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

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Message-ID: <c31ee9e4-1878-c0ae-70e6-42af5fd838c7@linaro.org>
Date:   Tue, 13 Jun 2023 01:40:49 +0300
From:   Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To:     Bjorn Andersson <quic_bjorande@...cinc.com>,
        Rob Clark <robdclark@...il.com>,
        Abhinav Kumar <quic_abhinavk@...cinc.com>,
        Marijn Suijten <marijn.suijten@...ainline.org>,
        Kuogee Hsieh <quic_khsieh@...cinc.com>,
        Johan Hovold <johan+linaro@...nel.org>
Cc:     Sean Paul <sean@...rly.run>, David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Stephen Boyd <swboyd@...omium.org>,
        Vinod Polimera <quic_vpolimer@...cinc.com>,
        Douglas Anderson <dianders@...omium.org>,
        linux-arm-msm@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        freedreno@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm/msm/dp: Drop aux devices together with DP controller

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.

> 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@...cinc.com>

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>

> ---
>   drivers/gpu/drm/msm/dp/dp_display.c | 14 +++-----------
>   1 file changed, 3 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/gpu/drm/msm/dp/dp_display.c b/drivers/gpu/drm/msm/dp/dp_display.c
> index 3d8fa2e73583..bbb0550a022b 100644
> --- a/drivers/gpu/drm/msm/dp/dp_display.c
> +++ b/drivers/gpu/drm/msm/dp/dp_display.c
> @@ -322,6 +322,8 @@ static void dp_display_unbind(struct device *dev, struct device *master,
>   
>   	kthread_stop(dp->ev_tsk);
>   
> +	of_dp_aux_depopulate_bus(dp->aux);
> +
>   	dp_power_client_deinit(dp->power);
>   	dp_unregister_audio_driver(dev, dp->audio);
>   	dp_aux_unregister(dp->aux);
> @@ -1521,11 +1523,6 @@ void msm_dp_debugfs_init(struct msm_dp *dp_display, struct drm_minor *minor)
>   	}
>   }
>   
> -static void of_dp_aux_depopulate_bus_void(void *data)
> -{
> -	of_dp_aux_depopulate_bus(data);
> -}
> -
>   static int dp_display_get_next_bridge(struct msm_dp *dp)
>   {
>   	int rc;
> @@ -1554,12 +1551,6 @@ static int dp_display_get_next_bridge(struct msm_dp *dp)
>   		of_node_put(aux_bus);
>   		if (rc)
>   			goto error;
> -
> -		rc = devm_add_action_or_reset(dp->drm_dev->dev,
> -						of_dp_aux_depopulate_bus_void,
> -						dp_priv->aux);
> -		if (rc)
> -			goto error;
>   	} else if (dp->is_edp) {
>   		DRM_ERROR("eDP aux_bus not found\n");
>   		return -ENODEV;
> @@ -1583,6 +1574,7 @@ static int dp_display_get_next_bridge(struct msm_dp *dp)
>   
>   error:
>   	if (dp->is_edp) {
> +		of_dp_aux_depopulate_bus(dp_priv->aux);
>   		disable_irq(dp_priv->irq);
>   		dp_display_host_phy_exit(dp_priv);
>   		dp_display_host_deinit(dp_priv);

-- 
With best wishes
Dmitry