linux-kernel - Re: [PATCH] mac80211: fix race in ieee80211_register

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Message-ID: <CAFA6WYMxKUXjhAfK6pTu9merNwUaKmQp6_FaTmW4e=kfTMthmw@mail.gmail.com>
Date:   Mon, 6 Apr 2020 18:36:06 +0530
From:   Sumit Garg <sumit.garg@...aro.org>
To:     Johannes Berg <johannes@...solutions.net>
Cc:     Kalle Valo <kvalo@...eaurora.org>, linux-wireless@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>, kuba@...nel.org,
        netdev@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Matthias-Peter Schöpfer 
        <matthias.schoepfer@...inx.io>,
        "Berg Philipp (HAU-EDS)" <Philipp.Berg@...bherr.com>,
        "Weitner Michael (HAU-EDS)" <Michael.Weitner@...bherr.com>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Loic Poulain <loic.poulain@...aro.org>, stable@...r.kernel.org
Subject: Re: [PATCH] mac80211: fix race in ieee80211_register_hw()

On Mon, 6 Apr 2020 at 18:17, Johannes Berg <johannes@...solutions.net> wrote:
>
> 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?

Indeed, it's a call to "struct ieee80211_ops::start" just after wiphy
is registered that causes the frame to be received leading to RX IRQ.

>
> The "wlan0" is kinda wrong there, should be "phy0" I guess, and then
> interface added from iw?

Okay, will update the sequence diagram.

-Sumit

>
> johannes
>