[PATCH v3 1/2] drm/bridge: parade-ps8640: Never store more than msg->size bytes in AUX xfer

[Guenter Roeck]

On Thu, Dec 14, 2023 at 12:38 PM Douglas Anderson <dianders at chromium.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 at chromium.org>

Reviewed-by: Guenter Roeck <groeck at chromium.org>

> ---
>
> Changes in v3:
> - Never return more than msg->size as the number of bytes we read.
>
> Changes in v2:
> - Still read all the bytes; just don't write them all to the buffer.
>
>  drivers/gpu/drm/bridge/parade-ps8640.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/bridge/parade-ps8640.c b/drivers/gpu/drm/bridge/parade-ps8640.c
> index 8161b1a1a4b1..d264b80d909d 100644
> --- a/drivers/gpu/drm/bridge/parade-ps8640.c
> +++ b/drivers/gpu/drm/bridge/parade-ps8640.c
> @@ -330,11 +330,12 @@ static ssize_t ps8640_aux_transfer_msg(struct drm_dp_aux *aux,
>                                 return ret;
>                         }
>
> -                       buf[i] = data;
> +                       if (i < msg->size)
> +                               buf[i] = data;
>                 }
>         }
>
> -       return len;
> +       return min(len, msg->size);
>  }
>
>  static ssize_t ps8640_aux_transfer(struct drm_dp_aux *aux,
> --
> 2.43.0.472.g3155946c3a-goog
>