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SubjectRe: [PATCH v3 17/24] locking/lockdep: Free lock classes that are no longer in use
On Thu, Dec 06, 2018 at 05:11:41PM -0800, Bart Van Assche wrote:
> + if (WARN_ON_ONCE(!hlock_class(prev)->hash_entry.pprev) ||
> + WARN_ONCE(!hlock_class(next)->hash_entry.pprev,
> + KERN_INFO "Detected use-after-free of lock class %s\n",
> + hlock_class(next)->name)) {
> + return 2;
> + }

Ah, this is that UaF on ->name, but it only happens when there's already
been a UaF, so that's fine I suppose. Still a note on that earlier
Changelog would've been nice I suppose.

> +/* Must be called with the graph lock held. */
> +static void remove_class_from_lock_chain(struct lock_chain *chain,
> + struct lock_class *class)
> +{
> + u64 chain_key;
> + int i;
> +
> +#ifdef CONFIG_PROVE_LOCKING
> + for (i = chain->base; i < chain->base + chain->depth; i++) {
> + if (chain_hlocks[i] != class - lock_classes)
> + continue;
> + if (--chain->depth > 0)
{
> + memmove(&chain_hlocks[i], &chain_hlocks[i + 1],
> + (chain->base + chain->depth - i) *
> + sizeof(chain_hlocks[0]));
}

Also, I suppose a comment here that notes we 'leak' chain_hlock[]
entries would be appropriate here.

If Waiman cares, it is possible to reclaim then by extending the above
memmove() to cover the _entire_ tail of the array and then going around
and fixing up all the chain->base 'pointers' that are in the moved part.

> + /*
> + * Each lock class occurs at most once in a
> + * lock chain so once we found a match we can
> + * break out of this loop.
> + */
> + goto recalc;
> + }
> + /* Since the chain has not been modified, return. */
> + return;
> +
> +recalc:
> + /*
> + * Note: calling hlist_del_rcu() from inside a
> + * hlist_for_each_entry_rcu() loop is safe.
> + */
> + if (chain->depth == 0) {
> + /* To do: decrease chain count. See also inc_chains(). */
> + hlist_del_rcu(&chain->entry);
> + return;
> + }
> + chain_key = 0;
> + for (i = chain->base; i < chain->base + chain->depth; i++)
> + chain_key = iterate_chain_key(chain_key, chain_hlocks[i] + 1);
> + if (chain->chain_key == chain_key)
> + return;
> + hlist_del_rcu(&chain->entry);
> + chain->chain_key = chain_key;
> + hlist_add_head_rcu(&chain->entry, chainhashentry(chain_key));

I think this is broken; while hlist_del_rcu() and hlist_add_head_rcu()
are individually safe against concurrent hlist_for_each_entry_rcu(),
they cannot be used consecutively like this.

The thing is, hlist_del_rcu() preserves the ->next pointer such that
a concurrent hlist_for_each_entry_rcu() that happens to be at that entry
will be able to continue.

But your hlist_add_head_rcu() will overwrite that, so the concurrent
iterator, which started on one list will continue on another.

There must be an RCU-GP between del_rcu and add_rcu such that the above
situation cannot happen.


> +/*
> + * Free all lock classes that are on the zapped_classes list. Called as an
> + * RCU callback function.
> + */
> +static void free_zapped_classes(struct callback_head *ch)
> +{
> + struct lock_class *class;
> + unsigned long flags;
> + int locked;
> +
> + raw_local_irq_save(flags);
> + locked = graph_lock();

If this fails; you must not touch any of the state. You don't hold the
lock after all.

> + rcu_callback_scheduled = false;
> + list_for_each_entry(class, &zapped_classes, lock_entry) {
> + reinit_class(class);
> + nr_lock_classes--;
> + }
> + list_splice_init(&zapped_classes, &free_lock_classes);
> + if (locked)
> + graph_unlock();
> + raw_local_irq_restore(flags);
> +}
> +
> +/* Must be called with the graph lock held. */
> +static void schedule_free_zapped_classes(void)
> +{
> + if (rcu_callback_scheduled)
> + return;
> + rcu_callback_scheduled = true;
> + call_rcu(&free_zapped_classes_rcu_head, free_zapped_classes);
> +}

But I fear this is fundamentally broken too. In exactly the manner I
tried to explain earlier.

(sorry, wide(r) terminal required)

It starts out good:

CPU0 CPU1 CPU2

rcu_read_lock()
// use class
zap_class()
schedule_free_zapped_classes();
call_rcu();

rcu_read_lock()

/* since none of the rcu_read_lock()
* sections we started out with are
* still active; this is where the
* callback _can_ happen */

But then, another user and and removal come in:

rcu_read_lock();

// use class zap_class()
list_add(&entry, &zapped_classes)
schedule_free_zapped_classes()
/* no-op, still pending */

free_zapped_classes()
list_splice(&zapped_classes, &free_lock_classes)

*whoops* we just splice'd a class
onto the free list that is still in
use !!


rcu_read_unlock()


There's two possible solutions:

- the second schedule_free_zapped_classes() must somehow ensure the
call_rcu() callback re-queues itself and waits for yet another GP, or

- you must add classes zapped after the first invocation to a second
list.


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