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

[Sumit Garg]

On Mon, 6 Apr 2020 at 18:14, Johannes Berg <johannes@xxxxxxxxxxxxxxxx> wrote:
>
> On Mon, 2020-04-06 at 17:51 +0530, Sumit Garg wrote:
> > 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.
>
> Indeed, oops.
>
> > 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()    |
> >        |                    |             |
>
> Makes sense.
>
> > @@ -1254,6 +1250,14 @@ int ieee80211_register_hw(struct ieee80211_hw *hw)
> >               local->sband_allocated |= BIT(band);
> >       }
> >
> > +     rtnl_unlock();
> > +
> > +     result = wiphy_register(local->hw.wiphy);
> > +     if (result < 0)
> > +             goto fail_wiphy_register;
> > +
> > +     rtnl_lock();
>
> I'm a bit worried about this unlock/relock here though.
>
> I think we only need the RTNL for the call to
> ieee80211_init_rate_ctrl_alg() and then later ieee80211_if_add(), so
> perhaps we can move that a little closer?
>

Sure, will move rtnl_unlock() to just after call to
ieee80211_init_rate_ctrl_alg().

> All the stuff between is really just setting up local stuff, so doesn't
> really need to worry?
>

Okay.

-Sumit

> johannes
>
>