netdev - Re: [PATCHv5 net-next] vxlan: virtual extensible lan

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Date:	Thu, 27 Sep 2012 16:00:54 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	David Miller <davem@...emloft.net>
Cc:	jesse@...ira.com, chrisw@...hat.com, netdev@...r.kernel.org
Subject: Re: [PATCHv5 net-next] vxlan: virtual extensible lan

On Thu, 27 Sep 2012 18:47:40 -0400 (EDT)
David Miller <davem@...emloft.net> wrote:

> From: Stephen Hemminger <shemminger@...tta.com>
> Date: Tue, 25 Sep 2012 15:09:57 -0700
> 
> > +/* Hash Ethernet address */
> > +static u32 eth_hash(const unsigned char *addr)
> > +{
> > +	/* could be optimized for unaligned access */
> > +	u32 a = addr[5] << 8 | addr[4];
> > +	u32 b = addr[3] << 24 | addr[2] << 16 | addr[1] << 8 | addr[0];
> > +
> > +	return jhash_2words(a, b, vxlan_salt);
> > +}
> > +
> > +/* Hash chain to use given mac address */
> > +static inline struct hlist_head *vxlan_fdb_head(struct vxlan_dev *vxlan,
> > +						const u8 *mac)
> > +{
> > +	return &vxlan->fdb_head[hash_32(eth_hash(mac), FDB_HASH_BITS)];
> > +}
> 
> This hashing is way overkill, and in fact is a mis-use of Jenkins.
> 
> Jenkins can work fine with power-of-two hash sizes, so using hash_32()
> on the Jenkins hash result is nothing short of silly.
> 
> I would go one step further and change this to:
> 
> 	(a ^ b) * vxlan_salt

Ok, but doubt that it makes much difference.

> 
> much like how we hash IP addresses in the ipv4 ARP tables.  That
> function is a universal hash, and therefore provides whatever kind
> of protection you're trying to obtain using the salt.
> 
> But I wonder if this matters at all, the administrator controls
> the contents of this table, rather than external entitites.

The table includes values learned from packets received. Like a bridge,
a malicious attacker who can forge MAC sourc addresses can overload one
chain by swamping the table with bogus values.  Probably needs a table limit.
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