net: stmmac: fix kernel panic due to NULL pointer dereference of xsk_pool

After free xsk_pool, there is possibility that napi polling is still
running in the middle, thus causes a kernel crash due to kernel NULL
pointer dereference of rx_q->xsk_pool and tx_q->xsk_pool.

Fix this by changing the XDP pool setup sequence to:
 1. disable napi before free xsk_pool
 2. enable napi after init xsk_pool

The following kernel panic is observed without this patch:

RIP: 0010:xsk_uses_need_wakeup+0x5/0x10
Call Trace:
stmmac_napi_poll_rxtx+0x3a9/0xae0 [stmmac]
__napi_poll+0x27/0x130
net_rx_action+0x233/0x280
__do_softirq+0xe2/0x2b6
run_ksoftirqd+0x1a/0x20
smpboot_thread_fn+0xac/0x140
? sort_range+0x20/0x20
kthread+0x124/0x150
? set_kthread_struct+0x40/0x40
ret_from_fork+0x1f/0x30
---[ end trace a77c8956b79ac107 ]---

Fixes: bba2556e ("net: stmmac: Enable RX via AF_XDP zero-copy")
Cc: <stable@vger.kernel.org> # 5.13.x
Signed-off-by: Song Yoong Siang <yoong.siang.song@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/stmicro/stmmac/stmmac_xdp.c

@@ -34,18 +34,18 @@ static int stmmac_xdp_enable_pool(struct stmmac_priv *priv,
 	need_update = netif_running(priv->dev) && stmmac_xdp_is_enabled(priv);
 
 	if (need_update) {
-		stmmac_disable_rx_queue(priv, queue);
-		stmmac_disable_tx_queue(priv, queue);
 		napi_disable(&ch->rx_napi);
 		napi_disable(&ch->tx_napi);
+		stmmac_disable_rx_queue(priv, queue);
+		stmmac_disable_tx_queue(priv, queue);
 	}
 
 	set_bit(queue, priv->af_xdp_zc_qps);
 
 	if (need_update) {
-		napi_enable(&ch->rxtx_napi);
 		stmmac_enable_rx_queue(priv, queue);
 		stmmac_enable_tx_queue(priv, queue);
+		napi_enable(&ch->rxtx_napi);
 
 		err = stmmac_xsk_wakeup(priv->dev, queue, XDP_WAKEUP_RX);
 		if (err)
@@ -72,10 +72,10 @@ static int stmmac_xdp_disable_pool(struct stmmac_priv *priv, u16 queue)
 	need_update = netif_running(priv->dev) && stmmac_xdp_is_enabled(priv);
 
 	if (need_update) {
+		napi_disable(&ch->rxtx_napi);
 		stmmac_disable_rx_queue(priv, queue);
 		stmmac_disable_tx_queue(priv, queue);
 		synchronize_rcu();
-		napi_disable(&ch->rxtx_napi);
 	}
 
 	xsk_pool_dma_unmap(pool, STMMAC_RX_DMA_ATTR);
@@ -83,10 +83,10 @@ static int stmmac_xdp_disable_pool(struct stmmac_priv *priv, u16 queue)
 	clear_bit(queue, priv->af_xdp_zc_qps);
 
 	if (need_update) {
-		napi_enable(&ch->rx_napi);
-		napi_enable(&ch->tx_napi);
 		stmmac_enable_rx_queue(priv, queue);
 		stmmac_enable_tx_queue(priv, queue);
+		napi_enable(&ch->rx_napi);
+		napi_enable(&ch->tx_napi);
 	}
 
 	return 0;