[PATCH mt76-next 28/29] wifi: mt76: serialize scan-link teardown with dev->mutex

[Felix Fietkau <nbd@nbd.name>]

From: Chad Monroe <chad@monroe.io>

The offchannel scan link is allocated in mt76_hw_scan() under
dev->mutex, but torn down without it: mt76_scan_complete() runs from
mt76_scan_work() on the mac80211 workqueue, or from mt76_abort_scan(),
and calls mt76_put_vif_phy_link(), whose vif_link_remove clears the
per-phy omac_mask and the device-wide vif_mask/mld_idx_mask with plain
read-modify-write. A vif link add or remove for another interface,
running concurrently under dev->mutex, can interleave with these
unlocked writes and lose an update: a cleared bit belonging to a live
link gets handed out again (two links sharing an omac/bss/wcid index,
breaking own-MAC unicast RX for the first one), or a freed bit stays
set until reboot and eventually exhausts the index space.

Take dev->mutex around the scan completion, mirroring the ROC teardown
in mt76_roc_complete_work()/mt76_abort_roc(), and switch the channel
restore to __mt76_set_channel() since the caller now holds the lock.
mt76_abort_scan() keeps cancelling the scan work before taking the
mutex, so the work-vs-abort ordering is unchanged.

Signed-off-by: Chad Monroe <chad@monroe.io>
Signed-off-by: Felix Fietkau <nbd@nbd.name>
---
 drivers/net/wireless/mediatek/mt76/scan.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/net/wireless/mediatek/mt76/scan.c b/drivers/net/wireless/mediatek/mt76/scan.c
index 3cb11689d4bf..3594b599662d 100644
--- a/drivers/net/wireless/mediatek/mt76/scan.c
+++ b/drivers/net/wireless/mediatek/mt76/scan.c
@@ -11,6 +11,8 @@ static void mt76_scan_complete(struct mt76_dev *dev, bool abort)
 		.aborted = abort,
 	};
 
+	lockdep_assert_held(&dev->mutex);
+
 	if (!phy)
 		return;
 
@@ -24,7 +26,7 @@ static void mt76_scan_complete(struct mt76_dev *dev, bool abort)
 	    !test_bit(MT76_MCU_RESET, &dev->phy.state)) {
 		bool offchannel = phy->offchannel;
 
-		mt76_set_channel(phy, &phy->main_chandef, false);
+		__mt76_set_channel(phy, &phy->main_chandef, false);
 		if (offchannel)
 			mt76_offchannel_notify(phy, false);
 	}
@@ -41,7 +43,10 @@ void mt76_abort_scan(struct mt76_dev *dev)
 	spin_unlock_bh(&dev->scan_lock);
 
 	cancel_delayed_work_sync(&dev->scan_work);
+
+	mutex_lock(&dev->mutex);
 	mt76_scan_complete(dev, true);
+	mutex_unlock(&dev->mutex);
 }
 EXPORT_SYMBOL_GPL(mt76_abort_scan);
 
@@ -139,7 +144,9 @@ void mt76_scan_work(struct work_struct *work)
 		goto probe;
 
 	if (dev->scan.chan_idx >= req->n_channels) {
+		mutex_lock(&dev->mutex);
 		mt76_scan_complete(dev, false);
+		mutex_unlock(&dev->mutex);
 		return;
 	}
 
-- 
2.53.0