Re: [syzbot] [bluetooth?] [bcachefs?] KASAN: slab-use-after-free Read in hci_uart_write_work

[Hillf Danton <hdanton@sina.com>]

> Date: Mon, 14 Jul 2025 10:09:31 -0700	[thread overview]
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    3f31a806a62e Merge tag 'mm-hotfixes-stable-2025-07-11-16-1..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=174b07d4580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=b309c907eaab29da
> dashboard link: https://syzkaller.appspot.com/bug?extid=fde6bd779f78e6e0992e
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=127ece8c580000

#syz test

--- x/drivers/bluetooth/hci_ldisc.c
+++ y/drivers/bluetooth/hci_ldisc.c
@@ -100,13 +100,9 @@ static inline struct sk_buff *hci_uart_d
 	struct sk_buff *skb = hu->tx_skb;
 
 	if (!skb) {
-		percpu_down_read(&hu->proto_lock);
-
 		if (test_bit(HCI_UART_PROTO_READY, &hu->flags) ||
 		    test_bit(HCI_UART_PROTO_INIT, &hu->flags))
 			skb = hu->proto->dequeue(hu);
-
-		percpu_up_read(&hu->proto_lock);
 	} else {
 		hu->tx_skb = NULL;
 	}
@@ -154,6 +150,11 @@ static void hci_uart_write_work(struct w
 	/* REVISIT: should we cope with bad skbs or ->write() returning
 	 * and error value ?
 	 */
+	percpu_down_read(&hu->proto_lock);
+	if (!test_bit(HCI_UART_PROTO_READY, &hu->flags)) {
+		percpu_up_read(&hu->proto_lock);
+		return;
+	}
 
 restart:
 	clear_bit(HCI_UART_TX_WAKEUP, &hu->tx_state);
@@ -179,6 +180,7 @@ restart:
 	if (test_bit(HCI_UART_TX_WAKEUP, &hu->tx_state))
 		goto restart;
 
+	percpu_up_read(&hu->proto_lock);
 	wake_up_bit(&hu->tx_state, HCI_UART_SENDING);
 }
 
@@ -231,9 +233,12 @@ static int hci_uart_flush(struct hci_dev
 
 	BT_DBG("hdev %p tty %p", hdev, tty);
 
+	percpu_down_write(&hu->proto_lock);
 	if (hu->tx_skb) {
-		kfree_skb(hu->tx_skb); hu->tx_skb = NULL;
+		kfree_skb(hu->tx_skb);
+		hu->tx_skb = NULL;
 	}
+	percpu_up_write(&hu->proto_lock);
 
 	/* Flush any pending characters in the driver and discipline. */
 	tty_ldisc_flush(tty);
--