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Commit c74aef2d authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Thomas Gleixner
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futex: Fix pi_state->owner serialization 



There was a reported suspicion about a race between exit_pi_state_list()
and put_pi_state(). The same report mentioned the comment with
put_pi_state() said it should be called with hb->lock held, and it no
longer is in all places.

As it turns out, the pi_state->owner serialization is indeed broken. As per
the new rules:

  734009e9 ("futex: Change locking rules")

pi_state->owner should be serialized by pi_state->pi_mutex.wait_lock.
For the sites setting pi_state->owner we already hold wait_lock (where
required) but exit_pi_state_list() and put_pi_state() were not and
raced on clearing it.

Fixes: 734009e9 ("futex: Change locking rules")
Reported-by: default avatarGratian Crisan <gratian.crisan@ni.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Cc: dvhart@infradead.org
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20170922154806.jd3ffltfk24m4o4y@hirez.programming.kicks-ass.net
parent e19b205b

kernel/futex.c

+22 −11
Original line number Diff line number Diff line
@@ -821,8 +821,6 @@ static void get_pi_state(struct futex_pi_state *pi_state) 
/*

 * Drops a reference to the pi_state object and frees or caches it

 * when the last reference is gone.

 *

 * Must be called with the hb lock held.

 */

static void put_pi_state(struct futex_pi_state *pi_state)

{
@@ -837,16 +835,22 @@ static void put_pi_state(struct futex_pi_state *pi_state) 
	 * and has cleaned up the pi_state already

	 */

	if (pi_state->owner) {

		raw_spin_lock_irq(&pi_state->owner->pi_lock);

		list_del_init(&pi_state->list);

		raw_spin_unlock_irq(&pi_state->owner->pi_lock);

		struct task_struct *owner;



		rt_mutex_proxy_unlock(&pi_state->pi_mutex, pi_state->owner);

		raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);

		owner = pi_state->owner;

		if (owner) {

			raw_spin_lock(&owner->pi_lock);

			list_del_init(&pi_state->list);

			raw_spin_unlock(&owner->pi_lock);

		}

		rt_mutex_proxy_unlock(&pi_state->pi_mutex, owner);

		raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);

	}



	if (current->pi_state_cache)

	if (current->pi_state_cache) {

		kfree(pi_state);

	else {

	} else {

		/*

		 * pi_state->list is already empty.

		 * clear pi_state->owner.
@@ -907,13 +911,14 @@ void exit_pi_state_list(struct task_struct *curr) 
		raw_spin_unlock_irq(&curr->pi_lock);



		spin_lock(&hb->lock);



		raw_spin_lock_irq(&curr->pi_lock);

		raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);

		raw_spin_lock(&curr->pi_lock);

		/*

		 * We dropped the pi-lock, so re-check whether this

		 * task still owns the PI-state:

		 */

		if (head->next != next) {

			raw_spin_unlock(&pi_state->pi_mutex.wait_lock);

			spin_unlock(&hb->lock);

			continue;

		}
@@ -922,9 +927,10 @@ void exit_pi_state_list(struct task_struct *curr) 
		WARN_ON(list_empty(&pi_state->list));

		list_del_init(&pi_state->list);

		pi_state->owner = NULL;

		raw_spin_unlock_irq(&curr->pi_lock);

		raw_spin_unlock(&curr->pi_lock);



		get_pi_state(pi_state);

		raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);

		spin_unlock(&hb->lock);



		rt_mutex_futex_unlock(&pi_state->pi_mutex);
@@ -1208,6 +1214,10 @@ static int attach_to_pi_owner(u32 uval, union futex_key *key, 


	WARN_ON(!list_empty(&pi_state->list));

	list_add(&pi_state->list, &p->pi_state_list);

	/*

	 * Assignment without holding pi_state->pi_mutex.wait_lock is safe

	 * because there is no concurrency as the object is not published yet.

	 */

	pi_state->owner = p;

	raw_spin_unlock_irq(&p->pi_lock);
@@ -2878,6 +2888,7 @@ static int futex_unlock_pi(u32 __user *uaddr, unsigned int flags) 
		raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);

		spin_unlock(&hb->lock);



		/* drops pi_state->pi_mutex.wait_lock */

		ret = wake_futex_pi(uaddr, uval, pi_state);



		put_pi_state(pi_state);