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

[David Miller <davem@davemloft.net>]

From: Christoph Paasch <christoph.paasch@gmail.com>
Date: Wed, 16 Sep 2015 22:41:14 -0700

> Hello,
> 
> On 10/08/15 - 11:00:15, David Miller wrote:
>> From: Daniel Borkmann <daniel@iogearbox.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@linux-foundation.org>
>> > Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
>> 
>> Applied and queued up for -stable, thanks.
> 
> can this patch get queued up for 4.1 as well?
> It seems to fix a similar issue in 4.1.6.

Done.