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Message-ID: <ee168acb768d87776db2be4e978616f9187908d0.camel@sipsolutions.net>
Subject: Re: [PATCH] mac80211: fix race in ieee80211_register_hw()
From:   Johannes Berg <johannes@sipsolutions.net>
To:     Kalle Valo <kvalo@codeaurora.org>
Cc:     Sumit Garg <sumit.garg@linaro.org>, linux-wireless@vger.kernel.org,
        davem@davemloft.net, kuba@kernel.org, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, matthias.schoepfer@ithinx.io,
        Philipp.Berg@liebherr.com, Michael.Weitner@liebherr.com,
        daniel.thompson@linaro.org, loic.poulain@linaro.org,
        stable@vger.kernel.org
Date:   Mon, 06 Apr 2020 15:07:57 +0200
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On Mon, 2020-04-06 at 16:04 +0300, Kalle Valo wrote:
> Johannes Berg <johannes@sipsolutions.net> writes:
> 
> > On Mon, 2020-04-06 at 15:52 +0300, Kalle Valo wrote:
> > > Johannes Berg <johannes@sipsolutions.net> writes:
> > > 
> > > > On Mon, 2020-04-06 at 15:44 +0300, Kalle Valo wrote:
> > > > > >     user-space  ieee80211_register_hw()  RX IRQ
> > > > > >     +++++++++++++++++++++++++++++++++++++++++++++
> > > > > >        |                    |             |
> > > > > >        |<---wlan0---wiphy_register()      |
> > > > > >        |----start wlan0---->|             |
> > > > > >        |                    |<---IRQ---(RX packet)
> > > > > >        |              Kernel crash        |
> > > > > >        |              due to unallocated  |
> > > > > >        |              workqueue.          |
> > > > 
> > > > [snip]
> > > > 
> > > > > 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.)
> > > > 
> > > > True, but I think he's saying that you can actually add and configure an
> > > > interface as soon as the wiphy is registered?
> > > 
> > > With '<---IRQ---(RX packet)' I assumed wcn36xx is delivering a frame to
> > > mac80211 using ieee80211_rx(), but of course I'm just guessing here.
> > 
> > Yeah, but that could be legitimate?
> 
> Ah, I misunderstood then. The way I have understood is that no rx frames
> should be delivered (= calling ieee80211_rx()_ before start() is called,
> but if that's not the case please ignore me :)

No no, that _is_ the case. But I think the "start wlan0" could end up
calling it?

johannes