Re: [PATCH v2] Bluetooth: hci_uart: fix UAF in hci_uart_tty_close()

[Greg KH <greg@kroah.com>]

On Thu, May 14, 2026 at 11:17:22PM +0800, w15303746062@163.com wrote:
> From: Mingyu Wang <25181214217@stu.xidian.edu.cn>
> 
> A Use-After-Free (UAF) vulnerability and a subsequent kernel panic were
> observed in hci_uart_write_work() due to a race condition between the
> initialization of the HCI UART line discipline and concurrent TTY hangup.
> 
> This issue was triggered by our custom device emulation and fuzzing
> framework (DevGen) on the v6.18 kernel. Due to the highly timing-dependent
> nature of this race condition (requiring a precise interleaving of
> TIOCVHANGUP and protocol setup), Syzkaller failed to extract a reliable
> standalone C reproducer (reproducer is too unreliable: 0.00).
> 
> The crash trace is as follows:
>   ODEBUG: free active (active state 0) object: ffff88804024e870 object type: work_struct hint: hci_uart_write_work+0x0/0x940
>   WARNING: CPU: 0 PID: 338273 at lib/debugobjects.c:612 debug_print_object+0x1a2/0x2b0
>   ...
>   Call Trace:
>    <TASK>
>    debug_check_no_obj_freed+0x3ec/0x520
>    kfree+0x3f0/0x6c0
>    hci_uart_tty_close+0x127/0x2a0
>    tty_ldisc_close+0x113/0x1a0
>    tty_ldisc_kill+0x8e/0x150
>    tty_ldisc_hangup+0x3c1/0x730
>    __tty_hangup.part.0+0x3fd/0x8a0
>    tty_ioctl+0x120f/0x1690
>    __x64_sys_ioctl+0x18f/0x210
>    do_syscall_64+0xcb/0xfa0
>    entry_SYSCALL_64_after_hwframe+0x77/0x7f
>    </TASK>
> 
> The issue arises because the workqueues (init_ready and write_work) are
> only cancelled if the HCI_UART_PROTO_READY flag is set. However, during
> the protocol initialization phase (HCI_UART_PROTO_INIT), the underlying
> protocol may schedule work. If a hangup occurs before the setup completes
> and the READY flag is set, hci_uart_tty_close() skips the cancel_work_sync()
> calls and proceeds to free the `hu` struct. When the delayed workqueue
> executes, it blindly dereferences the freed `hu` struct.
> 
> Fix this by moving the cancel_work_sync() calls outside the
> HCI_UART_PROTO_READY check, ensuring that any pending works are
> unconditionally cancelled before the hci_uart structure is freed.
> Note that hu->init_ready and hu->write_work are initialized in
> hci_uart_tty_open(), so it is always safe to call cancel_work_sync()
> on them in hci_uart_tty_close(), even if the protocol was never
> fully attached.
> 
> Fixes: 3b799254cf6f ("Bluetooth: hci_uart: Cancel init work before unregistering")
> 
> Signed-off-by: Mingyu Wang <25181214217@stu.xidian.edu.cn>
> ---
> Changes in v2:
> - Added KASAN/ODEBUG crash trace.
> - Added explanation for the absence of a standalone reproducer (highly timing-dependent race condition).
> - Added Fixes tag pointing to commit 3b799254cf6f.
> 
>  drivers/bluetooth/hci_ldisc.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/bluetooth/hci_ldisc.c b/drivers/bluetooth/hci_ldisc.c
> index 275ea865bc29..566e1c525ee2 100644
> --- a/drivers/bluetooth/hci_ldisc.c
> +++ b/drivers/bluetooth/hci_ldisc.c
> @@ -544,14 +544,18 @@ static void hci_uart_tty_close(struct tty_struct *tty)
>  	if (hdev)
>  		hci_uart_close(hdev);
>  
> +	/*
> +	 * Always cancel workqueues unconditionally before freeing the hu
> +	 * struct, as they might be active during the PROTO_INIT phase.
> +	 */
> +	cancel_work_sync(&hu->init_ready);
> +	cancel_work_sync(&hu->write_work);
> +
>  	if (test_bit(HCI_UART_PROTO_READY, &hu->flags)) {
>  		percpu_down_write(&hu->proto_lock);
>  		clear_bit(HCI_UART_PROTO_READY, &hu->flags);
>  		percpu_up_write(&hu->proto_lock);
>  
> -		cancel_work_sync(&hu->init_ready);
> -		cancel_work_sync(&hu->write_work);
> -
>  		if (hdev) {
>  			if (test_bit(HCI_UART_REGISTERED, &hu->flags))
>  				hci_unregister_dev(hdev);
> -- 
> 2.34.1
> 
> 

Hi,

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