netdev - Re: [PATCH 3/3] netlink: Lock out table resizes while dumping Netlink sockets

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Date:	Wed, 21 Jan 2015 10:00:32 +0000
From:	Patrick McHardy <kaber@...sh.net>
To:	Thomas Graf <tgraf@...g.ch>
Cc:	Herbert Xu <herbert@...dor.apana.org.au>, davem@...emloft.net,
	paulmck@...ux.vnet.ibm.com, ying.xue@...driver.com,
	netdev@...r.kernel.org, netfilter-devel@...r.kernel.org
Subject: Re: [PATCH 3/3] netlink: Lock out table resizes while dumping
 Netlink sockets

On 21.01, Thomas Graf wrote:
> On 01/21/15 at 08:14pm, Herbert Xu wrote:
> > On Wed, Jan 21, 2015 at 04:15:55PM +1100, Herbert Xu wrote:
> > > I failed to address the security aspect of this approach.  Obviously
> > > this can only work if the only entites doing the walk are trusted.
> > > Which means that the vast majority (if not all) hash table users
> > > would be excluded, in particular, netlink.
> > 
> > OK maybe we can get around this.  So we will postpone the resize
> > or rehash when a walk is in place, however, when we hit a hard
> > limit, i.e., when insert would otherwise fail, then we do the
> > rehash regardless of any outstanding walks.  The walks will just
> > behave erratically or we could force them to start from scratch
> > again.
> 
> Exactly. I think we also need a timer to abort walks because if
> only a single walker is allowed, an attacker could start a walk and
> not complete it to block out everybody else.

Restarting them automatically sounds reasonable, duplicate entries have
always been possible. That makes me think, we could have userspace
indicate support for NLM_F_DUMP_INTR and otherwise always restart.
That would solve the problem very easily.
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