drm/dp_mst: Fix down request message timeout handling

If receiving a reply for an MST down request message times out, the
thread receiving the reply in drm_dp_mst_handle_down_rep() could try to
dereference the drm_dp_sideband_msg_tx txmsg request message after the
thread waiting for the reply - calling drm_dp_mst_wait_tx_reply() - has
timed out and freed txmsg, hence leading to a use-after-free in
drm_dp_mst_handle_down_rep().

Prevent the above by holding the drm_dp_mst_topology_mgr::qlock in
drm_dp_mst_handle_down_rep() for the whole duration txmsg is looked up
from the request list and dereferenced.

v2: Fix unlocking mgr->qlock after verify_rx_request_type() fails.

Cc: Lyude Paul <lyude@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241203174632.2941402-1-imre.deak@intel.com

Author: ideak

drivers/gpu/drm/display/drm_dp_mst_topology.c

@@ -3984,9 +3984,9 @@ static int drm_dp_mst_handle_down_rep(struct drm_dp_mst_topology_mgr *mgr)
 
 	/* find the message */
 	mutex_lock(&mgr->qlock);
+
 	txmsg = list_first_entry_or_null(&mgr->tx_msg_downq,
 					 struct drm_dp_sideband_msg_tx, next);
-	mutex_unlock(&mgr->qlock);
 
 	/* Were we actually expecting a response, and from this mstb? */
 	if (!txmsg || txmsg->dst != mstb) {
@@ -3995,11 +3995,17 @@ static int drm_dp_mst_handle_down_rep(struct drm_dp_mst_topology_mgr *mgr)
 		hdr = &msg->initial_hdr;
 		drm_dbg_kms(mgr->dev, "Got MST reply with no msg %p %d %d %02x %02x\n",
 			    mstb, hdr->seqno, hdr->lct, hdr->rad[0], msg->msg[0]);
+
+		mutex_unlock(&mgr->qlock);
+
 		goto out_clear_reply;
 	}
 
-	if (!verify_rx_request_type(mgr, txmsg, msg))
+	if (!verify_rx_request_type(mgr, txmsg, msg)) {
+		mutex_unlock(&mgr->qlock);
+
 		goto out_clear_reply;
+	}
 
 	drm_dp_sideband_parse_reply(mgr, msg, &txmsg->reply);
 
@@ -4013,9 +4019,9 @@ static int drm_dp_mst_handle_down_rep(struct drm_dp_mst_topology_mgr *mgr)
 			    txmsg->reply.u.nak.nak_data);
 	}
 
-	mutex_lock(&mgr->qlock);
 	txmsg->state = DRM_DP_SIDEBAND_TX_RX;
 	list_del(&txmsg->next);
+
 	mutex_unlock(&mgr->qlock);
 
 	wake_up_all(&mgr->tx_waitq);