linux-kernel - Re: [PATCH v2] drm/mediatek: Clean dangling pointer on bind error path

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Date:   Wed, 23 Nov 2022 10:15:25 +0100
From:   AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>
To:     Nícolas F. R. A. Prado 
        <nfraprado@...labora.com>, Chun-Kuang Hu <chunkuang.hu@...nel.org>
Cc:     kernel@...labora.com, "Nancy . Lin" <nancy.lin@...iatek.com>,
        CK Hu <ck.hu@...iatek.com>,
        Daniel Kurtz <djkurtz@...omium.org>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...il.com>,
        Mao Huang <littlecvr@...omium.org>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        YT Shen <yt.shen@...iatek.com>,
        dri-devel@...ts.freedesktop.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mediatek@...ts.infradead.org
Subject: Re: [PATCH v2] drm/mediatek: Clean dangling pointer on bind error
 path

Il 22/11/22 15:39, Nícolas F. R. A. Prado ha scritto:
> mtk_drm_bind() can fail, in which case drm_dev_put() is called,
> destroying the drm_device object. However a pointer to it was still
> being held in the private object, and that pointer would be passed along
> to DRM in mtk_drm_sys_prepare() if a suspend were triggered at that
> point, resulting in a panic. Clean the pointer when destroying the
> object in the error path to prevent this from happening.
> 
> Fixes: 119f5173628a ("drm/mediatek: Add DRM Driver for Mediatek SoC MT8173.")
> Signed-off-by: Nícolas F. R. A. Prado <nfraprado@...labora.com>
> 
> ---
> 
> Changes in v2:
> - Added Fixes tag
> 
>   drivers/gpu/drm/mediatek/mtk_drm_drv.c | 1 +
>   1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/gpu/drm/mediatek/mtk_drm_drv.c b/drivers/gpu/drm/mediatek/mtk_drm_drv.c
> index 39a42dc8fb85..a21ff1b3258c 100644
> --- a/drivers/gpu/drm/mediatek/mtk_drm_drv.c
> +++ b/drivers/gpu/drm/mediatek/mtk_drm_drv.c
> @@ -514,6 +514,7 @@ static int mtk_drm_bind(struct device *dev)
>   err_deinit:
>   	mtk_drm_kms_deinit(drm);
>   err_free:
> +	private->drm = NULL;

Sorry for not noticing that in v1, but I've rechecked this function and, while this
commit does indeed actually solve the described issue, I think it's incomplete.

A few lines before, we have a loop that sets

		private->all_drm_private[i]->drm = drm;

...so here you should do...

	private->drm = NULL;

	while (i--) /* a for loop will also do, your choice */
		private->all_drm_private[i]->drm = NULL;
		
That makes sure that you cleanup *everything* :-)

Cheers,
Angelo