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futex: Fix pi_state->owner serialization

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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:

  734009e96d ("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: 734009e96d ("futex: Change locking rules")
Reported-by: Gratian Crisan <gratian.crisan@ni.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas 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
This commit is contained in:
Peter Zijlstra 2017-09-22 17:48:06 +02:00 committed by Thomas Gleixner
parent e19b205be4
commit c74aef2d06
1 changed files with 22 additions and 11 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

33
kernel/futex.c
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@ -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);