[Bridge] Use-after-free in br_multicast_rcv

Nikolay Aleksandrov nikolay at cumulusnetworks.com
Mon Jul 1 17:37:24 UTC 2019

Hi again,
On 01/07/2019 20:31, Martin Weinelt wrote:
> Hi Nik,
> On 7/1/19 7:03 PM, Nikolay Aleksandrov wrote:
>> Hi Martin,
>> On 01/07/2019 19:53, Martin Weinelt wrote:
>>> Hi Nik,
>>> more info below.
>>> On 6/29/19 3:11 PM, nikolay at cumulusnetworks.com wrote:
>>>> On 29 June 2019 14:54:44 EEST, Martin Weinelt <martin at linuxlounge.net> wrote:
>>>>> Hello,
>>>>> we've recently been experiencing memory leaks on our Linux-based
>>>>> routers,
>>>>> at least as far back as v4.19.16.
>>>>> After rebuilding with KASAN it found a use-after-free in 
>>>>> br_multicast_rcv which I could reproduce on v5.2.0-rc6. 
>>>>> Please find the KASAN report below, I'm anot sure what else to provide
>>>>> so
>>>>> feel free to ask.
>>>>> Best,
>>>>>  Martin
>>>> Hi Martin, 
>>>> I'll look into this, are there any specific steps to reproduce it? 
>>>> Thanks, 
>>>>    Nik
>>> Each server is a KVM Guest and has 18 bridges with the same master/slave
>>> relationships:
>>>   bridge -> batman-adv -> {l2 tunnel, virtio device}
>>> Linus Lüssing from the batman-adv asked me to apply this patch to help
>>> debugging.
>>> v5.2-rc6-170-g728254541ebc with this patch yielded the following KASAN 
>>> report, not sure if the additional information at the end is a result of
>>> the added patch though.
>>> Best,
>>>   Martin
>> I see a couple of issues that can cause out-of-bounds accesses in br_multicast.c
>> more specifically there're pskb_may_pull calls and accesses to stale skb pointers.
>> I've had these on my "to fix" list for some time now, will prepare, test the fixes and
>> send them for review. In a few minutes I'll send a test patch for you.
>> That being said, I thought you said you've been experiencing memory leaks, but below
>> reports are for out-of-bounds accesses, could you please clarify if you were
>> speaking about these or is there another issue as well ?
>> If you're experiencing memory leaks, are you sure they're related to the bridge ?
>> You could try kmemleak for those.
>> Thank you,
>>  Nik
> we had been experiencing memory leaks on v4.19.37, thats why we started to turn on
> KASAN and kmemleak in the first place. This is when we found this use-after-free.
> The memory leak exists, and is a separate issue. Apparently kmemleak does not work,
> I suspect the early log size is too small
> root at gw02:~# echo scan > /sys/kernel/debug/kmemleak                                                                                                                                                                                 -bash: echo: write error: Device or resource busy
> I'll increase the early log size with the next build to try and get more information
> on the memory leak, I'll open a separate thread for that then.
> Thanks,
>   Martin

I see, thanks for clarifying this. So on the KASAN could you please try the attached patch ?
Also could you please run the br_multicast_rcv+xxx addresses through
linux/scripts/faddr2line for your kernel/bridge:
usage: faddr2line [--list] <object file> <func+offset> <func+offset>...

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