linux-kernel - Re: [syzbot] general protection fault in hci_release

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Message-Id: <F5A39008-A2F6-4DE6-A23F-92ACA1E6CE9C@holtmann.org>
Date:   Mon, 2 Aug 2021 13:38:58 +0200
From:   Marcel Holtmann <marcel@...tmann.org>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:     Hillf Danton <hdanton@...a.com>,
        Luiz Augusto von Dentz <luiz.dentz@...il.com>,
        Johan Hedberg <johan.hedberg@...il.com>,
        BlueZ <linux-bluetooth@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Luiz Augusto von Dentz <luiz.von.dentz@...el.com>,
        netdev@...r.kernel.org, syzkaller-bugs@...glegroups.com,
        syzbot <syzbot+47c6d0efbb7fe2f7a5b8@...kaller.appspotmail.com>
Subject: Re: [syzbot] general protection fault in hci_release_dev

Hi Tetsuo,

>> To fix what was addressed in e305509e678b3a4a, defer putting hdev until
>> sock is released with sock locked.
>> 
>> Now only for thoughts.
> 
> Thanks for your analysis.
> 
> hci_alloc_dev() is called from hci_uart_register_dev() from  hci_uart_set_proto()
> from hci_uart_tty_ioctl(HCIUARTSETPROTO) via ld->ops->ioctl() from tty_ioctl(),
> and bt_host_release() is called from device_release() from kobject_put() from
> hci_uart_tty_close() from tty_ldisc_kill() from tty_ldisc_release() from
> tty_release_struct() from tty_release() from __fput().
> 
> The problem is that bt_host_release() is expecting that hci_register_dev()
> was called if "struct hci_dev" was allocated by hci_alloc_dev(). In other
> words, hci_register_dev() might not be called before bt_host_release().
> 
> Then, the fix I think is not to call hci_release_dev() when hci_unregister_dev()
> was not called. That is,
> 
> static void bt_host_release(struct device *dev)
> {
>        struct hci_dev *hdev = to_hci_dev(dev);
> +
> +       if (hci_dev_test_flag(hdev, HCI_UNREGISTER))
> +               hci_release_dev(hdev);
>        kfree(hdev);
>        module_put(THIS_MODULE);
> }
> 
> and remove kfree(hdev) from hci_release_dev(), for HCI_UNREGISTER flag is
> set if hci_unregister_dev() was called before bt_host_release() is called.

actually I am wondering if we should just remove the HCI LDISC support. All the tests
are focusing around the fact that you can create a line discipline as unprivileged
user.

To be honest the HCI LDISC support is not in use anymore for anything deployed after
we got around to establish TTY serdev support.

I am worried that we are trying hard to fix something in the Bluetooth core that
is actually a bug in the hci_uart driver and should be fixed solely there. Or that
driver needs to be deprecated. Are other drivers and their lifetime rules also
exhibiting these issues?

Regards

Marcel