netdev - ip6_rt_gc_elasticity and ip6_rt_gc_min_interval race in rt6_alloc

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Date:	Fri, 30 Nov 2012 19:43:36 +0100
From:	Jiri Bohac <jbohac@...e.cz>
To:	netdev@...r.kernel.org
Cc:	mkubecek@...e.cz
Subject: ip6_rt_gc_elasticity and ip6_rt_gc_min_interval race in
 rt6_alloc_cow() ?

Hi,

I came across this code in rt6_alloc_cow():

	if (rt6_bind_neighbour(rt, rt->dst.dev)) {
		struct net *net = dev_net(rt->dst.dev);
		int saved_rt_min_interval =
			net->ipv6.sysctl.ip6_rt_gc_min_interval;
		int saved_rt_elasticity =
			net->ipv6.sysctl.ip6_rt_gc_elasticity;

		if (attempts-- > 0) {
			net->ipv6.sysctl.ip6_rt_gc_elasticity = 1;
			net->ipv6.sysctl.ip6_rt_gc_min_interval = 0;

			ip6_dst_gc(&net->ipv6.ip6_dst_ops);

			net->ipv6.sysctl.ip6_rt_gc_elasticity =
				saved_rt_elasticity;
			net->ipv6.sysctl.ip6_rt_gc_min_interval =
				saved_rt_min_interval;
			goto retry;
		}

		net_warn_ratelimited("Neighbour table overflow\n");
		dst_free(&rt->dst);
		return NULL;
	}

I believe there is a race condition in the setting of
ip6_rt_gc_elasticity and ip6_rt_gc_min_interval. I have not seen
happen, though:

1) a setting made by the user via sysctl while ip6_dst_gc() is
   running will get lost

2) another instance of rt6_alloc_cow() may save the temporary
   values in the saved_* variables, making those permanent.

Am I overlooking some synchronization or should I send a
patch to fix this?

Thanks,

-- 
Jiri Bohac <jbohac@...e.cz>
SUSE Labs, SUSE CZ

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