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

From: Marcelo Ricardo Leitner <[email-masked]>
Date: Wed, 11 Jul 2018 23:29:34 -0300

On Mon, Jul 09, 2018 at 11:44:38PM +0300, Vlad Buslov wrote:
> 
> On Mon 09 Jul 2018 at 20:34, Marcelo Ricardo Leitner <[email-masked]> 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 <[email-masked]>
> >> Signed-off-by: Vlad Buslov <[email-masked]>
> >> ---
> >>  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?

Talked offline with Vlad and I agree that this is fine as is.

Reviewed-by: Marcelo Ricardo Leitner <[email-masked]>

Thanks,
Marcelo