Bluetooth: btmtksdio: fix use-after-free at btmtksdio_recv_event

We should not access skb buffer data anymore after hci_recv_frame was
called.

[   39.634809] BUG: KASAN: use-after-free in btmtksdio_recv_event+0x1b0
[   39.634855] Read of size 1 at addr ffffff80cf28a60d by task kworker
[   39.634962] Call trace:
[   39.634974]  dump_backtrace+0x0/0x3b8
[   39.634999]  show_stack+0x20/0x2c
[   39.635016]  dump_stack_lvl+0x60/0x78
[   39.635040]  print_address_description+0x70/0x2f0
[   39.635062]  kasan_report+0x154/0x194
[   39.635079]  __asan_report_load1_noabort+0x44/0x50
[   39.635099]  btmtksdio_recv_event+0x1b0/0x1c4
[   39.635129]  btmtksdio_txrx_work+0x6cc/0xac4
[   39.635157]  process_one_work+0x560/0xc5c
[   39.635177]  worker_thread+0x7ec/0xcc0
[   39.635195]  kthread+0x2d0/0x3d0
[   39.635215]  ret_from_fork+0x10/0x20
[   39.635247] Allocated by task 0:
[   39.635260] (stack is not available)
[   39.635281] Freed by task 2392:
[   39.635295]  kasan_save_stack+0x38/0x68
[   39.635319]  kasan_set_track+0x28/0x3c
[   39.635338]  kasan_set_free_info+0x28/0x4c
[   39.635357]  ____kasan_slab_free+0x104/0x150
[   39.635374]  __kasan_slab_free+0x18/0x28
[   39.635391]  slab_free_freelist_hook+0x114/0x248
[   39.635410]  kfree+0xf8/0x2b4
[   39.635427]  skb_free_head+0x58/0x98
[   39.635447]  skb_release_data+0x2f4/0x410
[   39.635464]  skb_release_all+0x50/0x60
[   39.635481]  kfree_skb+0xc8/0x25c
[   39.635498]  hci_event_packet+0x894/0xca4 [bluetooth]
[   39.635721]  hci_rx_work+0x1c8/0x68c [bluetooth]
[   39.635925]  process_one_work+0x560/0xc5c
[   39.635951]  worker_thread+0x7ec/0xcc0
[   39.635970]  kthread+0x2d0/0x3d0
[   39.635990]  ret_from_fork+0x10/0x20
[   39.636021] The buggy address belongs to the object at ffffff80cf28a600
                which belongs to the cache kmalloc-512 of size 512
[   39.636039] The buggy address is located 13 bytes inside of
                512-byte region [ffffff80cf28a600, ffffff80cf28a800)

Fixes: 9aebfd4a ("Bluetooth: mediatek: add support for MediaTek MT7663S and MT7668S SDIO devices")
Co-developed-by: Yake Yang <yake.yang@mediatek.com>
Signed-off-by: Yake Yang <yake.yang@mediatek.com>
Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

drivers/bluetooth/btmtksdio.c

@@ -379,6 +379,7 @@ static int btmtksdio_recv_event(struct hci_dev *hdev, struct sk_buff *skb)
 {
 	struct btmtksdio_dev *bdev = hci_get_drvdata(hdev);
 	struct hci_event_hdr *hdr = (void *)skb->data;
+	u8 evt = hdr->evt;
 	int err;
 
 	/* When someone waits for the WMT event, the skb is being cloned
@@ -396,7 +397,7 @@ static int btmtksdio_recv_event(struct hci_dev *hdev, struct sk_buff *skb)
 	if (err < 0)
 		goto err_free_skb;
 
-	if (hdr->evt == HCI_EV_WMT) {
+	if (evt == HCI_EV_WMT) {
 		if (test_and_clear_bit(BTMTKSDIO_TX_WAIT_VND_EVT,
 				       &bdev->tx_state)) {
 			/* Barrier to sync with other CPUs */