linux-kernel - Re: [PATCHv4 4/6] drm/omap: fix incorrect union usage

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Message-ID: <f64b580d-14ae-42df-34b9-a1f7c87e5f37@ti.com>
Date:   Fri, 16 Nov 2018 15:41:24 +0200
From:   Tomi Valkeinen <tomi.valkeinen@...com>
To:     Sebastian Reichel <sebastian.reichel@...labora.com>,
        Sebastian Reichel <sre@...nel.org>,
        Tony Lindgren <tony@...mide.com>, Pavel Machek <pavel@....cz>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>
CC:     "H. Nikolaus Schaller" <hns@...delico.com>,
        <dri-devel@...ts.freedesktop.org>, <linux-omap@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <kernel@...labora.com>
Subject: Re: [PATCHv4 4/6] drm/omap: fix incorrect union usage

On 16/11/18 01:06, Sebastian Reichel wrote:
> The DSI encoder sets dssdev->ops->dsi.set_config, which is stored at the
> same offset as dssdev->ops->hdmi.set_hdmi_mode. The code in omap_encoder
> only checks if dssdev->ops->hdmi.set_hdmi_mode is NULL. Due to the way
> union works, it won't be NULL if dsi.set_config is set. This means
> dsi_set_config will be called with config=hdmi_mode=false=NULL parameter
> resulting in a NULL dereference. Also the dereference happens while
> console is locked, so kernel hangs without any debug output (can be
> avoided by fbmem's lockless_register_fb=1 parameter).
> 
> This fixes the issue by exiting early if the output type definitely
> has no hdmi_set operations.
> 
> Fixes: 83910ad3f51fb ("drm/omap: Move most omap_dss_driver operations to omap_dss_device_ops")
> Signed-off-by: Sebastian Reichel <sebastian.reichel@...labora.com>
> ---
>  drivers/gpu/drm/omapdrm/omap_encoder.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/drivers/gpu/drm/omapdrm/omap_encoder.c b/drivers/gpu/drm/omapdrm/omap_encoder.c
> index 32bbe3a80e7d..ba0099f0644c 100644
> --- a/drivers/gpu/drm/omapdrm/omap_encoder.c
> +++ b/drivers/gpu/drm/omapdrm/omap_encoder.c
> @@ -122,6 +122,14 @@ static void omap_encoder_mode_set(struct drm_encoder *encoder,
>  
>  	dssdev = omap_encoder->output;
>  
> +	/* The following operations access dssdev->ops->hdmi, which is a union
> +	 * also used by DSI. This ensures, that the field does not have data
> +	 * for DSI (or any other future output type).
> +	 */
> +	if (dssdev->output_type != OMAP_DISPLAY_TYPE_HDMI &&
> +	    dssdev->output_type != OMAP_DISPLAY_TYPE_DVI)

Good catch.

Why DVI?

I think the whole code block starting from

/* Set the HDMI mode and HDMI infoframe if applicable. */

to the end of the function should be inside

if (dssdev->output_type == OMAP_DISPLAY_TYPE_HDMI)

 Tomi

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