Re: [PATCH] lockdep: Speed up lockdep_unregister_key() with expedited RCU synchronization

Boqun Feng
2025-03-25T03:51:13Z

On Mon, Mar 24, 2025 at 09:56:25PM -0400, Waiman Long wrote:
> On 3/24/25 8:47 PM, Boqun Feng wrote:
> > On Mon, Mar 24, 2025 at 12:30:10PM -0700, Boqun Feng wrote:
> > > On Mon, Mar 24, 2025 at 12:21:07PM -0700, Boqun Feng wrote:
> > > > On Mon, Mar 24, 2025 at 01:23:50PM +0100, Eric Dumazet wrote:
> > > > [...]
> > > > > > > ---
> > > > > > > kernel/locking/lockdep.c | 6 ++++--
> > > > > > > 1 file changed, 4 insertions(+), 2 deletions(-)
> > > > > > >
> > > > > > > diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> > > > > > > index 4470680f02269..a79030ac36dd4 100644
> > > > > > > --- a/kernel/locking/lockdep.c
> > > > > > > +++ b/kernel/locking/lockdep.c
> > > > > > > @@ -6595,8 +6595,10 @@ void lockdep_unregister_key(struct lock_class_key *key)
> > > > > > > if (need_callback)
> > > > > > > call_rcu(&delayed_free.rcu_head, free_zapped_rcu);
> > > > > > >
> > > > > > > - /* Wait until is_dynamic_key() has finished accessing k->hash_entry. */
> > > > > > > - synchronize_rcu();
> > > > I feel a bit confusing even for the old comment, normally I would expect
> > > > the caller of lockdep_unregister_key() should guarantee the key has been
> > > > unpublished, in other words, there is no way a lockdep_unregister_key()
> > > > could race with a register_lock_class()/lockdep_init_map_type(). The
> > > > synchronize_rcu() is not needed then.
> > > >
> > > > Let's say someone breaks my assumption above, then when doing a
> > > > register_lock_class() with a key about to be unregister, I cannot see
> > > > anything stops the following:
> > > >
> > > > CPU 0 CPU 1
> > > > ===== =====
> > > > register_lock_class():
> > > > ...
> > > > } else if (... && !is_dynamic_key(lock->key)) {
> > > > // ->key is not unregistered yet, so this branch is not
> > > > // taken.
> > > > return NULL;
> > > > }
> > > > lockdep_unregister_key(..);
> > > > // key unregister, can be free
> > > > // any time.
> > > > key = lock->key->subkeys + subclass; // BOOM! UAF.

This is not a UAF :(

> > > >
> > > > So either we don't need the synchronize_rcu() here or the
> > > > synchronize_rcu() doesn't help at all. Am I missing something subtle
> > > > here?
> > > >
> > > Oh! Maybe I was missing register_lock_class() must be called with irq
> > > disabled, which is also an RCU read-side critical section.
> > >
> > Since register_lock_class() will be call with irq disabled, maybe hazard
> > pointers [1] is better because most of the case we only have nr_cpus
> > readers, so the potential hazard pointer slots are fixed.
> >
> > So the below patch can reduce the time of the tc command from real ~1.7
> > second (v6.14) to real ~0.05 second (v6.14 + patch) in my test env,
> > which is not surprising given it's a dedicated hazard pointers for
> > lock_class_key.
> >
> > Thoughts?
>
> My understanding is that it is not a race between register_lock_class() and
> lockdep_unregister_key(). It is the fact that the structure that holds the
> lock_class_key may be freed immediately after return from
> lockdep_unregister_key(). So any processes that are in the process of
> iterating the hash_list containing the hash_entry to be unregistered may hit

You mean the lock_keys_hash table, right? I used register_lock_class()
as an example, because it's one of the places that iterates
lock_keys_hash. IIUC lock_keys_hash is only used in
lockdep_{un,}register_key() and is_dynamic_key() (which are only called
by lockdep_init_map_type() and register_lock_class()).

> a UAF problem. See commit 61cc4534b6550 ("locking/lockdep: Avoid potential
> access of invalid memory in lock_class") for a discussion of this kind of
> UAF problem.
>

That commit seemed fixing a race between disabling lockdep and
unregistering key, and most importantly, call zap_class() for the
unregistered key even if lockdep is disabled (debug_locks = 0). It might
be related, but I'm not sure that's the reason of putting
synchronize_rcu() there. Say you want to synchronize between
/proc/lockdep and lockdep_unregister_key(), and you have
synchronize_rcu() in lockdep_unregister_key(), what's the RCU read-side
critical section at /proc/lockdep?

Regards,
Boqun

> As suggested by Eric, one possible solution is to add a
> lockdep_unregister_key() variant function that presumes the structure
> holding the key won't be freed until after a RCU delay. In this case, we can
> skip the last synchronize_rcu() call. Any callers that need immediate return
> should use kfree_rcu() to free the structure after calling the
> lockdep_unregister_key() variant.
>
> Cheers,
> Longman
>