diff mbox series

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

Message ID 20231214123205.v2.1.I9d1afcaad76a3e2c0ca046dc4adbc2b632c22eda@changeid
State New
Headers
Series [v2,1/2] drm/bridge: parade-ps8640: Never store more than msg->size bytes in AUX xfer |

Commit Message

Doug Anderson Dec. 14, 2023, 8:32 p.m. UTC 
  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@chromium.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(-)

Comments

Doug Anderson Dec. 14, 2023, 8:33 p.m. UTC | #1 
Hi,

On Thu, Dec 14, 2023 at 12:32 PM Douglas Anderson <dianders@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@chromium.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
diff mbox series

Patch

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;
 		}
 	}