linux-kernel - Re: [PATCH v6 00/16] locking/lockdep: Add support for dynamic keys

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Date:   Mon, 14 Jan 2019 13:52:35 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Bart Van Assche <bvanassche@....org>
Cc:     mingo@...hat.com, tj@...nel.org, longman@...hat.com,
        johannes.berg@...el.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 00/16] locking/lockdep: Add support for dynamic keys

On Fri, Jan 11, 2019 at 09:01:41AM -0800, Bart Van Assche wrote:
> On Fri, 2019-01-11 at 17:55 +0100, Peter Zijlstra wrote:
> > On Fri, Jan 11, 2019 at 07:55:03AM -0800, Bart Van Assche wrote:
> > > On Fri, 2019-01-11 at 13:48 +0100, Peter Zijlstra wrote:
> > > > I spotted this new v6 in my inbox and have rebased to it.
> > > 
> > > Thanks!
> > > 
> > > > On Wed, Jan 09, 2019 at 01:01:48PM -0800, Bart Van Assche wrote:
> > > > 
> > > > > The changes compared to v5 are:
> > > > > - Modified zap_class() such that it doesn't try to free a list entry that
> > > > >   is already being freed.
> > > > 
> > > > I however have a question on this; this seems wrong. Once a list entry
> > > > is enqueued it should not be reachable anymore. If we can reach an entry
> > > > after call_rcu() happened, we've got a problem.
> > > 
> > > Apparently I confused you - sorry that I was not more clear. What I meant is
> > > that I changed a single if test into a loop. The graph lock is held while that
> > > loop is being executed so the code below is serialized against the code called
> > > from inside the RCU callback:
> > > 
> > > @@ -4574,8 +4563,9 @@ static void zap_class(struct pending_free *pf, struct lock
> > > _class *class)
> > >                 entry = list_entries + i;
> > >                 if (entry->class != class && entry->links_to != class)
> > >                         continue;
> > > -               if (__test_and_set_bit(i, pf->list_entries_being_freed))
> > > +               if (list_entry_being_freed(i))
> > >                         continue;
> > 
> > Yes, it is the above change that caught my eye.. That checks _both_ your
> > lists. One is your current open one (@pf), but the other could already
> > be pending the call_rcu().
> > 
> > So my question is why do we have to check both ?! How come the old code,
> > that only checked @pf, is wrong?
> > 
> > > +               set_bit(i, pf->list_entries_being_freed);
> > >                 nr_list_entries--;
> > >                 list_del_rcu(&entry->entry);
> > >         }
> 
> The list_del_rcu() call must only happen once. 

Yes; obviously. But if we need to check all @pf's, that means the entry
is still reachable after a single reset_lock()/free_key_range(), which
is a bug.

> I ran into complaints reporting that
> the list_del_rcu() call triggered list corruption. This change made these complaints
> disappear.

I'm saying this solution buggy, because that means the entry is still
reachable after we do call_rcu() (which is a straight up UAF).

Also put it differently, what guarantees checking those two @pf's is
sufficient. Suppose your earlier @pf already did the RCU callback and
freed stuff while the second is in progress. Then you're poking into
dead space.