netdev - Re: [PATCH] Fix xfrm hash collisions by changing __xfrm4_daddr_saddr

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Date:	Thu, 13 Aug 2009 12:06:06 +1000
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Jussi Maki <joamaki@...il.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org
Subject: Re: [PATCH] Fix xfrm hash collisions by changing
	__xfrm4_daddr_saddr_hash to hash addresses with addition

Jussi Maki <joamaki@...il.com> wrote:
>
> diff --git a/net/xfrm/xfrm_hash.h b/net/xfrm/xfrm_hash.h
> index d401dc8..e5195c9 100644
> --- a/net/xfrm/xfrm_hash.h
> +++ b/net/xfrm/xfrm_hash.h
> @@ -16,7 +16,7 @@ static inline unsigned int __xfrm6_addr_hash(xfrm_address_t *addr)
> 
> static inline unsigned int __xfrm4_daddr_saddr_hash(xfrm_address_t *daddr, xfrm_address_t *saddr)
> {
> -       return ntohl(daddr->a4 ^ saddr->a4);
> +       return ntohl(daddr->a4 + saddr->a4);
> }

What if the other side intentionally picks a destination addresses
to create collisions? Actually it's even easier than that.  We
don't include the SPI in the hash so regardless of how we hash
it, our peer can simply continue to create SAs with the same
descriptors and they'll all hash to the same bucket.

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
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