Subject: Fw: [Bug 199109] New: pptp: kernel printk "recursion detected", and then reboot itself

From: Stephen Hemminger <[email-masked]>
Date: Wed, 14 Mar 2018 08:02:17 -0700

Begin forwarded message:

Date: Wed, 14 Mar 2018 06:56:09 +0000
From: [email-masked]
To: [email-masked]
Subject: [Bug 199109] New: pptp: kernel printk "recursion detected", and then reboot itself

https://bugzilla.kernel.org/show_bug.cgi?id=199109

            Bug ID: 199109
           Summary: pptp: kernel printk "recursion detected", and then
                    reboot itself
           Product: Networking
           Version: 2.5
    Kernel Version: 4.9.77
          Hardware: Mips32
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: high
          Priority: P1
         Component: IPV4
          Assignee: [email-masked]
          Reporter: [email-masked]
        Regression: No

Created attachment 274715
  --> https://bugzilla.kernel.org/attachment.cgi?id=274715&action=edit  
system log

Use openwrt LEDE, (gcc version 5.5.0 (OpenWrt GCC 5.5.0 r5932-9c4fe10) ).
When use pptp, make WAN(eth1) down and then make WAN(eth1) up, pptp will redial
itself. Do this in a loop.

After a while, kernel print "recursion detected" and "net_ratelimit: 49422
callbacks suppressed", print this many times, then system reboot.

When add more printk in driver "driver/net/ppp/ppp_generic.c", fond it fall in
loop in "static void __ppp_xmit_process(struct ppp *ppp)", which called by
"static void __ppp_channel_push(struct channel *pch)". 
__ppp_channel_push() locked ppp->xmit_recursion, and loop for long time( while
(!ppp->xmit_pending && (skb = skb_dequeue(&ppp->file.xq))) ppp_send_frame(ppp,
skb); ). Some other thread want to lock ppp->xmit_recursion, but failed.

static void __ppp_channel_push(struct channel *pch)
{
        struct sk_buff *skb;
        struct ppp *ppp;

        spin_lock_bh(&pch->downl);
        if (pch->chan) {
                while (!skb_queue_empty(&pch->file.xq)) {
                        skb = skb_dequeue(&pch->file.xq);
                        if (!pch->chan->ops->start_xmit(pch->chan, skb)) {
                                /* put the packet back and try again later */
                                skb_queue_head(&pch->file.xq, skb);
                                break;
                        }
                }
        } else {
                /* channel got deregistered */
                skb_queue_purge(&pch->file.xq);
        }
        spin_unlock_bh(&pch->downl);
        /* see if there is anything from the attached unit to be sent */
        if (skb_queue_empty(&pch->file.xq)) {
                ppp = pch->ppp;
                if (ppp)
                        __ppp_xmit_process(ppp);
        }
}

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