ppp: avoid loop in xmit recursion detection code

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: 55454a56 ("ppp: avoid dealock on recursive xmit")
Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>