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:

  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.kicks-ass.net

1 files changed, 22 insertions, 11 deletions

diff --git a/kernel/futex.c b/kernel/futex.c
index 3d38eaf05492..0518a0bfc746 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -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 @@ retry:
                 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);