netdev - Re: [PATCH net] ipv4: igmp: fix a use after free

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Date:   Fri, 9 Jun 2017 10:01:18 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Xin Long <lucien.xin@...il.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        "David S. Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net] ipv4: igmp: fix a use after free

On Fri, Jun 9, 2017 at 8:56 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Fri, 2017-06-09 at 14:24 +0800, Xin Long wrote:
>> On Fri, Jun 9, 2017 at 8:59 AM, Cong Wang <xiyou.wangcong@...il.com> wrote:
>>
>> > On Thu, Jun 8, 2017 at 1:33 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>> >> I mentioned (in https://lkml.org/lkml/2017/5/31/619 ) that we might need
>> >> to defer freeing after rcu grace period but for some reason decided it
>> >> was not needed.
>> Yes, this one could fix it.
>>
>> >
>> > This one makes sense, it is the second time I saw the use-after-free
>> > in igmp code, both are because we don't respect the RCU rule to free
>> > an element in the list.
>> >
>> >>
>> >> What about :
>> >
>> > But not sure if all ip_ma_put() callers want ip_mc_clear_src().
>> If that's problem, there may be another way:
>>
>>   leave ip_mc_clear_src as it is, just add pmc->lock to protect this call.
>>
>> this use-after-free was actually caused by using pmc->sources/tomb
>> in add_grec while ip_mc_clear_src is freeing them. add_grec is already
>> under pmc->lock, so to add pmc->lock for ip_mc_clear_src should be
>> enough to protect the list pmc->sources/tomb.
>>
>> wdyt ?
>
> This would we weird.
>
> When we free skb components, we do not grab a spinlock.
>
> When we free something, just make sure we must be the last user of it.
>
> RCU rules -> Must respect RCU grace period before delete.
>
> No need for extra spinlock.

This is what I thought in my first response, until I realized
it is not pure RCU, otherwise pmc->lock should not be taken
in igmpv3_send_cr(). It seems the code is mixing the use
of spinlock and RCU.

We need RCU anyway, ip_check_mc_rcu() is the real fast
path where we don't take spinlock. I think we will need more
work.