Date:   Mon, 24 Apr 2017 17:53:25 +0300
From:   Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
To:     Xin Long <lucien.xin@...il.com>
Cc:     network dev <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Stephen Hemminger <stephen@...workplumber.org>,
        "bridge@...ts.linux-foundation.org" 
        <bridge@...ts.linux-foundation.org>
Subject: Re: [PATCH net] bridge: shutdown bridge device before removing it

On 24/04/17 17:41, Xin Long wrote:
> On Mon, Apr 24, 2017 at 8:07 PM, Nikolay Aleksandrov
> <nikolay@...ulusnetworks.com> wrote:
>> On 24/04/17 14:01, Nikolay Aleksandrov wrote:
>>> On 24/04/17 10:25, Xin Long wrote:
>>>> During removing a bridge device, if the bridge is still up, a new mdb entry
>>>> still can be added in br_multicast_add_group() after all mdb entries are
>>>> removed in br_multicast_dev_del(). Like the path:
>>>>
>>>>   mld_ifc_timer_expire ->
>>>>     mld_sendpack -> ...
>>>>       br_multicast_rcv ->
>>>>         br_multicast_add_group
>>>>
>>>> The new mp's timer will be set up. If the timer expires after the bridge
>>>> is freed, it may cause use-after-free panic in br_multicast_group_expired.
>>>> This can happen when ip link remove a bridge or destroy a netns with a
>>>> bridge device inside.
>>>>
>>>> As we can see in br_del_bridge, brctl is also supposed to remove a bridge
>>>> device after it's shutdown.
>>>>
>>>> This patch is to call dev_close at the beginning of br_dev_delete so that
>>>> netif_running check in br_multicast_add_group can avoid this issue. But
>>>> to keep consistent with before, it will not remove the IFF_UP check in
>>>> br_del_bridge for brctl.
>>>>
>>>> Reported-by: Jianwen Ji <jiji@...hat.com>
>>>> Signed-off-by: Xin Long <lucien.xin@...il.com>
>>>> ---
>>>>  net/bridge/br_if.c | 2 ++
>>>>  1 file changed, 2 insertions(+)
>>>>
>>>
>>> +CC bridge maintainers
>>>
>>> I can see how this could happen, could you also provide the traceback ?
>>>
>>> The patch looks good to me, actually I think it fixes another issue with
>>> mcast stats where the percpu pointer can be accessed after it's freed if
>>> an mcast packet can get sent via br->dev after the br_multicast_dev_del() call.
>>> This is definitely stable material, if I'm not mistaken the issue is there since
>>> the introduction of br_dev_delete:
>>> commit e10177abf842
>>> Author: Satish Ashok <sashok@...ulusnetworks.com>
>>> Date:   Wed Jul 15 07:16:51 2015 -0700
>>>
>>>     bridge: multicast: fix handling of temp and perm entries
>>>
>>>
>>>
>>> Acked-by: Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
>>>
>>
>> Actually I have a better idea for a fix because dev_close() for a single device is rather heavy.
>> Why don't you move the mdb flush logic in the bridge's ndo_uninit() callback ?
>> That should have the same effect and be much faster.
> Yes. But it seems that all cleanups for bridge should be done after
> it's shutdown since beginning according to brctl. I'm not sure if there
> are still other problems caused by this. maybe safer to use dev_close.
> I need to check more to confirm this.
> 

ndo_uninit() is after the device has been stopped, so it is the same as
your fix as I said.

> I also have another question about mp->timer removing.
> As we can see, now it removes this timer with del_timer, instead of
> del_timer_sync. What if the timer is running when del_timer ?
> How can we be sure that br_multicast_group_expired will be done
> before the bridge dev is freed. synchronize_net ?
> 

Yeah, I've been thinking about that and the only race is that the timer
might have fired and waiting for the lock while the mdb is being flushed
thus the cancel_timer() won't affect it and then it will enter and see
that !netif_running(br->dev), but unfortunately there's a bug because we
cannot guarantee that br->dev still exists at that point.
This is a different bug though.

>>
>> By the way I just noticed that there's also a memory leak - the mdb hash is reallocated
>> and not freed due to the mdb rehash, here's also kmemleak's object:
>>
> yeps, ;-)
> 
>> unreferenced object 0xffff8800540ba800 (size 2048):
>>   comm "softirq", pid 0, jiffies 4520588901 (age 5787.284s)
>>   hex dump (first 32 bytes):
>>     00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
>>     00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
>>   backtrace:
>>     [<ffffffff816e2287>] kmemleak_alloc+0x67/0xc0
>>     [<ffffffff81260bea>] __kmalloc+0x1ba/0x3e0
>>     [<ffffffffa05c60ee>] br_mdb_rehash+0x5e/0x340 [bridge]
>>     [<ffffffffa05c74af>] br_multicast_new_group+0x43f/0x6e0 [bridge]
>>     [<ffffffffa05c7aa3>] br_multicast_add_group+0x203/0x260 [bridge]
>>     [<ffffffffa05ca4b5>] br_multicast_rcv+0x945/0x11d0 [bridge]
>>     [<ffffffffa05b6b10>] br_dev_xmit+0x180/0x470 [bridge]
>>     [<ffffffff815c781b>] dev_hard_start_xmit+0xbb/0x3d0
>>     [<ffffffff815c8743>] __dev_queue_xmit+0xb13/0xc10
>>     [<ffffffff815c8850>] dev_queue_xmit+0x10/0x20
>>     [<ffffffffa02f8d7a>] ip6_finish_output2+0x5ca/0xac0 [ipv6]
>>     [<ffffffffa02fbfc6>] ip6_finish_output+0x126/0x2c0 [ipv6]
>>     [<ffffffffa02fc245>] ip6_output+0xe5/0x390 [ipv6]
>>     [<ffffffffa032b92c>] NF_HOOK.constprop.44+0x6c/0x240 [ipv6]
>>     [<ffffffffa032bd16>] mld_sendpack+0x216/0x3e0 [ipv6]
>>     [<ffffffffa032d5eb>] mld_ifc_timer_expire+0x18b/0x2b0 [ipv6]
>>
>>
>>

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