Re: [PATCH net v3] ppp: fix refcount underflow on channel unbridge

[Tom Parkin <tparkin@katalix.com>]

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On  Fri, Jan 08, 2021 at 21:57:50 +0100, Guillaume Nault wrote:
> On Thu, Jan 07, 2021 at 06:13:15PM +0000, Tom Parkin wrote:
> > When setting up a channel bridge, ppp_bridge_channels sets the
> > pch->bridge field before taking the associated reference on the bridge
> > file instance.
> > 
> > This opens up a refcount underflow bug if ppp_bridge_channels called
> > via. iotcl runs concurrently with ppp_unbridge_channels executing via.
> > file release.
> > 
> > The bug is triggered by ppp_bridge_channels taking the error path
> > through the 'err_unset' label.  In this scenario, pch->bridge is set,
> > but the reference on the bridged channel will not be taken because
> > the function errors out.  If ppp_unbridge_channels observes pch->bridge
> > before it is unset by the error path, it will erroneously drop the
> > reference on the bridged channel and cause a refcount underflow.
> > 
> > To avoid this, ensure that ppp_bridge_channels holds a reference on
> > each channel in advance of setting the bridge pointers.
> 
> Thanks for following up on this!
> 
> Acked-by: Guillaume Nault <gnault@redhat.com>

Thanks again for reviewing, Guillaume.

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