netdev - Re: [PATCH net-next] net: sched: fix unprotected access to rcu cookie pointer

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Date:   Mon, 09 Jul 2018 23:44:38 +0300
From:   Vlad Buslov <vladbu@...lanox.com>
To:     Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, jhs@...atatu.com,
        xiyou.wangcong@...il.com, jiri@...nulli.us
Subject: Re: [PATCH net-next] net: sched: fix unprotected access to rcu cookie pointer


On Mon 09 Jul 2018 at 20:34, Marcelo Ricardo Leitner <marcelo.leitner@...il.com> wrote:
> On Mon, Jul 09, 2018 at 08:26:47PM +0300, Vlad Buslov wrote:
>> Fix action attribute size calculation function to take rcu read lock and
>> access act_cookie pointer with rcu dereference.
>> 
>> Fixes: eec94fdb0480 ("net: sched: use rcu for action cookie update")
>> Reported-by: Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
>> Signed-off-by: Vlad Buslov <vladbu@...lanox.com>
>> ---
>>  net/sched/act_api.c | 9 +++++++--
>>  1 file changed, 7 insertions(+), 2 deletions(-)
>> 
>> diff --git a/net/sched/act_api.c b/net/sched/act_api.c
>> index 66dc19746c63..148a89ab789b 100644
>> --- a/net/sched/act_api.c
>> +++ b/net/sched/act_api.c
>> @@ -149,10 +149,15 @@ EXPORT_SYMBOL(__tcf_idr_release);
>>  
>>  static size_t tcf_action_shared_attrs_size(const struct tc_action *act)
>>  {
>> +	struct tc_cookie *act_cookie;
>>  	u32 cookie_len = 0;
>>  
>> -	if (act->act_cookie)
>> -		cookie_len = nla_total_size(act->act_cookie->len);
>> +	rcu_read_lock();
>> +	act_cookie = rcu_dereference(act->act_cookie);
>> +
>> +	if (act_cookie)
>> +		cookie_len = nla_total_size(act_cookie->len);
>> +	rcu_read_unlock();
>
> I am not sure if this is enough to fix the entire issue. Now it will
> fetch the length correctly but, what guarantees that when it tries to
> actually copy the key (tcf_action_dump_1), the same act_cookie pointer
> will be used? As in, can't the new re-fetch be different/smaller than
> the object used here?

I checked the code of nlmsg_put() and similar functions, and they check
that there is enough free space at skb tailroom. If not, they fail
gracefully and return error. Am I missing something?

>
>>  
>>  	return  nla_total_size(0) /* action number nested */
>>  		+ nla_total_size(IFNAMSIZ) /* TCA_ACT_KIND */
>> -- 
>> 2.7.5
>>