netdev - Re: [PATCH] net: sched: fix memory leak in tcindex_set

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Date:   Sun, 6 Nov 2022 09:49:16 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Hawkins Jiawei <yin31149@...il.com>
Cc:     18801353760@....com, davem@...emloft.net, edumazet@...gle.com,
        jhs@...atatu.com, jiri@...nulli.us, kuba@...nel.org,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        pabeni@...hat.com,
        syzbot+232ebdbd36706c965ebf@...kaller.appspotmail.com,
        syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH] net: sched: fix memory leak in tcindex_set_parms

On Sun, Nov 06, 2022 at 10:55:31PM +0800, Hawkins Jiawei wrote:
> Hi Cong,
> 
> >
> >
> > diff --git a/net/sched/cls_tcindex.c b/net/sched/cls_tcindex.c
> > index 1c9eeb98d826..00a6c04a4b42 100644
> > --- a/net/sched/cls_tcindex.c
> > +++ b/net/sched/cls_tcindex.c
> > @@ -479,6 +479,7 @@ tcindex_set_parms(struct net *net, struct tcf_proto *tp, unsigned long base,
> >         }
> >
> >         if (old_r && old_r != r) {
> > +               tcf_exts_destroy(&old_r->exts);
> >                 err = tcindex_filter_result_init(old_r, cp, net);
> >                 if (err < 0) {
> >                         kfree(f);
> 
> As for the position of the tcf_exts_destroy(), should we
> call it after the RCU updating, after
> `rcu_assign_pointer(tp->root, cp)` ?
> 
> Or the concurrent RCU readers may derefer this freed memory
> (Please correct me If I am wrong).

I don't think so, because we already have tcf_exts_change() in multiple
places within tcindex_set_parms(). Even if this is really a problem,
moving it after rcu_assign_pointer() does not help, you need to wait for
a grace period.

Thanks.