Re: [PATCH] drm/msm/dp: Drop aux devices together with DP controller

From: Doug Anderson <dianders@chromium.org>
To: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>,
	Abhinav Kumar <quic_abhinavk@quicinc.com>
Cc: freedreno@lists.freedesktop.org,
	Sankeerth Billakanti <quic_sbillaka@quicinc.com>,
	Bjorn Andersson <quic_bjorande@quicinc.com>,
	linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org,
	Kuogee Hsieh <quic_khsieh@quicinc.com>,
	linux-arm-msm@vger.kernel.org,
	Marijn Suijten <marijn.suijten@somainline.org>,
	Stephen Boyd <swboyd@chromium.org>, Sean Paul <sean@poorly.run>,
	Vinod Polimera <quic_vpolimer@quicinc.com>,
	Johan Hovold <johan+linaro@kernel.org>
Subject: Re: [PATCH] drm/msm/dp: Drop aux devices together with DP controller
Date: Tue, 13 Jun 2023 12:33:59 -0700	[thread overview]
Message-ID: <CAD=FV=Whn3GDDMQiJGYbSK8LF-OO247m69Sqcf7dMw6HFthgqw@mail.gmail.com> (raw)
In-Reply-To: <c31ee9e4-1878-c0ae-70e6-42af5fd838c7@linaro.org>

Hi,

On Mon, Jun 12, 2023 at 3:40 PM Dmitry Baryshkov
<dmitry.baryshkov@linaro.org> wrote:
>
> On 13/06/2023 01:01, Bjorn Andersson wrote:
> > Using devres to depopulate the aux bus made sure that upon a probe
> > deferral the EDP panel device would be destroyed and recreated upon next
> > attempt.
> >
> > But the struct device which the devres is tied to is the DPUs
> > (drm_dev->dev), which may be happen after the DP controller is torn
> > down.
> >
> > Indications of this can be seen in the commonly seen EDID-hexdump full
> > of zeros in the log, or the occasional/rare KASAN fault where the
> > panel's attempt to read the EDID information causes a use after free on
> > DP resources.
> >
> > It's tempting to move the devres to the DP controller's struct device,
> > but the resources used by the device(s) on the aux bus are explicitly
> > torn down in the error path.
>
> I hoped that proper usage of of_dp_aux_populate_bus(), with the callback
> function being non-NULL would have solved at least this part. But it
> seems I'll never see this patch.

Agreed. This has been pending for > 1 year now with no significant
progress. Abhinav: Is there anything that can be done about this? Not
following up on agreed-to cleanups in a timely manner doesn't set a
good precedent. Next time the Qualcomm display wants to land something
and promises to land a followup people will be less likely to believe
them...

> > The KASAN-reported use-after-free also
> > remains, as the DP aux "module" explicitly frees its devres-allocated
> > memory in this code path.
> >
> > As such, explicitly depopulate the aux bus in the error path, and in the
> > component unbind path, to avoid these issues.
> >
> > Fixes: 2b57f726611e ("drm/msm/dp: fix aux-bus EP lifetime")
> > Signed-off-by: Bjorn Andersson <quic_bjorande@quicinc.com>
>
> Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>

Reviewed-by: Douglas Anderson <dianders@chromium.org>