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

[Johannes Berg <johannes@sipsolutions.net>]

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?

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

johannes