netdev - Re: [PATCH net] ipv4: try to cache dst_entries which would cause a redirect

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Date:	Wed, 21 Jan 2015 10:56:16 +0200 (EET)
From:	Julian Anastasov <ja@....bg>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
cc:	netdev@...r.kernel.org, Marcelo Leitner <mleitner@...hat.com>,
	Florian Westphal <fw@...len.de>
Subject: Re: [PATCH net] ipv4: try to cache dst_entries which would cause a
 redirect


	Hello,

On Tue, 20 Jan 2015, Hannes Frederic Sowa wrote:

> Not caching dst_entries which cause redirects could be exploited by hosts
> on the same subnet, causing a severe DoS attack. This effect aggravated
> since commit f88649721268999 ("ipv4: fix dst race in sk_dst_get()").
> 
> Lookups causing redirects will be allocated with DST_NOCACHE set which
> will force dst_release to free them via RCU.  Unfortunately waiting for
> RCU grace period just takes too long, we can end up with >1M dst_entries
> waiting to be released and the system will run OOM. rcuos threads cannot
> catch up under high softirq load.
> 
> Attaching the flag to emit a redirect later on to the specific skb allows
> us to cache those dst_entries thus reducing the pressure on allocation
> and deallocation.
> 
> This issue was discovered by Marcelo Leitner.

	Change looks good to me but additional place
should be changed too: inet_rtm_getroute() will call
ip_route_input() and later rt_fill_info() will put
rt_flags in rtm_flags. We have to set RTCF_DOREDIRECT
just in rtm_flags depending on IPSKB_DOREDIRECT becuase
iproute needs to print "redirect". You can test it with
ip route get ... iif INDEV

Regards

--
Julian Anastasov <ja@....bg>
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