Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <01315373-4c02-539b-9586-d764053bd8ba@collabora.com>
Date:   Mon, 28 Nov 2022 14:09:19 +0100
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101
 Thunderbird/102.5.0
Subject: Re: [PATCH v2] drm/mediatek: Clean dangling pointer on bind error
 path
Content-Language: en-US
To:     =?UTF-8?B?TsOtY29sYXMgRi4gUi4gQS4gUHJhZG8=?= 
        <nfraprado@collabora.com>
Cc:     Chun-Kuang Hu <chunkuang.hu@kernel.org>, kernel@collabora.com,
        "Nancy . Lin" <nancy.lin@mediatek.com>, CK Hu <ck.hu@mediatek.com>,
        Daniel Kurtz <djkurtz@chromium.org>,
        Daniel Vetter <daniel@ffwll.ch>,
        David Airlie <airlied@gmail.com>,
        Mao Huang <littlecvr@chromium.org>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        Philipp Zabel <p.zabel@pengutronix.de>,
        YT Shen <yt.shen@mediatek.com>,
        dri-devel@lists.freedesktop.org,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-mediatek@lists.infradead.org
References: <20221122143949.3493104-1-nfraprado@collabora.com>
 <a67594cf-eb7f-873f-1c11-ccb4317b6bdf@collabora.com>
 <20221125163413.jwutjr5uxey6b32o@notapiano>
From:   AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@collabora.com>
In-Reply-To: <20221125163413.jwutjr5uxey6b32o@notapiano>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit
Precedence: bulk

Il 25/11/22 17:34, Nícolas F. R. A. Prado ha scritto:
> On Wed, Nov 23, 2022 at 10:15:25AM +0100, AngeloGioacchino Del Regno wrote:
>> 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@collabora.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;
> 
> Actually that line only exists with [1] applied, but that commit hasn't been
> merged as of yet. I debated whether it would be better to send this fix as is,
> or ask Nancy Lin to add the tweaked fix you mention below to that series, but
> sending this fix as is seemed better since it can be backported to older kernel
> versions.
> 
> So assuming this commit gets merged first, Nancy should make that tweak you
> mentioned below to ensure all the pointers are zeroed out, which is why I've
> added Nancy to CC. (As a side note, given that only the mmsys with drm_master =
> true would ever call the drm suspend helper, even this patch as is is enough to
> avoid the panic even with that series applied, still we should zero out all
> pointers for good measure)
> 
> [1] https://lore.kernel.org/linux-mediatek/20221107072413.16178-6-nancy.lin@mediatek.com/
> 

Sorry about that, you're right - the proposed fix is for the commit you
linked, I got confused somehow.

This means that you get my

Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>

Cheers!