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

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Message-ID: <1497023774.736.92.camel@edumazet-glaptop3.roam.corp.google.com>
Date:   Fri, 09 Jun 2017 08:56:14 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Xin Long <lucien.xin@...il.com>
Cc:     Cong Wang <xiyou.wangcong@...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, 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.