drm/dp_mst: Verify request type in the corresponding down message reply

After receiving the response for an MST down request message, the
response should be accepted/parsed only if the response type matches
that of the request. Ensure this by checking if the request type code
stored both in the request and the reply match, dropping the reply in
case of a mismatch.

This fixes the topology detection for an MST hub, as described in the
Closes link below, where the hub sends an incorrect reply message after
a CLEAR_PAYLOAD_TABLE -> LINK_ADDRESS down request message sequence.

Cc: Lyude Paul <lyude@redhat.com>
Cc: <stable@vger.kernel.org>
Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/12804
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241203160223.2926014-3-imre.deak@intel.com

Author: ideak

drivers/gpu/drm/display/drm_dp_mst_topology.c

@@ -3941,6 +3941,34 @@ drm_dp_get_one_sb_msg(struct drm_dp_mst_topology_mgr *mgr, bool up,
 	return true;
 }
 
+static int get_msg_request_type(u8 data)
+{
+	return data & 0x7f;
+}
+
+static bool verify_rx_request_type(struct drm_dp_mst_topology_mgr *mgr,
+				   const struct drm_dp_sideband_msg_tx *txmsg,
+				   const struct drm_dp_sideband_msg_rx *rxmsg)
+{
+	const struct drm_dp_sideband_msg_hdr *hdr = &rxmsg->initial_hdr;
+	const struct drm_dp_mst_branch *mstb = txmsg->dst;
+	int tx_req_type = get_msg_request_type(txmsg->msg[0]);
+	int rx_req_type = get_msg_request_type(rxmsg->msg[0]);
+	char rad_str[64];
+
+	if (tx_req_type == rx_req_type)
+		return true;
+
+	drm_dp_mst_rad_to_str(mstb->rad, mstb->lct, rad_str, sizeof(rad_str));
+	drm_dbg_kms(mgr->dev,
+		    "Got unexpected MST reply, mstb: %p seqno: %d lct: %d rad: %s rx_req_type: %s (%02x) != tx_req_type: %s (%02x)\n",
+		    mstb, hdr->seqno, mstb->lct, rad_str,
+		    drm_dp_mst_req_type_str(rx_req_type), rx_req_type,
+		    drm_dp_mst_req_type_str(tx_req_type), tx_req_type);
+
+	return false;
+}
+
 static int drm_dp_mst_handle_down_rep(struct drm_dp_mst_topology_mgr *mgr)
 {
 	struct drm_dp_sideband_msg_tx *txmsg;
@@ -3970,6 +3998,9 @@ static int drm_dp_mst_handle_down_rep(struct drm_dp_mst_topology_mgr *mgr)
 		goto out_clear_reply;
 	}
 
+	if (!verify_rx_request_type(mgr, txmsg, msg))
+		goto out_clear_reply;
+
 	drm_dp_sideband_parse_reply(mgr, msg, &txmsg->reply);
 
 	if (txmsg->reply.reply_type == DP_SIDEBAND_REPLY_NAK) {