[PATCH v3 2/2] drm/bridge: ti-sn65dsi86: 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:
>
> For aux reads, the value `msg->size` indicates the size of the buffer
> provided by `msg->buffer`. We should never in any circumstances write
> more bytes to the buffer since it may overflow the buffer.
>
> In the ti-sn65dsi86 driver there is one code path that reads the
> transfer length from hardware. Even though it's never been seen to be
> a problem, we should make extra sure that the hardware isn't
> increasing the length since doing so would cause us to overrun the
> buffer.
>
> Fixes: 982f589bde7a ("drm/bridge: ti-sn65dsi86: Update reply on aux failures")
> Signed-off-by: Douglas Anderson <dianders at chromium.org>

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

> ---
>
> (no changes since v2)
>
> Changes in v2:
> - Updated patch subject to match ps8640 patch.
>
>  drivers/gpu/drm/bridge/ti-sn65dsi86.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/bridge/ti-sn65dsi86.c b/drivers/gpu/drm/bridge/ti-sn65dsi86.c
> index 9095d1453710..62cc3893dca5 100644
> --- a/drivers/gpu/drm/bridge/ti-sn65dsi86.c
> +++ b/drivers/gpu/drm/bridge/ti-sn65dsi86.c
> @@ -527,6 +527,7 @@ static ssize_t ti_sn_aux_transfer(struct drm_dp_aux *aux,
>         u32 request_val = AUX_CMD_REQ(msg->request);
>         u8 *buf = msg->buffer;
>         unsigned int len = msg->size;
> +       unsigned int short_len;
>         unsigned int val;
>         int ret;
>         u8 addr_len[SN_AUX_LENGTH_REG + 1 - SN_AUX_ADDR_19_16_REG];
> @@ -600,7 +601,8 @@ static ssize_t ti_sn_aux_transfer(struct drm_dp_aux *aux,
>         }
>
>         if (val & AUX_IRQ_STATUS_AUX_SHORT) {
> -               ret = regmap_read(pdata->regmap, SN_AUX_LENGTH_REG, &len);
> +               ret = regmap_read(pdata->regmap, SN_AUX_LENGTH_REG, &short_len);
> +               len = min(len, short_len);
>                 if (ret)
>                         goto exit;
>         } else if (val & AUX_IRQ_STATUS_NAT_I2C_FAIL) {
> --
> 2.43.0.472.g3155946c3a-goog
>