Re: [PATCH nf 1/2] netfilter: nfnetlink_acct: fix race between nfacct del and xt_nfacct destroy

[Pablo Neira Ayuso <pablo@netfilter.org>]

On Sat, Aug 13, 2016 at 11:13:01PM +0800, Liping Zhang wrote:
> From: Liping Zhang <liping.zhang@spreadtrum.com>
> 
> Suppose that we input the following commands at first:
>   # nfacct add test
>   # iptables -A INPUT -m nfacct --nfacct-name test
> 
> And now "test" acct's refcnt is 2, but later when we try to delete the
> "test" nfacct and the related iptables rule at the same time, race maybe
> happen:
>       CPU0                                    CPU1
>   nfnl_acct_try_del                      nfnl_acct_put
>   atomic_dec_and_test //ref=1,testfail          -
>        -                                 atomic_dec_and_test //ref=0,testok
>        -                                 kfree_rcu
>   atomic_inc //ref=1                            -
> 
> So after the rcu grace period, nf_acct will be freed but it is still linked
> in the nfnl_acct_list, and we can access it later, then oops will happen.
> 
> Convert atomic_dec_and_test and atomic_inc combinaiton to one atomic
> operation atomic_cmpxchg here to fix this problem.

Applied, thanks.