Re: [PATCH net] ppp: avoid loop in xmit recursion detection code

[David Miller <davem@davemloft.net>]

From: Guillaume Nault <g.nault@alphalink.fr>
Date: Tue, 20 Mar 2018 16:49:26 +0100

> We already detect situations where a PPP channel sends packets back to
> its upper PPP device. While this is enough to avoid deadlocking on xmit
> locks, this doesn't prevent packets from looping between the channel
> and the unit.
> 
> The problem is that ppp_start_xmit() enqueues packets in ppp->file.xq
> before checking for xmit recursion. Therefore, __ppp_xmit_process()
> might dequeue a packet from ppp->file.xq and send it on the channel
> which, in turn, loops it back on the unit. Then ppp_start_xmit()
> queues the packet back to ppp->file.xq and __ppp_xmit_process() picks
> it up and sends it again through the channel. Therefore, the packet
> will loop between __ppp_xmit_process() and ppp_start_xmit() until some
> other part of the xmit path drops it.
> 
> For L2TP, we rapidly fill the skb's headroom and pppol2tp_xmit() drops
> the packet after a few iterations. But PPTP reallocates the headroom
> if necessary, letting the loop run and exhaust the machine resources
> (as reported in https://bugzilla.kernel.org/show_bug.cgi?id=199109).
> 
> Fix this by letting __ppp_xmit_process() enqueue the skb to
> ppp->file.xq, so that we can check for recursion before adding it to
> the queue. Now ppp_xmit_process() can drop the packet when recursion is
> detected.
> 
> __ppp_channel_push() is a bit special. It calls __ppp_xmit_process()
> without having any actual packet to send. This is used by
> ppp_output_wakeup() to re-enable transmission on the parent unit (for
> implementations like ppp_async.c, where the .start_xmit() function
> might not consume the skb, leaving it in ppp->xmit_pending and
> disabling transmission).
> Therefore, __ppp_xmit_process() needs to handle the case where skb is
> NULL, dequeuing as many packets as possible from ppp->file.xq.
> 
> Reported-by: xu heng <xuheng333@zoho.com>
> Fixes: 55454a565836 ("ppp: avoid dealock on recursive xmit")
> Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>

Applied and queued up for -stable, thank you.