mac802154: Fix potential RCU dereference issue in mac802154_scan_worker

In the `mac802154_scan_worker` function, the `scan_req->type` field was
accessed after the RCU read-side critical section was unlocked. According
to RCU usage rules, this is illegal and can lead to unpredictable
behavior, such as accessing memory that has been updated or causing
use-after-free issues.

This possible bug was identified using a static analysis tool developed
by myself, specifically designed to detect RCU-related issues.

To address this, the `scan_req->type` value is now stored in a local
variable `scan_req_type` while still within the RCU read-side critical
section. The `scan_req_type` is then used after the RCU lock is released,
ensuring that the type value is safely accessed without violating RCU
rules.

Fixes: e2c3e6f ("mac802154: Handle active scanning")
Cc: [email protected]
Signed-off-by: Jiawei Ye <[email protected]>
Acked-by: Miquel Raynal <[email protected]>
Reviewed-by: Przemek Kitszel <[email protected]>
Link: https://lore.kernel.org/[email protected]
Signed-off-by: Stefan Schmidt <[email protected]>

Author: pandayeah

net/mac802154/scan.c

@@ -176,6 +176,7 @@ void mac802154_scan_worker(struct work_struct *work)
 	struct ieee802154_local *local =
 		container_of(work, struct ieee802154_local, scan_work.work);
 	struct cfg802154_scan_request *scan_req;
+	enum nl802154_scan_types scan_req_type;
 	struct ieee802154_sub_if_data *sdata;
 	unsigned int scan_duration = 0;
 	struct wpan_phy *wpan_phy;
@@ -209,6 +210,7 @@ void mac802154_scan_worker(struct work_struct *work)
 	}
 
 	wpan_phy = scan_req->wpan_phy;
+	scan_req_type = scan_req->type;
 	scan_req_duration = scan_req->duration;
 
 	/* Look for the next valid chan */
@@ -246,7 +248,7 @@ void mac802154_scan_worker(struct work_struct *work)
 		goto end_scan;
 	}
 
-	if (scan_req->type == NL802154_SCAN_ACTIVE) {
+	if (scan_req_type == NL802154_SCAN_ACTIVE) {
 		ret = mac802154_transmit_beacon_req(local, sdata);
 		if (ret)
 			dev_err(&sdata->dev->dev,