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

[Doug Anderson]

Hi,

On Thu, Dec 14, 2023 at 12:38 PM Douglas Anderson <dianders@xxxxxxxxxxxx> 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@xxxxxxxxxxxx>

Since the patch fixes a crash, has two Reviews (even if they're both
from @chromium), and doesn't seem controversial, I didn't want a full
week and just landed it in drm-misc-fixes. If anyone is upset by this
then please shout and we can revert or I can post a followup patch.

3164c8a70073 drm/bridge: parade-ps8640: Never store more than
msg->size bytes in AUX xfer