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

[David Miller <davem@davemloft.net>]

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 :-)

Applied and queued up for -stable, thanks for fixing this.