linux-kernel - Re: [PATCH v2 1/2] drm/bridge: parade-ps8640: Never store more than msg->size bytes in AUX xfer

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Date: Thu, 14 Dec 2023 12:33:49 -0800
From: Doug Anderson <dianders@...omium.org>
To: dri-devel@...ts.freedesktop.org
Cc: Guenter Roeck <groeck@...omium.org>, Andrzej Hajda <andrzej.hajda@...el.com>, 
	Daniel Vetter <daniel@...ll.ch>, David Airlie <airlied@...il.com>, 
	Jernej Skrabec <jernej.skrabec@...il.com>, Jonas Karlman <jonas@...boo.se>, 
	Laurent Pinchart <Laurent.pinchart@...asonboard.com>, 
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>, 
	Neil Armstrong <neil.armstrong@...aro.org>, Philip Chen <philipchen@...omium.org>, 
	Robert Foss <rfoss@...nel.org>, Sam Ravnborg <sam@...nborg.org>, Stephen Boyd <swboyd@...omium.org>, 
	Thomas Zimmermann <tzimmermann@...e.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] drm/bridge: parade-ps8640: Never store more than
 msg->size bytes in AUX xfer

Hi,

On Thu, Dec 14, 2023 at 12:32 PM Douglas Anderson <dianders@...omium.org> wrote:
>
> While testing, I happened to notice a random crash that looked like:
>
>   Kernel panic - not syncing: stack-protector:
>   Kernel stack is corrupted in: drm_dp_dpcd_probe+0x120/0x120
>
> Analysis of drm_dp_dpcd_probe() shows that we pass in a 1-byte buffer
> (allocated on the stack) to the aux->transfer() function. Presumably
> if the aux->transfer() writes more than one byte to this buffer then
> we're in a bad shape.
>
> Dropping into kgdb, I noticed that "aux->transfer" pointed at
> ps8640_aux_transfer().
>
> Reading through ps8640_aux_transfer(), I can see that there are cases
> where it could write more bytes to msg->buffer than were specified by
> msg->size. This could happen if the hardware reported back something
> bogus to us. Let's fix this so we never write more than msg->size
> bytes. We'll still read all the bytes from the hardware just in case
> the hardware requires it since the aux transfer data comes through an
> auto-incrementing register.
>
> NOTE: I have no actual way to reproduce this issue but it seems likely
> this is what was happening in the crash I looked at.
>
> Fixes: 13afcdd7277e ("drm/bridge: parade-ps8640: Add support for AUX channel")
> Signed-off-by: Douglas Anderson <dianders@...omium.org>
> ---
>
> Changes in v2:
> - Still read all the bytes; just don't write them all to the buffer.
>
>  drivers/gpu/drm/bridge/parade-ps8640.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/bridge/parade-ps8640.c b/drivers/gpu/drm/bridge/parade-ps8640.c
> index 8161b1a1a4b1..dfb97fbb5be6 100644
> --- a/drivers/gpu/drm/bridge/parade-ps8640.c
> +++ b/drivers/gpu/drm/bridge/parade-ps8640.c
> @@ -330,7 +330,8 @@ static ssize_t ps8640_aux_transfer_msg(struct drm_dp_aux *aux,
>                                 return ret;
>                         }
>
> -                       buf[i] = data;
> +                       if (i < msg->size)
> +                               buf[i] = data;

Ah crud. I just read over Stephen's response again and realized that
in this case I'm returning the wrong length from the function. A v3
will come shortly.

-Doug