[PATCH] drm/bridge: tc358767: clamp the reported AUX read size to the request

[Maoyi Xie]

tc_aux_transfer() clamps an AUX read to the payload limit:

	size_t size = min_t(size_t, DP_AUX_MAX_PAYLOAD_BYTES - 1, msg->size);

After the transfer it replaces size with the byte count the controller
reports in AUX_BYTES:

	if (size)
		size = FIELD_GET(AUX_BYTES, auxstatus);

AUX_BYTES is GENMASK(15, 8), so it can be up to 255. Nothing clamps it
back to the request. tc_aux_read_data() reads that many bytes into the
16-byte auxrdata stack buffer, then copies them into the caller buffer. A
reported count of 255 makes the read run to 256 bytes and overruns both.

The controller should never report more than it was asked to transfer, so
this is defense in depth rather than a live hole. The reported count is
only lightly trusted, and the check is cheap. Clamp it back to the request,
the same way ti-sn65dsi86 does in commit aca58eac52b8 ("drm/bridge:
ti-sn65dsi86: Never store more than msg->size bytes in AUX xfer").

Fixes: 12dfe7c4d9c5 ("drm/bridge: tc358767: Use reported AUX transfer size")
Co-developed-by: Kaixuan Li <kaixuan.li@xxxxxxxxxx>
Signed-off-by: Kaixuan Li <kaixuan.li@xxxxxxxxxx>
Signed-off-by: Maoyi Xie <maoyixie.tju@xxxxxxxxx>
---
 drivers/gpu/drm/bridge/tc358767.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/bridge/tc358767.c b/drivers/gpu/drm/bridge/tc358767.c
index 4097fef4b8..26ba5a6c1b 100644
--- a/drivers/gpu/drm/bridge/tc358767.c
+++ b/drivers/gpu/drm/bridge/tc358767.c
@@ -527,7 +527,7 @@ static ssize_t tc_aux_transfer(struct drm_dp_aux *aux,
 	 * address-only transfer
 	 */
 	if (size)
-		size = FIELD_GET(AUX_BYTES, auxstatus);
+		size = min_t(size_t, size, FIELD_GET(AUX_BYTES, auxstatus));
 	msg->reply = FIELD_GET(AUX_STATUS, auxstatus);
 
 	switch (request) {
-- 
2.34.1