regression in 6.17-rc4 that apparently breaks boot
for some users. To make things worse, the offending commit was also
backported to the stable trees.

You need to get this one into mainline as soon as possible, that is,
this week and into 6.18-rc4.

Johan