Re: [PATCH net v2] net: hdlc_ppp: Fix issues when mod_timer is called while timer is running
On Tue, Dec 29, 2020 at 12:10 AM Hillf Danton wrote: > > >The code path that calls add_timer is for sending keep-alive packets > >when operating in the OPENED state. If we have just changed to the > >OPENED state in ppp_cp_event, we should wait for the amount of time > >set by the (2nd) mod_timer call in ppp_cp_event, before firing the > > What if your change also covers the first case of mod_timer() in > ppp_cp_event()? Yes, for the 1st mod_timer call in ppp_cp_event, the situation is the same. If it is called, it means we are sending out a Configure Request or Terminate Request. In this case, we should wait for the amount of time set by this mod_timer call, before firing the timer. We shouldn't fire the timer immediately because this is not the intention of this mod_timer call. > >timer. We shouldn't fire the timer immediately after we change to the > >OPENED state. This is not the intention of the (2nd) mod_timer call in > >ppp_cp_event. Therefore aborting ppp_timer is the correct solution. > > > Given an expiring timer, is it the right time to call ppp_tx_flush() in > addition to add/mod_timer()? Do you mean when we are aborting ppp_timer, whether we need to call ppp_tx_flush in the aborted ppp_timer? I don't think so. Because when ppp_rx (which directly or indirectly calls ppp_cp_event) releases the lock, it will call ppp_tx_flush. So we don't need to call it again.
Re: [PATCH net v2] net: hdlc_ppp: Fix issues when mod_timer is called while timer is running
From: Xie He
Date: Sun, 27 Dec 2020 18:53:39 -0800
> ppp_cp_event is called directly or indirectly by ppp_rx with "ppp->lock"
> held. It may call mod_timer to add a new timer. However, at the same time
> ppp_timer may be already running and waiting for "ppp->lock". In this
> case, there's no need for ppp_timer to continue running and it can just
> exit.
>
> If we let ppp_timer continue running, it may call add_timer. This causes
> kernel panic because add_timer can't be called with a timer pending.
> This patch fixes this problem.
>
> Fixes: e022c2f07ae5 ("WAN: new synchronous PPP implementation for generic
> HDLC.")
> Cc: Krzysztof Halasa
> Signed-off-by: Xie He
Applied and queued up for -stable, thanks!
Re: [PATCH net v2] net: hdlc_ppp: Fix issues when mod_timer is called while timer is running
On Mon, Dec 28, 2020 at 1:17 AM Hillf Danton wrote: > > On Sun, 27 Dec 2020 18:53:39 -0800 Xie He wrote: > > ppp_cp_event is called directly or indirectly by ppp_rx with "ppp->lock" > > held. It may call mod_timer to add a new timer. However, at the same time > > ppp_timer may be already running and waiting for "ppp->lock". In this > > case, there's no need for ppp_timer to continue running and it can just > > exit. > > Because the timer callback loses the race in acquiring the ppp->lock > does not mean it should abort. I think aborting ppp_timer is the correct solution. When mod_timer is called by ppp_cp_event, which is (directly or indirectly) called by ppp_rx, this means we received something on the line that makes the original timer no longer necessary. If the timer is pending, mod_timer will delete it. If the timer is running and waiting for the lock, I think it should abort. This way it would appear that the timer hasn't fired and is deleted before it fires. > > If we let ppp_timer continue running, it may call add_timer. This causes > > kernel panic because add_timer can't be called with a timer pending. > > Meanwhie we can defuse the peril following add_timer() added in > e022c2f07ae5, say by replacing add_timer() with mod_timer(). The code path that calls add_timer is for sending keep-alive packets when operating in the OPENED state. If we have just changed to the OPENED state in ppp_cp_event, we should wait for the amount of time set by the (2nd) mod_timer call in ppp_cp_event, before firing the timer. We shouldn't fire the timer immediately after we change to the OPENED state. This is not the intention of the (2nd) mod_timer call in ppp_cp_event. Therefore aborting ppp_timer is the correct solution.
[PATCH net v2] net: hdlc_ppp: Fix issues when mod_timer is called while timer is running
ppp_cp_event is called directly or indirectly by ppp_rx with "ppp->lock"
held. It may call mod_timer to add a new timer. However, at the same time
ppp_timer may be already running and waiting for "ppp->lock". In this
case, there's no need for ppp_timer to continue running and it can just
exit.
If we let ppp_timer continue running, it may call add_timer. This causes
kernel panic because add_timer can't be called with a timer pending.
This patch fixes this problem.
Fixes: e022c2f07ae5 ("WAN: new synchronous PPP implementation for generic
HDLC.")
Cc: Krzysztof Halasa
Signed-off-by: Xie He
---
drivers/net/wan/hdlc_ppp.c | 7 +++
1 file changed, 7 insertions(+)
diff --git a/drivers/net/wan/hdlc_ppp.c b/drivers/net/wan/hdlc_ppp.c
index 64f855651336..261b53fc8e04 100644
--- a/drivers/net/wan/hdlc_ppp.c
+++ b/drivers/net/wan/hdlc_ppp.c
@@ -569,6 +569,13 @@ static void ppp_timer(struct timer_list *t)
unsigned long flags;
spin_lock_irqsave(&ppp->lock, flags);
+ /* mod_timer could be called after we entered this function but
+* before we got the lock.
+*/
+ if (timer_pending(&proto->timer)) {
+ spin_unlock_irqrestore(&ppp->lock, flags);
+ return;
+ }
switch (proto->state) {
case STOPPING:
case REQ_SENT:
--
2.27.0
