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Date: Wed, 6 Jan 2021 12:33:53 -0800 From: Jakub Kicinski <kuba@...nel.org> To: Tom Parkin <tparkin@...alix.com> Cc: netdev@...r.kernel.org, gnault@...hat.com, jchapman@...alix.com Subject: Re: [PATCH] ppp: fix refcount underflow on channel unbridge On Tue, 5 Jan 2021 21:17:43 +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. Please prefix the subject with [PATCH net v3] for v3, add a fixes tag, and make sure to include you sign-off.
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