netdev - Re: [PATCH net v2] packet: fix use after free race in send path when dev is released

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Message-ID: <1385047624.10637.48.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Thu, 21 Nov 2013 07:27:04 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Daniel Borkmann <dborkman@...hat.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org,
	Salam Noureddine <noureddine@...stanetworks.com>,
	Ben Greear <greearb@...delatech.com>
Subject: Re: [PATCH net v2] packet: fix use after free race in send path
 when dev is released

On Thu, 2013-11-21 at 16:02 +0100, Daniel Borkmann wrote:

> 
> That was also my first thought, but Salam pointed out to me, that in case
> we have a situation such as ...
> 
> in packet_cached_dev_get():
>      rcu_read_lock();
>      dev = rcu_dereference(po->cached_dev);      in packet_notifier():
>                                                  ---> CPU1: dev_put(po->prot_hook.dev);
> ---> CPU0:
>      if (dev)
> 	dev_hold(dev);
>      rcu_read_unlock();
> 
> ... we could reach a refcount of 0, before we increase it back to 1. Not sure
> if this can actually happen, maybe in preemptible RCU where read-side critical
> sections to be preempted? So with this rather paranoid approach we make sure
> to avoid such a situation as we wait a grace period when readers finished.

There is no need, because we respect a rcu grace period at dismantle
time already.

Nothing bad can happen inside the rcu_read_lock()/rcu_read_unlock() pair
in packed_cached_dev_get()

Note that dev_put() does not take any immediate action, it only
decrements the refcount.

So if CPU1 does the dev_put(po->prot_hook.dev) after
setting cached_dev to NULL, we should be safe.



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