Date:   Fri, 11 May 2018 07:40:59 -0400
From:   Dan Streetman <ddstreet@...e.org>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Tommi Rantala <tommi.t.rantala@...ia.com>,
        Neil Horman <nhorman@...driver.com>,
        Xin Long <lucien.xin@...il.com>,
        David Ahern <dsahern@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Cong Wang <xiyou.wangcong@...il.com>,
        David Miller <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        syzkaller <syzkaller@...glegroups.com>,
        Dan Streetman <ddstreet@...onical.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Alexey Kodanev <alexey.kodanev@...cle.com>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        linux-sctp@...r.kernel.org
Subject: Re: net: hang in unregister_netdevice: waiting for lo to become free

On Fri, May 11, 2018 at 5:19 AM, Dmitry Vyukov <dvyukov@...gle.com> wrote:
> On Thu, May 10, 2018 at 12:23 PM, Dan Streetman <ddstreet@...e.org> wrote:
>>>>>>>> <tommi.t.rantala@...ia.com> wrote:
>>>>>>>>> On 20.02.2018 18:26, Neil Horman wrote:
>>>>>>>>>>
>>>>>>>>>> On Tue, Feb 20, 2018 at 09:14:41AM +0100, Dmitry Vyukov wrote:
>>>>>>>>>>>
>>>>>>>>>>> On Tue, Feb 20, 2018 at 8:56 AM, Tommi Rantala
>>>>>>>>>>> <tommi.t.rantala@...ia.com> wrote:
>>>>>>>>>>>>
>>>>>>>>>>>> On 19.02.2018 20:59, Dmitry Vyukov wrote:
>>>>>>>>>>>>>
>>>>>>>>>>>>> Is this meant to be fixed already? I am still seeing this on the
>>>>>>>>>>>>> latest upstream tree.
>>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> These two commits are in v4.16-rc1:
>>>>>>>>>>>>
>>>>>>>>>>>> commit 4a31a6b19f9ddf498c81f5c9b089742b7472a6f8
>>>>>>>>>>>> Author: Tommi Rantala <tommi.t.rantala@...ia.com>
>>>>>>>>>>>> Date:   Mon Feb 5 21:48:14 2018 +0200
>>>>>>>>>>>>
>>>>>>>>>>>>      sctp: fix dst refcnt leak in sctp_v4_get_dst
>>>>>>>>>>>> ...
>>>>>>>>>>>>      Fixes: 410f03831 ("sctp: add routing output fallback")
>>>>>>>>>>>>      Fixes: 0ca50d12f ("sctp: fix src address selection if using
>>>>>>>>>>>> secondary
>>>>>>>>>>>> addresses")
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> commit 957d761cf91cdbb175ad7d8f5472336a4d54dbf2
>>>>>>>>>>>> Author: Alexey Kodanev <alexey.kodanev@...cle.com>
>>>>>>>>>>>> Date:   Mon Feb 5 15:10:35 2018 +0300
>>>>>>>>>>>>
>>>>>>>>>>>>      sctp: fix dst refcnt leak in sctp_v6_get_dst()
>>>>>>>>>>>> ...
>>>>>>>>>>>>      Fixes: dbc2b5e9a09e ("sctp: fix src address selection if using
>>>>>>>>>>>> secondary
>>>>>>>>>>>> addresses for ipv6")
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> I guess we missed something if it's still reproducible.
>>>>>>>>>>>>
>>>>>>>>>>>> I can check it later this week, unless someone else beat me to it.
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> Hi Tommi,
>>>>>>>>>>>
>>>>>>>>>>> Hmmm, I can't claim that it's exactly the same bug. Perhaps it's
>>>>>>>>>>> another one then. But I am still seeing these:
>>>>>>>>>>>
>>>>>>>>>>> [   58.799130] unregister_netdevice: waiting for lo to become free.
>>>>>>>>>>> Usage count = 4
>>>>>>>>>>> [   60.847138] unregister_netdevice: waiting for lo to become free.
>>>>>>>>>>> Usage count = 4
>>>>>>>>>>> [   62.895093] unregister_netdevice: waiting for lo to become free.
>>>>>>>>>>> Usage count = 4
>>>>>>>>>>> [   64.943103] unregister_netdevice: waiting for lo to become free.
>>>>>>>>>>> Usage count = 4
>>>>>>>>>>>
>>>>>>>>>>> on upstream tree pulled ~12 hours ago.
