[PATCH 02/11] wifi: mt76: mt792x: fix NULL pointer and firmware reload issues

[Zac <zac@zacbowling.com>]

From: Zac Bowling <zac@zacbowling.com>

This patch combines two fixes for the shared mt792x code used by both
MT7921 and MT7925 drivers:

1. Fix NULL pointer dereference in TX path:

Add NULL pointer checks in mt792x_tx() to prevent kernel crashes when
transmitting packets during MLO link removal.

The function calls mt792x_sta_to_link() which can return NULL if the
link is being removed, but the return value was dereferenced without
checking. Similarly, the RCU-protected link_conf and link_sta pointers
were used without NULL validation.

This race can occur when:
- A packet is queued for transmission
- Concurrently, the link is being removed (mt7925_mac_link_sta_remove)
- mt792x_sta_to_link() returns NULL for the removed link
- Kernel crashes on wcid = &mlink->wcid dereference

Fix by checking mlink, conf, and link_sta before use, freeing the SKB
and returning early if any pointer is NULL.

2. Fix firmware reload failure after previous load crash:

If the firmware loading process crashes or is interrupted after
acquiring the patch semaphore but before releasing it, subsequent
firmware load attempts will fail with 'Failed to get patch semaphore'.

Apply the same fix from MT7915 (commit 79dd14f): release the patch
semaphore before starting firmware load and restart MCU firmware to
ensure clean state.

Fixes: c74df1c067f2 ("wifi: mt76: mt792x: introduce mt792x-lib module")
Fixes: 583204ae70f9 ("wifi: mt76: mt792x: move mt7921_load_firmware in mt792x-lib module")
Link: https://github.com/openwrt/mt76/commit/79dd14f2e8161b656341b6653261779199aedbe4
Signed-off-by: Zac Bowling <zac@zacbowling.com>
---
 .../net/wireless/mediatek/mt76/mt792x_core.c  | 27 +++++++++++++++++--
 1 file changed, 25 insertions(+), 2 deletions(-)

diff --git a/drivers/net/wireless/mediatek/mt76/mt792x_core.c b/drivers/net/wireless/mediatek/mt76/mt792x_core.c
index f2ed16feb6c1..05598202b488 100644
--- a/drivers/net/wireless/mediatek/mt76/mt792x_core.c
+++ b/drivers/net/wireless/mediatek/mt76/mt792x_core.c
@@ -95,6 +95,8 @@ void mt792x_tx(struct ieee80211_hw *hw, struct ieee80211_tx_control *control,
 				       IEEE80211_TX_CTRL_MLO_LINK);
 		sta = (struct mt792x_sta *)control->sta->drv_priv;
 		mlink = mt792x_sta_to_link(sta, link_id);
+		if (!mlink)
+			goto free_skb;
 		wcid = &mlink->wcid;
 	}
 
@@ -113,9 +115,12 @@ void mt792x_tx(struct ieee80211_hw *hw, struct ieee80211_tx_control *control,
 		link_id = wcid->link_id;
 		rcu_read_lock();
 		conf = rcu_dereference(vif->link_conf[link_id]);
-		memcpy(hdr->addr2, conf->addr, ETH_ALEN);
-
 		link_sta = rcu_dereference(control->sta->link[link_id]);
+		if (!conf || !link_sta) {
+			rcu_read_unlock();
+			goto free_skb;
+		}
+		memcpy(hdr->addr2, conf->addr, ETH_ALEN);
 		memcpy(hdr->addr1, link_sta->addr, ETH_ALEN);
 
 		if (vif->type == NL80211_IFTYPE_STATION)
@@ -136,6 +141,10 @@ void mt792x_tx(struct ieee80211_hw *hw, struct ieee80211_tx_control *control,
 	}
 
 	mt76_connac_pm_queue_skb(hw, &dev->pm, wcid, skb);
+	return;
+
+free_skb:
+	ieee80211_free_txskb(hw, skb);
 }
 EXPORT_SYMBOL_GPL(mt792x_tx);
 
@@ -927,6 +936,20 @@ int mt792x_load_firmware(struct mt792x_dev *dev)
 {
 	int ret;
 
+	/* Release semaphore if taken by previous failed load attempt.
+	 * This prevents "Failed to get patch semaphore" errors when
+	 * recovering from firmware crashes or suspend/resume failures.
+	 */
+	ret = mt76_connac_mcu_patch_sem_ctrl(&dev->mt76, false);
+	if (ret < 0)
+		dev_dbg(dev->mt76.dev, "Semaphore release returned %d (may be expected)\n", ret);
+
+	/* Always restart MCU to ensure clean state before loading firmware */
+	mt76_connac_mcu_restart(&dev->mt76);
+
+	/* Wait for MCU to be ready after restart */
+	msleep(100);
+
 	ret = mt76_connac2_load_patch(&dev->mt76, mt792x_patch_name(dev));
 	if (ret)
 		return ret;
-- 
2.52.0