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

Boqun Feng
2025-03-25T18:45:15Z

On Tue, Mar 25, 2025 at 10:52:16AM -0400, Waiman Long wrote:
> On 3/24/25 11:41 PM, Boqun Feng wrote:
> > 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 unpublished, in other words, there is no way a lockdep_unregister_key()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?
> I agree that the commit that I mentioned is not relevant to the current
> case. You are right that is_dynamic_key() is the only function that is
> problematic, the other two are protected by the lockdep_lock. So they are
> safe. Anyway, I believe that the actual race happens in the iteration of the
> hashed list in is_dynamic_key(). The key that you save in the
> lockdep_key_hazptr in your proposed patch should never be the key (dead_key)

The key stored in lockdep_key_hazptr is the one that the rest of the
function will use after is_dynamic_key() return true. That is,

CPU 0 CPU 1
===== =====
WRITE_ONCE(*lockdep_key_hazptr, key);
smp_mb();

is_dynamic_key():
hlist_for_each_entry_rcu(k, hash_head, hash_entry) {
if (k == key) {
found = true;
break;
}
}
lockdep_unregister_key():
hlist_for_each_entry_rcu(k, hash_head, hash_entry) {
if (k == key) {
hlist_del_rcu(&k->hash_entry);
found = true;
break;
}
}

smp_mb();

synchronize_lockdep_key_hazptr():
for_each_possible_cpu(cpu) {
<wait for the hazptr slot on
that CPU to be not equal to
the removed key>
}

, so that if is_dynamic_key() finds a key was in the hash, even though
later on the key would be removed by lockdep_unregister_key(), the
hazard pointers guarantee lockdep_unregister_key() would wait for the
hazard pointer to release.

> that is passed to lockdep_unregister_key(). In is_dynamic_key():
>
>     hlist_for_each_entry_rcu(k, hash_head, hash_entry) {
>                 if (k == key) {
>                         found = true;
>                         break;
>                 }
>         }
>
> key != k (dead_key), but before accessing its content to get to hash_entry,

It is the dead_key.

> an interrupt/NMI can happen. In the mean time, the structure holding the key
> is freed and its content can be overwritten with some garbage. When
> interrupt/NMI returns, hash_entry can point to anything leading to crash or
> an infinite loop.  Perhaps we can use some kind of synchronization mechanism

No, hash_entry cannot be freed or overwritten because the user has
protect the key with hazard pointers, only when the user reset the
hazard pointer to NULL, lockdep_unregister_key() can then return and the
key can be freed.

> between is_dynamic_key() and lockdep_unregister_key() to prevent this kind
> of racing. For example, we can have an atomic counter associated with each

The hazard pointer I proposed provides the exact synchronization ;-)

Regards,
Boqun

> head of the hashed table. is_dynamic_key() can increment the counter if it
> is not zero to proceed and lockdep_unregister_key() have to make sure it can
> safely decrement the counter to 0 before going ahead. Just a thought!
>
> Cheers,
> Longman