>>>>>>>>>>>
>>>>>>>>>> Can you write a systemtap script to probe dev_hold, and dev_put, printing
>>>>>>>>>> out a
>>>>>>>>>> backtrace if the device name matches "lo".  That should tell us
>>>>>>>>>> definitively if
>>>>>>>>>> the problem is in the same location or not
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> Hi Dmitry, I tested with the reproducer and the kernel .config file that you
>>>>>>>>> sent in the first email in this thread:
>>>>>>>>>
>>>>>>>>> With 4.16-rc2 unable to reproduce.
>>>>>>>>>
>>>>>>>>> With 4.15-rc9 bug reproducible, and I get "unregister_netdevice: waiting for
>>>>>>>>> lo to become free. Usage count = 3"
>>>>>>>>>
>>>>>>>>> With 4.15-rc9 and Alexey's "sctp: fix dst refcnt leak in sctp_v6_get_dst()"
>>>>>>>>> cherry-picked on top, unable to reproduce.
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> Is syzkaller doing something else now to trigger the bug...?
>>>>>>>>> Can you still trigger the bug with the same reproducer?
>>>>>>>>
>>>>>>>> Hi Neil, Tommi,
>>>>>>>>
>>>>>>>> Reviving this old thread about "unregister_netdevice: waiting for lo
>>>>>>>> to become free. Usage count = 3" hangs.
>>>>>>>> I still did not have time to deep dive into what happens there (too
>>>>>>>> many bugs coming from syzbot). But this still actively happens and I
>>>>>>>> suspect accounts to a significant portion of various hang reports,
>>>>>>>> which are quite unpleasant.
>>>>>>>>
>>>>>>>> One idea that could make it all simpler:
>>>>>>>>
>>>>>>>> Is this wait loop in netdev_wait_allrefs() supposed to wait for any
>>>>>>>> prolonged periods of time under any non-buggy conditions? E.g. more
>>>>>>>> than 1-2 minutes?
>>>>>>>> If it only supposed to wait briefly for things that already supposed
>>>>>>>> to be shutting down, and we add a WARNING there after some timeout,
>>>>>>>> then syzbot will report all info how/when it happens, hopefully
>>>>>>>> extracting reproducers, and all the nice things.
>>>>>>>> But this WARNING should not have any false positives under any
>>>>>>>> realistic conditions (e.g. waiting for arrival of remote packets with
>>>>>>>> large timeouts).
>>>>>>>>
>>>>>>>> Looking at some task hung reports, it seems that this code holds some
>>>>>>>> mutexes, takes workqueue thread and prevents any progress with
>>>>>>>> destruction of other devices (and net namespace creation/destruction),
>>>>>>>> so I guess it should not wait for any indefinite periods of time?
>>>>>>>
>>>>>>> I'm working on this currently:
>>>>>>> https://bugs.launchpad.net/ubuntu/zesty/+source/linux/+bug/1711407
>>>>>>>
>>>>>>> I added a summary of what I've found to be the cause (or at least, one
>>>>>>> possible cause) of this:
>>>>>>> https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1711407/comments/72
>>>>>>>
>>>>>>> I'm working on a patch to work around the main side-effect of this,
>>>>>>> which is hanging while holding the global net mutex.  Hangs will still
>>>>>>> happen (e.g. if a dst leaks) but should not affect anything else,
>>>>>>> other than a leak of the dst and its net namespace.
>>>>>>>
>>>>>>> Fixing the dst leaks is important too, of course, but a dst leak (or
>>>>>>> other cause) shouldn't break the entire system.
>>>>>>
>>>>>> Leaking some memory is definitely better than hanging the system.
>>>>>>
>>>>>> So I've made syzkaller to recognize "unregister_netdevice: waiting for
>>>>>> (.*) to become free" as a kernel bug:
>>>>>> https://github.com/google/syzkaller/commit/7a67784ca8bdc3b26cce2f0ec9a40d2dd9ec9396
>>>>>> Unfortunately it does not make it catch these bugs because creating a
>>>>>> net namespace per test is too damn slow, so namespaces are reused for
>>>>>> lots of tests and when/if it's eventually destroyed it's already too
>>>>>> late to find root cause.
