Re: [PATCH] mac80211: fix race in ieee80211_register_hw()

[Kalle Valo <kvalo@codeaurora.org>]

Sumit Garg <sumit.garg@linaro.org> writes:

> A race condition leading to a kernel crash is observed during invocation
> of ieee80211_register_hw() on a dragonboard410c device having wcn36xx
> driver built as a loadable module along with a wifi manager in user-space
> waiting for a wifi device (wlanX) to be active.
>
> Sequence diagram for a particular kernel crash scenario:
>
>     user-space  ieee80211_register_hw()  RX IRQ
>     +++++++++++++++++++++++++++++++++++++++++++++
>        |                    |             |
>        |<---wlan0---wiphy_register()      |
>        |----start wlan0---->|             |
>        |                    |<---IRQ---(RX packet)
>        |              Kernel crash        |
>        |              due to unallocated  |
>        |              workqueue.          |
>        |                    |             |
>        |       alloc_ordered_workqueue()  |
>        |                    |             |
>        |              Misc wiphy init.    |
>        |                    |             |
>        |            ieee80211_if_add()    |
>        |                    |             |
>
> As evident from above sequence diagram, this race condition isn't specific
> to a particular wifi driver but rather the initialization sequence in
> ieee80211_register_hw() needs to be fixed. So re-order the initialization
> sequence and the updated sequence diagram would look like:
>
>     user-space  ieee80211_register_hw()  RX IRQ
>     +++++++++++++++++++++++++++++++++++++++++++++
>        |                    |             |
>        |       alloc_ordered_workqueue()  |
>        |                    |             |
>        |              Misc wiphy init.    |
>        |                    |             |
>        |<---wlan0---wiphy_register()      |
>        |----start wlan0---->|             |
>        |                    |<---IRQ---(RX packet)
>        |                    |             |
>        |            ieee80211_if_add()    |
>        |                    |             |
>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Sumit Garg <sumit.garg@linaro.org>

I have understood that no frames should be received until mac80211 calls
struct ieee80211_ops::start:

 * @start: Called before the first netdevice attached to the hardware
 *         is enabled. This should turn on the hardware and must turn on
 *         frame reception (for possibly enabled monitor interfaces.)
   
So I would claim that this is a bug in wcn36xx.

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