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

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Date:   Fri, 9 Jun 2017 14:24:00 +0800
From:   Xin Long <lucien.xin@...il.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Eric Dumazet <eric.dumazet@...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 8:59 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 ?