>>>>>>
>>>>>> But I've run a one-off experiment with prompt net namespace
>>>>>> destruction and syzkaller was able to easily extract a C reproducer:
>>>>>> https://gist.githubusercontent.com/dvyukov/d571e8fff24e127ca48a8c4790d42bfa/raw/52050e93ba9afbb5126b9d7bb39b7e71a82af016/gistfile1.txt
>>>>>>
>>>>>> On upstream 16e205cf42da1f497b10a4a24f563e6c0d574eec with this config:
>>>>>> https://gist.githubusercontent.com/dvyukov/9663c57443adb21f2795b92ef0829d62/raw/bbea0652e23746096dd56855a28f6c681aebcdee/gistfile1.txt
>>>>>>
>>>>>> this gives me:
>>>>>>
>>>>>> [   83.183198] unregister_netdevice: waiting for lo to become free.
>>>>>> Usage count = 9
>>>>>> [   85.231202] unregister_netdevice: waiting for lo to become free.
>>>>>> Usage count = 9
>>>>>> ...
>>>>>> [  523.511205] unregister_netdevice: waiting for lo to become free.
>>>>>> Usage count = 9
>>>>>> ...
>>>>>>
>>>>>> This is generated from this syzkaller program:
>>>>>>
>>>>>> r0 = socket$inet6(0xa, 0x1, 0x84)
>>>>>> setsockopt$inet6_IPV6_XFRM_POLICY(r0, 0x29, 0x23,
>>>>>> &(0x7f0000000380)={{{@...=@...ote={0xfe, 0x80, [], 0xbb},
>>>>>> @in=@...={0xac, 0x14, 0x14}, 0x0, 0x0, 0x0, 0x0, 0xa}, {}, {}, 0x0,
>>>>>> 0x0, 0x1}, {{@in=@...al={0xac, 0x14, 0x14, 0xaa}, 0x0, 0x32}, 0x0,
>>>>>> @in=@...al={0xac, 0x14, 0x14, 0xaa}, 0x3504}}, 0xe8)
>>>>>> bind$inet6(r0, &(0x7f0000000000)={0xa, 0x4e20}, 0x1c)
>>>>>> connect$inet(r0, &(0x7f0000000040)={0x2, 0x4e20, @dev={0xac, 0x14,
>>>>>> 0x14, 0xd}}, 0x10)
>>>>>> syz_emit_ethernet(0x3e, &(0x7f00000001c0)={@...al={[0xaa, 0xaa, 0xaa,
>>>>>> 0xaa, 0xaa], 0xaa}, @dev={[0xaa, 0xaa, 0xaa, 0xaa, 0xaa]}, [],
>>>>>> {@...6={0x86dd, {0x0, 0x6, "50a09c", 0x8, 0xffffff11, 0x0,
>>>>>> @remote={0xfe, 0x80, [], 0xbb}, @local={0xfe, 0x80, [], 0xaa}, {[],
>>>>>> @udp={0x0, 0x4e20, 0x8}}}}}}, &(0x7f0000000040))
>>>>>>
>>>>>> So this seems to be related to IPv6 and/or xfrm and is potentially
>>>>>> caused by external packets (that syz_emit_ethernet call).
>>>>>
>>>>>
>>>>>
>>>>> Here is another repro which seems to be a different bug (note that it
>>>>> requires fault injection):
>>>>>
>>>>> https://gist.githubusercontent.com/dvyukov/1c56623016cc4c24a69d433c5114ad5b/raw/530478f571b195193101b912aa646948528baa8e/gistfile1.txt
>>>>>
>>>>> Dan, do you mind taking a look at them? Fixing these should eliminate
>>>>> root causes of these hangs/leaks.
>>>>
>>>> Yep I will look at them, thanks for the reproducers.
>>>
>>> Hi Dan,
>>>
>>> Any updates on this? syzbot is hitting this all the time.
>>
>> Sorry, the recent changes from net_mutex -> net_rwsem/pernet_ops_rwsem
>> have complicated what I had done to workaround this, but I'm still
>> working on it.  Apologies for the delay.
>
> Are you looking at the mitigation? Or the bugs that trigger it? Or both?

Both - workaround first, as most important (and relatively easiest) is
allowing the system to continue to create and destroy net namespaces
once this happens (instead of hanging all further netns init/cleanup
until a system reboot).

Then, I do want to try to add some debug to make it easier to debug a
leaked dst (or other cause).

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