Re: [PATCH v2 3/3] pppoatm: protect against freeing of vcc

[Krzysztof Mazur <krzysiek@podlesie.net>]

On Tue, Nov 27, 2012 at 06:02:29PM +0000, David Woodhouse wrote:
> On Tue, 2012-11-27 at 18:39 +0100, Krzysztof Mazur wrote:
> > Yes, I missed that one - it's even worse, I introduced that bug 
> > in "[PATCH 1/7] atm: detach protocol before closing vcc". Before that
> > patch that scenario shouldn't happen because vcc was closed before
> > calling pppoatm_send(vcc, NULL) - the driver should provide appropriate
> > synchronization.
> > 
> > I think that we should just drop that patch. With later changes it's not
> > necessary - the pppoatm_send() can be safely called while closing vcc.
> 
> I'm not running with that patch. This bug exists for br2684 even before
> it, and I think also for pppoatm.
> 
> In solos-pci at least, the ops->close() function doesn't flush all
> pending skbs for this vcc before returning. So can be a tasklet
> somewhere which has loaded the address of the vcc->pop function from one
> of them, and is going to call it in some unspecified amount of time.
> 
> Should we make the device's ->close function wait for all TX and RX skbs
> for this vcc to complete? 

Yes, the ->close() can sleep and after vcc is closed the ->pop() shouldn't be
called.


While reviewing your br2684 patch I also found that some ATM drivers does
not call ->pop() when ->send() fails, they should do:

	if (vcc->pop)
		vcc->pop(vcc, skb);
	else
		dev_kfree_skb(skb);

but some drivers just call dev_kfree_skb(skb).

I think that we should add atm_pop() function that does that and fix all
drivers.

Krzysiek