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

[Hannes Frederic Sowa <hannes@stressinduktion.org>]

Hi Julian,

On Mi, 2015-01-21 at 10:56 +0200, Julian Anastasov wrote:
> 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

Very good catch, thanks. Will post v2 soon.

Thanks,
Hannes