[PATCH 1/2] drm/bridge: parade-ps8640: Never increase the length when reading from AUX

[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 increase the length.

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>
---

 drivers/gpu/drm/bridge/parade-ps8640.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/bridge/parade-ps8640.c b/drivers/gpu/drm/bridge/parade-ps8640.c
index 8161b1a1a4b1..fb2ec4264549 100644
--- a/drivers/gpu/drm/bridge/parade-ps8640.c
+++ b/drivers/gpu/drm/bridge/parade-ps8640.c
@@ -302,7 +302,7 @@ static ssize_t ps8640_aux_transfer_msg(struct drm_dp_aux *aux,
 
 		fallthrough;
 	case SWAUX_STATUS_ACKM:
-		len = data & SWAUX_M_MASK;
+		len = min(len, (unsigned int)(data & SWAUX_M_MASK));
 		break;
 	case SWAUX_STATUS_DEFER:
 	case SWAUX_STATUS_I2C_DEFER:
@@ -310,7 +310,7 @@ static ssize_t ps8640_aux_transfer_msg(struct drm_dp_aux *aux,
 			msg->reply |= DP_AUX_NATIVE_REPLY_DEFER;
 		else
 			msg->reply |= DP_AUX_I2C_REPLY_DEFER;
-		len = data & SWAUX_M_MASK;
+		len = min(len, (unsigned int)(data & SWAUX_M_MASK));
 		break;
 	case SWAUX_STATUS_INVALID:
 		return -EOPNOTSUPP;
-- 
2.43.0.472.g3155946c3a-goog