[PATCH mt76 v2] wifi: mt76: fix unlocked wcid check in mt76_tx_status_skb_add()

[Ryan Leung]

mt76_tx_status_skb_add() checks rcu_access_pointer(dev->wcid[wcid->idx])
before taking dev->status_lock, then allocates a pktid afterwards. A
concurrent mt76_wcid_cleanup() can run between the check and the lock,
letting a stale wcid be re-added to dev->wcid_list after teardown.
Worse, the check only tests for a non-NULL slot, so it also passes if a
different wcid has since been published at the same idx.

Move the check under status_lock, alongside idr_alloc(), and compare
identity (rcu_access_pointer(dev->wcid[wcid->idx]) != wcid) instead of
just testing for non-NULL, so that a reused index is correctly rejected.

Fixes: fcfe1b5e162b ("mt76: fix tx status related use-after-free race on station removal")
Signed-off-by: Ryan Leung <untilscour at protonmail.com>
---
Changes in v2:
- check that dev->wcid[wcid->idx] still points at this wcid, not just
  that it is non-NULL, since the slot can be reused by a different
  station between the unlocked check and the lock being taken.
- Link to v1: https://patch.msgid.link/20260821-mt76-stale-wcid-race-v1-1-d6de3f842506@protonmail.com
---
 drivers/net/wireless/mediatek/mt76/tx.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/net/wireless/mediatek/mt76/tx.c b/drivers/net/wireless/mediatek/mt76/tx.c
index 3707ee19e4ae..0f74d808d328 100644
--- a/drivers/net/wireless/mediatek/mt76/tx.c
+++ b/drivers/net/wireless/mediatek/mt76/tx.c
@@ -129,7 +129,7 @@ mt76_tx_status_skb_add(struct mt76_dev *dev, struct mt76_wcid *wcid,
 
 	memset(cb, 0, sizeof(*cb));
 
-	if (!wcid || !rcu_access_pointer(dev->wcid[wcid->idx]))
+	if (!wcid)
 		return MT_PACKET_ID_NO_ACK;
 
 	if (info->flags & IEEE80211_TX_CTL_NO_ACK)
@@ -147,6 +147,11 @@ mt76_tx_status_skb_add(struct mt76_dev *dev, struct mt76_wcid *wcid,
 
 	spin_lock_bh(&dev->status_lock);
 
+	if (rcu_access_pointer(dev->wcid[wcid->idx]) != wcid) {
+		pid = MT_PACKET_ID_NO_ACK;
+		goto out;
+	}
+
 	pid = idr_alloc(&wcid->pktid, skb, MT_PACKET_ID_FIRST,
 			MT_PACKET_ID_MASK, GFP_ATOMIC);
 	if (pid < 0) {

---
base-commit: ca800a9302764c445de0da0e84d2252400a770ee
change-id: 20260821-mt76-stale-wcid-race-4d92f898437b

Best regards,
--  
Ryan Leung <untilscour at protonmail.com>