From: Peter Zijlstra peterz@infradead.org

4.9.115-rt94-rc1 stable review patch. If you have any objection to the inclusion of this patch, let me know.

--- 8< --- 8< --- 8< --- [ Upstream commit c74aef2d06a9f59cece89093eecc552933cba72a ]

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:

734009e96d19 ("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: 734009e96d19 ("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.... Signed-off-by: Sebastian Andrzej Siewior bigeasy@linutronix.de Signed-off-by: Julia Cartwright julia@ni.com --- kernel/futex.c | 34 ++++++++++++++++++++++------------ 1 file changed, 22 insertions(+), 12 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c index 8ab0ddd4cf8f..47e42faad6c5 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -819,8 +819,6 @@ 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) { @@ -835,16 +833,22 @@ 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. @@ -903,14 +907,15 @@ 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_irq(&curr->pi_lock); + raw_spin_unlock(&curr->pi_lock); + raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock); spin_unlock(&hb->lock); raw_spin_lock_irq(&curr->pi_lock); continue; @@ -920,9 +925,10 @@ 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); @@ -1204,6 +1210,10 @@ 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);