Re: [Patch net] net_sched: add a temporary refcnt for struct tcindex_data

["Paul E. McKenney" <paulmck@kernel.org>]

On Mon, Mar 30, 2020 at 04:24:42PM -0700, Cong Wang wrote:
> On Mon, Mar 30, 2020 at 2:35 PM Paul E. McKenney <paulmck@kernel.org> wrote:
> >
> > On Sat, Mar 28, 2020 at 12:12:59PM -0700, Cong Wang wrote:
> > > Although we intentionally use an ordered workqueue for all tc
> > > filter works, the ordering is not guaranteed by RCU work,
> > > given that tcf_queue_work() is esstenially a call_rcu().
> > >
> > > This problem is demostrated by Thomas:
> > >
> > >   CPU 0:
> > >     tcf_queue_work()
> > >       tcf_queue_work(&r->rwork, tcindex_destroy_rexts_work);
> > >
> > >   -> Migration to CPU 1
> > >
> > >   CPU 1:
> > >      tcf_queue_work(&p->rwork, tcindex_destroy_work);
> > >
> > > so the 2nd work could be queued before the 1st one, which leads
> > > to a free-after-free.
> > >
> > > Enforcing this order in RCU work is hard as it requires to change
> > > RCU code too. Fortunately we can workaround this problem in tcindex
> > > filter by taking a temporary refcnt, we only refcnt it right before
> > > we begin to destroy it. This simplifies the code a lot as a full
> > > refcnt requires much more changes in tcindex_set_parms().
> > >
> > > Reported-by: syzbot+46f513c3033d592409d2@syzkaller.appspotmail.com
> > > Fixes: 3d210534cc93 ("net_sched: fix a race condition in tcindex_destroy()")
> > > Cc: Thomas Gleixner <tglx@linutronix.de>
> > > Cc: Paul E. McKenney <paulmck@kernel.org>
> > > Cc: Jamal Hadi Salim <jhs@mojatatu.com>
> > > Cc: Jiri Pirko <jiri@resnulli.us>
> > > Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
> >
> > Looks plausible, but what did you do to verify that the structures
> > were in fact being freed?  See below for more detail.
> 
> I ran the syzbot reproducer for about 20 minutes, there was no
> memory leak reported after scanning.

And if you (say) set the initial reference count to two instead of one,
there is a memory leak reported, correct?

							Thanx, Paul