netdev - Re: Re: [PATCH net v5 2/2] nfc: nfcmrvl: main: reorder destructive operations in nfcmrvl_nci_unregister

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Date:   Mon, 2 May 2022 15:25:14 +0800 (GMT+08:00)
From:   duoming@....edu.cn
To:     "Krzysztof Kozlowski" <krzysztof.kozlowski@...aro.org>
Cc:     linux-kernel@...r.kernel.org, kuba@...nel.org,
        gregkh@...uxfoundation.org, davem@...emloft.net,
        edumazet@...gle.com, pabeni@...hat.com, alexander.deucher@....com,
        akpm@...ux-foundation.org, broonie@...nel.org,
        netdev@...r.kernel.org, linma@....edu.cn
Subject: Re: Re: [PATCH net v5 2/2] nfc: nfcmrvl: main: reorder destructive
 operations in nfcmrvl_nci_unregister_dev to avoid bugs




> -----原始邮件-----
> 发件人: "Krzysztof Kozlowski" <krzysztof.kozlowski@...aro.org>
> 发送时间: 2022-05-02 14:34:07 (星期一)
> 收件人: duoming@....edu.cn
> 抄送: linux-kernel@...r.kernel.org, kuba@...nel.org, gregkh@...uxfoundation.org, davem@...emloft.net, edumazet@...gle.com, pabeni@...hat.com, alexander.deucher@....com, akpm@...ux-foundation.org, broonie@...nel.org, netdev@...r.kernel.org, linma@....edu.cn
> 主题: Re: [PATCH net v5 2/2] nfc: nfcmrvl: main: reorder destructive operations in nfcmrvl_nci_unregister_dev to avoid bugs
> 
> On 29/04/2022 11:13, duoming@....edu.cn wrote:
> > Hello,
> > 
> > On Fri, 29 Apr 2022 09:27:48 +0200 Krzysztof wrote:
> > 
> >>> There are destructive operations such as nfcmrvl_fw_dnld_abort and
> >>> gpio_free in nfcmrvl_nci_unregister_dev. The resources such as firmware,
> >>> gpio and so on could be destructed while the upper layer functions such as
> >>> nfcmrvl_fw_dnld_start and nfcmrvl_nci_recv_frame is executing, which leads
> >>> to double-free, use-after-free and null-ptr-deref bugs.
> >>>
> >>> There are three situations that could lead to double-free bugs.
> >>>
> >>> The first situation is shown below:
> >>>
> >>>    (Thread 1)                 |      (Thread 2)
> >>> nfcmrvl_fw_dnld_start         |
> >>>  ...                          |  nfcmrvl_nci_unregister_dev
> >>>  release_firmware()           |   nfcmrvl_fw_dnld_abort
> >>>   kfree(fw) //(1)             |    fw_dnld_over
> >>>                               |     release_firmware
> >>>   ...                         |      kfree(fw) //(2)
> >>>                               |     ...
> >>>
> >>> The second situation is shown below:
> >>>
> >>>    (Thread 1)                 |      (Thread 2)
> >>> nfcmrvl_fw_dnld_start         |
> >>>  ...                          |
> >>>  mod_timer                    |
> >>>  (wait a time)                |
> >>>  fw_dnld_timeout              |  nfcmrvl_nci_unregister_dev
> >>>    fw_dnld_over               |   nfcmrvl_fw_dnld_abort
> >>>     release_firmware          |    fw_dnld_over
> >>>      kfree(fw) //(1)          |     release_firmware
> >>>      ...                      |      kfree(fw) //(2)
> >>
> >> How exactly the case here is being prevented?
> >>
> >> If nfcmrvl_nci_unregister_dev() happens slightly earlier, before
> >> fw_dnld_timeout() on the left side (T1), the T1 will still hit it, won't it?
> > 
> > I think it could be prevented. We use nci_unregister_device() to synchronize, if the
> > firmware download routine is running, the cleanup routine will wait it to finish. 
> > The flag "fw_download_in_progress" will be set to false, if the the firmware download
> > routine is finished. 
> 
> fw_download_in_progress is not synchronized in
> nfcmrvl_nci_unregister_dev(), so even if fw_dnld_timeout() set it to
> true, the nfcmrvl_nci_unregister_dev() happening concurrently will not
> see updated fw_download_in_progress.

The fw_download_in_progress is set to false in nfc_fw_download(). The nfc_fw_download() is
synchronized with nfc_unregister_device(). If nfc_fw_download() is running, nfc_unregister_device()
will wait nfc_fw_download() to finish. So the nfcmrvl_nci_unregister_dev() could see the updated
fw_download_in_progress. The process is shown below:

        (Thread 1)                                         |       (Thread 2)
 nfcmrvl_nci_unregister_dev                                | nfc_fw_download
   nci_unregister_device                                   |  ...
                                                           |  device_lock()
     ...                                                   |  dev->fw_download_in_progress = false; //(1)
                                                           |  device_unlock()
     nfc_unregister_device                                 | 
   if (priv->ndev->nfc_dev->fw_download_in_progress) //(2) | 
     nfcmrvl_fw_dnld_abort(priv); //not execute            |   

We set fw_download_in_progress to false in position (1) and the check in position (2) will fail,
the nfcmrvl_fw_dnld_abort() in nfcmrvl_nci_unregister_dev() will not execute. So the double-free
bugs could be prevented.

> > Although the timer handler fw_dnld_timeout() could be running, nfcmrvl_nci_unregister_dev()
> > will check the flag "fw_download_in_progress" which is already set to false and nfcmrvl_fw_dnld_abort()
> > in nfcmrvl_nci_unregister_dev() will not execute.
> 
> I am sorry, but you cannot move code around hoping it will by itself
> solve synchronization issues.

I think this solution sove synchronization issues. If you still have any questions welcome to ask me.

Best regards,
Duoming Zhou