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

[Douglas Anderson]

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>
---

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