Re: [PATCH net] l2tp: fix refcount leakage on PPPoL2TP sockets

[Guillaume Nault <g.nault@alphalink.fr>]

On Tue, Jun 05, 2018 at 09:41:24AM -0400, David Miller wrote:
> From: Guillaume Nault <g.nault@alphalink.fr>
> Date: Mon, 4 Jun 2018 18:52:19 +0200
> 
> > Commit d02ba2a6110c ("l2tp: fix race in pppol2tp_release with session
> > object destroy") tried to fix a race condition where a PPPoL2TP socket
> > would disappear while the L2TP session was still using it. However, it
> > missed the root issue which is that an L2TP session may accept to be
> > reconnected if its associated socket has entered the release process.
> > 
> > The tentative fix makes the session hold the socket it is connected to.
> > That saves the kernel from crashing, but introduces refcount leakage,
> > preventing the socket from completing the release process. Once stalled,
> > everything the socket depends on can't be released anymore, including
> > the L2TP session and the l2tp_ppp module.
>  ...
> > So it all boils down to pppol2tp_connect() failing to realise that the
> > session has already been connected. This patch drops the unneeded extra
> > reference counting (mostly reverting d02ba2a6110c) and checks that
> > neither ->sk nor ->__sk is set before allowing a session to be
> > connected.
> > 
> > Fixes: d02ba2a6110c ("l2tp: fix race in pppol2tp_release with session object destroy")
> > Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
> 
> So much fidgeting around in this area over the past year or two :-)
> 
Putting L2TP into production without adding custom workarounds has been
such a long journey, but we're almost there :-)

> Applied and queued up for -stable, thanks for fixing this.
> 
I still have a handful of issues to fix, though.