netdev - Re: [syzbot] general protection fault in hci_release

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Date:   Mon, 2 Aug 2021 20:02:24 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Hillf Danton <hdanton@...a.com>,
        Luiz Augusto von Dentz <luiz.dentz@...il.com>
Cc:     johan.hedberg@...il.com, linux-bluetooth@...r.kernel.org,
        linux-kernel@...r.kernel.org, luiz.von.dentz@...el.com,
        marcel@...tmann.org, netdev@...r.kernel.org,
        syzkaller-bugs@...glegroups.com,
        syzbot <syzbot+47c6d0efbb7fe2f7a5b8@...kaller.appspotmail.com>
Subject: Re: [syzbot] general protection fault in hci_release_dev

Hello.

On 2021/08/02 18:54, Hillf Danton wrote:
> 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.