netdev - Re: [BUG] deadlock in nl80211_vendor

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Date:   Fri, 25 Mar 2022 22:54:38 +0100
From:   Johannes Berg <johannes@...solutions.net>
To:     William McVicker <willmcvicker@...gle.com>
Cc:     Jakub Kicinski <kuba@...nel.org>, linux-wireless@...r.kernel.org,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Kalle Valo <kvalo@...eaurora.org>,
        "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        Amitkumar Karwar <amitkarwar@...il.com>,
        Ganapathi Bhat <ganapathi.bhat@....com>,
        Xinming Hu <huxinming820@...il.com>, kernel-team@...roid.com,
        Paolo Abeni <pabeni@...hat.com>
Subject: Re: [BUG] deadlock in nl80211_vendor_cmd

On Fri, 2022-03-25 at 22:16 +0100, Johannes Berg wrote:
> 
> > Thread 1                         Thread 2
> >  nl80211_pre_doit():
> >    rtnl_lock()
> >    wiphy_lock()                   nl80211_pre_doit():
> >                                     rtnl_lock() // blocked by Thread 1
> >  nl80211_vendor_cmd():
> >    doit()
> >      cfg80211_unregister_netdevice()
> >    rtnl_unlock():
> >      netdev_run_todo():
> >        __rtnl_unlock()
> >                                     <got RTNL lock>
> >                                     wiphy_lock() // blocked by Thread 1
> >        rtnl_lock(); // DEADLOCK
> >  nl80211_post_doit():
> >    wiphy_unlock();
> 
> 
> Right, this is what I had discussed in my other mails.
> 
> Basically, you're actually doing (some form of) unregister_netdevice()
> before rtnl_unlock().
> 
> Clearly this isn't possible in cfg80211 itself.
> 
> However, I couldn't entirely discount the possibility that this is
> possible:
> 
> Thread 1                   Thread 2
>                             rtnl_lock()
>                             unregister_netdevice()
>                             __rtnl_unlock()
> rtnl_lock()
> wiphy_lock()
> netdev_run_todo()
>  __rtnl_unlock()
>  // list not empty now    
>  // because of thread 2     rtnl_lock()
>  rtnl_lock()
>                             wiphy_lock()
> 
> ** DEADLOCK **
> 
> 
> Given my other discussion with Jakub though, it seems that we can indeed
> make sure that this cannot happen, and then this scenario is impossible
> without the unregistration you're doing.
> 

I just sent a patch for this then, forgot to CC everyone:

https://lore.kernel.org/r/20220325225055.37e89a72f814.Ic73d206e217db20fd22dcec14fe5442ca732804b@changeid

But basically it changes nothing, just adds a WARN_ON with documentation
ensuring that the invariant never breaks, i.e. that Thread 2 can't
happen.

And maybe I should've written that with 3 Threads, but the setup of
unregister_netdevice()/__rtnl_unlock() could happen anywhere in the
system anyway.


johannes