netdev - Re: [PATCH net] netlink: make sure -EBUSY won't escape from netlink

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Date:	Mon, 10 Aug 2015 11:00:15 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	daniel@...earbox.net
Cc:	herbert@...dor.apana.org.au, tgraf@...g.ch,
	torvalds@...ux-foundation.org, netdev@...r.kernel.org
Subject: Re: [PATCH net] netlink: make sure -EBUSY won't escape from
 netlink_insert

From: Daniel Borkmann <daniel@...earbox.net>
Date: Fri,  7 Aug 2015 00:26:41 +0200

> Linus reports the following deadlock on rtnl_mutex; triggered only
> once so far (extract):
 ...
> It seems so far plausible that the recursive call into rtnetlink_rcv()
> looks suspicious. One way, where this could trigger is that the senders
> NETLINK_CB(skb).portid was wrongly 0 (which is rtnetlink socket), so
> the rtnl_getlink() request's answer would be sent to the kernel instead
> to the actual user process, thus grabbing rtnl_mutex() twice.
> 
> One theory would be that netlink_autobind() triggered via netlink_sendmsg()
> internally overwrites the -EBUSY error to 0, but where it is wrongly
> originating from __netlink_insert() instead. That would reset the
> socket's portid to 0, which is then filled into NETLINK_CB(skb).portid
> later on. As commit d470e3b483dc ("[NETLINK]: Fix two socket hashing bugs.")
> also puts it, -EBUSY should not be propagated from netlink_insert().
> 
> It looks like it's very unlikely to reproduce. We need to trigger the
> rhashtable_insert_rehash() handler under a situation where rehashing
> currently occurs (one /rare/ way would be to hit ht->elasticity limits
> while not filled enough to expand the hashtable, but that would rather
> require a specifically crafted bind() sequence with knowledge about
> destination slots, seems unlikely). It probably makes sense to guard
> __netlink_insert() in any case and remap that error. It was suggested
> that EOVERFLOW might be better than an already overloaded ENOMEM.
> 
> Reference: http://thread.gmane.org/gmane.linux.network/372676
> Reported-by: Linus Torvalds <torvalds@...ux-foundation.org>
> Signed-off-by: Daniel Borkmann <daniel@...earbox.net>

Applied and queued up for -stable, thanks.
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