linux-kernel - Re: [PATCH v2] usb:xhci fix panic in xhci_free_virt_devices_depth

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Date:   Tue, 7 Nov 2017 09:55:23 +0800
From:   Chen Yu <chenyu56@...wei.com>
To:     Mathias Nyman <mathias.nyman@...ux.intel.com>,
        Greg KH <gregkh@...uxfoundation.org>
CC:     <wangbinghui@...ilicon.com>, <mathias.nyman@...el.com>,
        <linux-usb@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <fanning4@...ilicon.com>, <lirui39@...ilicon.com>,
        <yangdi10@...ilicon.com>, <groeck@...gle.com>,
        <john.stultz@...aro.org>
Subject: Re: [PATCH v2] usb:xhci fix panic in
 xhci_free_virt_devices_depth_first



On 2017/11/6 22:00, Mathias Nyman wrote:
> On 06.11.2017 14:36, Chen Yu wrote:
>>
>>
>> On 2017/11/6 19:32, Greg KH wrote:
>>>> A simple process is as below:
>>>>     xhci_plat_probe()
>>>>                 |
>>>>     usb_add_hcd()                    xhci_plat_remove()
>>>>            |                        |
>>>>     find some device                usb_remove_hcd()
>>>>            |                        |
>>>>     hub_port_connect() -> usb_alloc_dev()        usb_disconnect()
>>>>            |                        |
>>>>     before hub_enable_device()            xhci_stop()
>>>>                                 |
>>>>                             xhci_mem_cleanup()
>>>>                                 |
>>>>                             xhci_free_virt_devices_depth_first()
>>>>                                 |
>>>>                             real_port is 0 access xhci->rh_bw[vdev->real_port-1]
>>>>
>>>> The problem came from https://bugs.96boards.org/show_bug.cgi?id=535
>>>> Also look at crbug.com/700041
>>>
>>> Then the bug needs to be fixed, throwing a huge kernel trace message
>>> into the kernel log is not "fixing" the problem at all, right?
>>>
>>> thanks,
>>>
>>> greg k-h
>>>
>>> .
>>>
>>
>> You are right, the way that xhci_plat_remove() to be called needs to be fixed.
>> But there is still possibility for this crash.
>> What do you think if just add an "xhci_warn" instead of "WARN_ON"?
>> +       if (!vdev->real_port) {
>> +               xhci_warn(xhci, "Bad vdev->real_port\n");
>> +               goto out;
>> +       }
>> +
>>
> 
> This patch solves the issue, just drop all the error messages.
> 
> vdev->real_port is not set until the the device enable/address
> stage, and we know it won't have any children yet then, so no need to
> worry about a child having tt pointers to this device.
> 
> The "goto out" to xhci_free_virt_device() you do is fine here.
> 
> xhci_plat_remove() is the .remove callback for the xhci platform driver.
> It might get called before a device is properly enabled/addressed.
> Not really a error. A unlikely but possible situation.
> 
> xhci_free_tt_info() already has a similar check

Thanks a lot for your analysis and suggestion!
The check within xhci_free_tt_info() is a better one.
I will upload a new patch like the check in xhci_free_tt_info().

> 
> Thanks
> -Mathias
> 
> 
> .
>