linux-kernel - Re: [PATCH] drm: bridge: it66121: Fix invalid connector dereference

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Message-ID: <sfqdspjo5p4s3xpumgqqutgdcfyfwdtmvnrnfbft2d32mpqs7w@p2rs336yyfsj>
Date:   Fri, 1 Sep 2023 14:28:30 +0530
From:   Jai Luthra <j-luthra@...com>
To:     Aradhya Bhatia <a-bhatia1@...com>
CC:     Phong LE <ple@...libre.com>,
        Neil Armstrong <neil.armstrong@...aro.org>,
        Andrzej Hajda <andrzej.hajda@...el.com>,
        Robert Foss <rfoss@...nel.org>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Jonas Karlman <jonas@...boo.se>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Nicolas Belin <nbelin@...libre.com>,
        "Andy.Hsieh" <Andy.Hsieh@...iatek.com>,
        <dri-devel@...ts.freedesktop.org>, <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>, <devarsht@...com>,
        <nm@...com>, Helen Mae Koike Fornazier <helen.koike@...labora.com>
Subject: Re: [PATCH] drm: bridge: it66121: Fix invalid connector dereference

Hi Aradhya,

Thanks for the comments.

On Aug 28, 2023 at 16:34:30 +0530, Aradhya Bhatia wrote:
> Hi Jai,
> 
> Thanks for debugging the issue.
> 
> On 25-Aug-23 16:32, Jai Luthra wrote:
> > Fix the NULL pointer dereference when no monitor is connected, and the
> > sound card is opened from userspace.
> > 
> > Instead return an error as EDID information cannot be provided to
> > the sound framework if there is no connector attached.
> > 
> > Fixes: e0fd83dbe924 ("drm: bridge: it66121: Add audio support")
> > Reported-by: Nishanth Menon <nm@...com>
> > Closes: https://lore.kernel.org/all/20230825105849.crhon42qndxqif4i@gondola/
> > Signed-off-by: Jai Luthra <j-luthra@...com>
> > ---
> >  drivers/gpu/drm/bridge/ite-it66121.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/bridge/ite-it66121.c b/drivers/gpu/drm/bridge/ite-it66121.c
> > index 466641c77fe9..d6fa00dea464 100644
> > --- a/drivers/gpu/drm/bridge/ite-it66121.c
> > +++ b/drivers/gpu/drm/bridge/ite-it66121.c
> > @@ -1446,6 +1446,11 @@ static int it66121_audio_get_eld(struct device *dev, void *data,
> >  {
> >  	struct it66121_ctx *ctx = dev_get_drvdata(dev);
> >  
> > +	if (!ctx->connector) {
> > +		dev_dbg(dev, "No connector present, cannot provide EDID data");
> > +		return -EINVAL;
> > +	}
> > +
> 
> There are not many HDMI bridges that support codecs in the kernel, but
> upon a quick look, bridge/analogix/anx7625.c and
> bridge/synopsys/dw-hdmi* gracefully return a buffer of 0s when the
> connector is unavailable.

Interesting, that sounds cleaner to me.

> 
> I am not sure why that is done, but I also don't see the hdmi-codec
> driver handle the 0s situation properly. It is business as usual for the
> hdmi-codec.

Chasing this down through the hdmi-codec driver and sound framework, it 
will use sane defaults for supported channels and sample rates if the 
actual EDID struct we are passing is all 0s. I tested it out on the 
beagle play board and did not see any crashes.

> 
> Did you come across some observation when you were testing?

Yes, with the current state of the patch, if I plug-in a monitor after 
the soundcard is probed I cannot playback audio, because we returned an 
error initially.

This issue goes away if we return a buffer of 0s like those other 
bridges. Overall I'm happy with this idea, so I will send a v2 fixing 
this.

> 
> Regards
> Aradhya
> 
> >  	mutex_lock(&ctx->lock);
> >  
> >  	memcpy(buf, ctx->connector->eld,
> > 
> > ---
> > base-commit: 6269320850097903b30be8f07a5c61d9f7592393
> > change-id: 20230825-it66121_edid-6ee98517808b
> > 
> > Best regards,
> 

-- 
Thanks,
Jai

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