linux-kernel - Re: netfilter: nf_conntrack: there maybe a bug in __nf_conntrack_confirm, when it race against get_next

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Message-ID: <012601cff7d1$7ce2d620$76a88260$@gmail.com>
Date:	Tue, 4 Nov 2014 09:48:32 +0800
From:	"billbonaparte" <programme110@...il.com>
To:	<fw@...len.de>
Cc:	<linux-kernel@...r.kernel.org>,
	"Pablo Neira Ayuso" <pablo@...filter.org>,
	"Patrick McHardy" <kaber@...sh.net>, <kadlec@...ckhole.kfki.hu>,
	<davem@...emloft.net>, "Changli Gao" <xiaosuo@...il.com>,
	"Jesper Dangaard Brouer" <brouer@...hat.com>,
	"Andrey Vagin" <avagin@...nvz.org>
Subject: Re: netfilter: nf_conntrack: there maybe a bug in __nf_conntrack_confirm, when it race against get_next_corpse

(sorry to send this e-mail again, last mail is rejected by server due to
non-acceptable content)

Florian Westphal [mailto:fw@...len.de] wrote:
>Correct.  This is broken since the central spin lock removal, since 
>nf_conntrack_lock no longer protects both get_next_corpse and 
>conntrack_confirm.
> 
>Please send a patch, moving dying check after removal of conntrack from 
>the percpu list,
Since unconfirmed conntrack is stored in unconfirmed-list which is per-cpu
list and protected by per-cpu spin-lock, we can remove it from
uncomfirmed-list and insert it into ct-hash-table separately. that is to
say, we can remove it from uncomfirmed-list without holding corresponding
hash-lock, then check if it is dying.
if it is dying, we add it to the dying-list, then quit
__nf_conntrack_confirm. we do this to follow the rules that the conntrack
must alternatively at unconfirmed-list or dying-list when it is abort to be
destroyed.

>> 	2. operation on ct->status should be atomic, because it race aginst 
>> get_next_corpse.
>
>Alternatively we could also get rid of the unconfirmed list handling in
get_next_corpse, 
>it looks to me as if its simply not worth the trouble to also caring 
>about
unconfirmed lists.

yes, I think so. 
if there is a race at operating ct->status, there will be in alternative
case:
1) IPS_DYING bit which set in get_next_corpse override other bits (e.g.
IPS_SRC_NAT_DONE_BIT), or
2) other bits (e.g. IPS_SRC_NAT_DONE_BIT) which set in nf_nat_setup_info
override IPS_DYING bit.
but, any case seems to be okay.

Download attachment "fix_conntrack_confirm_race.patch" of type "application/octet-stream" (2623 bytes)