Re: [PATCH net] ipv6: fix a dst leak when removing its exception

["Mika Penttilä" <mika.penttila@nextfour.com>]

On 15.11.2018 20.17, David Ahern wrote:
> On 11/14/18 11:23 PM, Xin Long wrote:
>> On Thu, Nov 15, 2018 at 3:33 PM David Ahern <dsa@cumulusnetworks.com> wrote:
>>> On 11/14/18 11:03 AM, David Ahern wrote:
>>>> On 11/13/18 8:48 AM, Xin Long wrote:
>>>>> These is no need to hold dst before calling rt6_remove_exception_rt().
>>>>> The call to dst_hold_safe() in ip6_link_failure() was for ip6_del_rt(),
>>>>> which has been removed in Commit 93531c674315 ("net/ipv6: separate
>>>>> handling of FIB entries from dst based routes"). Otherwise, it will
>>>>> cause a dst leak.
>>>>>
>>>>> This patch is to simply remove the dst_hold_safe() call before calling
>>>>> rt6_remove_exception_rt() and also do the same in ip6_del_cached_rt().
>>>>> It's safe, because the removal of the exception that holds its dst's
>>>>> refcnt is protected by rt6_exception_lock.
>>>>>
>>>>> Fixes: 93531c674315 ("net/ipv6: separate handling of FIB entries from dst based routes")
>>>>> Fixes: 23fb93a4d3f1 ("net/ipv6: Cleanup exception and cache route handling")
>>>>> Reported-by: Li Shuang <shuali@redhat.com>
>>>>> Signed-off-by: Xin Long <lucien.xin@gmail.com>
>>>>> ---
>>>>>  net/ipv6/route.c | 7 +++----
>>>>>  1 file changed, 3 insertions(+), 4 deletions(-)
>>>> was this problem actually hit or is this patch based on a code analysis?
>>>>
>>> I ask because I have not been able to reproduce the leak using existing
>>> tests (e.g., pmtu) that I know create exceptions.
>>>
>>> If this problem was hit, it would be good to get a test case for it.
>> The attachment is the ip6_dst.sh with IPVS.
>>
>> # sh ip6_dst.sh
>>
>> But this one triggers the kernel warnings caused by 2 places:
>>    unregister_netdevice: waiting for br0 to become free. Usage count = 3
>>
>> 1. one is IPVS, I just posted the fix:
>> https://patchwork.ozlabs.org/patch/998123/  [1]
>> 2. the other one is IPv6,
>> ip6_link_failure() will be hit.
>>
>> So to make this reproduce clearly, you may want to apply
>> patch [1] firstly.
>>
> Thanks for the script. It does not replicate the problem using net-next
> tree as of
>
> commit 6d5db6c37929cb0a84e64ba0590a74593e5ce3b8
> Merge: 15cef30974c5 bd3b5d462add
> Author: David S. Miller <davem@davemloft.net>
> Date:   Wed Nov 14 08:51:28 2018 -0800
>
>     Merge branch 'nfp-abm-track-all-Qdiscs'
>
>
> I would be really surprised if the fib6_info change introduced a need to
> change the dst hold's for exception routes. I am not seeing the
> connection, so I really want to see it reproduced.
>
> Thanks


Maybe it's not 100% reproducer then, but I think the fix is obviously right.

--Mika