linux-kernel - Re: [PATCH] b43legacy: Fix a sleep-in-atomic bug in b43legacy_op_bss_info

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Date:   Wed, 31 May 2017 17:32:15 +0200
From:   Michael Büsch <m@...s.ch>
To:     Kalle Valo <kvalo@...eaurora.org>
Cc:     Jia-Ju Bai <baijiaju1990@....com>, netdev@...r.kernel.org,
        linux-wireless@...r.kernel.org, b43-dev@...ts.infradead.org,
        linux-kernel@...r.kernel.org, Larry.Finger@...inger.net
Subject: Re: [PATCH] b43legacy: Fix a sleep-in-atomic bug in
 b43legacy_op_bss_info_changed

On Wed, 31 May 2017 13:26:43 +0300
Kalle Valo <kvalo@...eaurora.org> wrote:

> Jia-Ju Bai <baijiaju1990@....com> writes:
> 
> > The driver may sleep under a spin lock, and the function call path is:
> > b43legacy_op_bss_info_changed (acquire the lock by spin_lock_irqsave)
> >   b43legacy_synchronize_irq
> >     synchronize_irq --> may sleep
> >
> > To fix it, the lock is released before b43legacy_synchronize_irq, and the 
> > lock is acquired again after this function.
> >
> > Signed-off-by: Jia-Ju Bai <baijiaju1990@....com>
> > ---
> >  drivers/net/wireless/broadcom/b43legacy/main.c |    2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/drivers/net/wireless/broadcom/b43legacy/main.c b/drivers/net/wireless/broadcom/b43legacy/main.c
> > index f1e3dad..31ead21 100644
> > --- a/drivers/net/wireless/broadcom/b43legacy/main.c
> > +++ b/drivers/net/wireless/broadcom/b43legacy/main.c
> > @@ -2859,7 +2859,9 @@ static void b43legacy_op_bss_info_changed(struct ieee80211_hw *hw,
> >  	b43legacy_write32(dev, B43legacy_MMIO_GEN_IRQ_MASK, 0);
> >  
> >  	if (changed & BSS_CHANGED_BSSID) {
> > +		spin_unlock_irqrestore(&wl->irq_lock, flags);
> >  		b43legacy_synchronize_irq(dev);
> > +		spin_lock_irqsave(&wl->irq_lock, flags);  
> 
> To me this looks like a fragile workaround and not a real fix. You can
> easily add new race conditions with releasing the lock like this.
> 


I think releasing the lock possibly is fine. It certainly is better than
sleeping with a lock held.
We disabled the device interrupts just before this line.

However I think the synchronize_irq should be outside of the
conditional right after the write to B43legacy_MMIO_GEN_IRQ_MASK. (So
two lines above)
I don't think it makes sense to only synchronize if BSS_CHANGED_BSSID
is set.


On the other hand b43 does not have this irq-disabling foobar anymore.
So somebody must have removed it. Maybe you can find the commit that
removed this stuff from b43 and port it to b43legacy?


So I would vote for moving the synchronize_irq up outside of the
conditional and put the unlock/lock sequence around it.
And as a second patch on top of that try to remove this stuff
altogether like b43 did.

-- 
Michael

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