futex: Avoid violating the 10th rule of futex

commit c1e2f0eaf015fb7076d51a339011f2383e6dd389 upstream.

Julia reported futex state corruption in the following scenario:

   waiter                                  waker                                            stealer (prio > waiter)

   futex(WAIT_REQUEUE_PI, uaddr, uaddr2,
         timeout=[N ms])
      futex_wait_requeue_pi()
         futex_wait_queue_me()
            freezable_schedule()
            <scheduled out>
                                           futex(LOCK_PI, uaddr2)
                                           futex(CMP_REQUEUE_PI, uaddr,
                                                 uaddr2, 1, 0)
                                              /* requeues waiter to uaddr2 */
                                           futex(UNLOCK_PI, uaddr2)
                                                 wake_futex_pi()
                                                    cmp_futex_value_locked(uaddr2, waiter)
                                                    wake_up_q()
           <woken by waker>
           <hrtimer_wakeup() fires,
            clears sleeper->task>
                                                                                           futex(LOCK_PI, uaddr2)
                                                                                              __rt_mutex_start_proxy_lock()
                                                                                                 try_to_take_rt_mutex() /* steals lock */
                                                                                                    rt_mutex_set_owner(lock, stealer)
                                                                                              <preempted>
         <scheduled in>
         rt_mutex_wait_proxy_lock()
            __rt_mutex_slowlock()
               try_to_take_rt_mutex() /* fails, lock held by stealer */
               if (timeout && !timeout->task)
                  return -ETIMEDOUT;
            fixup_owner()
               /* lock wasn't acquired, so,
                  fixup_pi_state_owner skipped */

   return -ETIMEDOUT;

   /* At this point, we've returned -ETIMEDOUT to userspace, but the
    * futex word shows waiter to be the owner, and the pi_mutex has
    * stealer as the owner */

   futex_lock(LOCK_PI, uaddr2)
     -> bails with EDEADLK, futex word says we're owner.

And suggested that what commit:

  73d786bd043e ("futex: Rework inconsistent rt_mutex/futex_q state")

removes from fixup_owner() looks to be just what is needed. And indeed
it is -- I completely missed that requeue_pi could also result in this
case. So we need to restore that, except that subsequent patches, like
commit:

  16ffa12d7425 ("futex: Pull rt_mutex_futex_unlock() out from under hb->lock")

changed all the locking rules. Even without that, the sequence:

-               if (rt_mutex_futex_trylock(&q->pi_state->pi_mutex)) {
-                       locked = 1;
-                       goto out;
-               }

-               raw_spin_lock_irq(&q->pi_state->pi_mutex.wait_lock);
-               owner = rt_mutex_owner(&q->pi_state->pi_mutex);
-               if (!owner)
-                       owner = rt_mutex_next_owner(&q->pi_state->pi_mutex);
-               raw_spin_unlock_irq(&q->pi_state->pi_mutex.wait_lock);
-               ret = fixup_pi_state_owner(uaddr, q, owner);

already suggests there were races; otherwise we'd never have to look
at next_owner.

So instead of doing 3 consecutive wait_lock sections with who knows
what races, we do it all in a single section. Additionally, the usage
of pi_state->owner in fixup_owner() was only safe because only the
rt_mutex owner would modify it, which this additional case wrecks.

Luckily the values can only change away and not to the value we're
testing, this means we can do a speculative test and double check once
we have the wait_lock.

Fixes: 73d786bd043e ("futex: Rework inconsistent rt_mutex/futex_q state")
Reported-by: Julia Cartwright <julia@ni.com>
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>
Tested-by: Julia Cartwright <julia@ni.com>
Tested-by: Gratian Crisan <gratian.crisan@ni.com>
Cc: Darren Hart <dvhart@infradead.org>
Link: https://lkml.kernel.org/r/20171208124939.7livp7no2ov65rrc@hirez.programming.kicks-ass.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/kernel/futex.c b/kernel/futex.c
index 76ed592..29ac5b6 100644 (file)
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -2294,21 +2294,17 @@ static void unqueue_me_pi(struct futex_q *q)
        spin_unlock(q->lock_ptr);
 }
 
-/*
- * Fixup the pi_state owner with the new owner.
- *
- * Must be called with hash bucket lock held and mm->sem held for non
- * private futexes.
- */
 static int fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q,
-                               struct task_struct *newowner)
+                               struct task_struct *argowner)
 {
-       u32 newtid = task_pid_vnr(newowner) | FUTEX_WAITERS;
        struct futex_pi_state *pi_state = q->pi_state;
        u32 uval, uninitialized_var(curval), newval;
-       struct task_struct *oldowner;
+       struct task_struct *oldowner, *newowner;
+       u32 newtid;
        int ret;
 
+       lockdep_assert_held(q->lock_ptr);
+
        raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
 
        oldowner = pi_state->owner;
@@ -2317,11 +2313,17 @@ static int fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q,
                newtid |= FUTEX_OWNER_DIED;
 
        /*
-        * We are here either because we stole the rtmutex from the
-        * previous highest priority waiter or we are the highest priority
-        * waiter but have failed to get the rtmutex the first time.
+        * We are here because either:
+        *
+        *  - we stole the lock and pi_state->owner needs updating to reflect
+        *    that (@argowner == current),
         *
-        * We have to replace the newowner TID in the user space variable.
+        * or:
+        *
+        *  - someone stole our lock and we need to fix things to point to the
+        *    new owner (@argowner == NULL).
+        *
+        * Either way, we have to replace the TID in the user space variable.
         * This must be atomic as we have to preserve the owner died bit here.
         *
         * Note: We write the user space value _before_ changing the pi_state
@@ -2334,6 +2336,42 @@ static int fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q,
         * in the PID check in lookup_pi_state.
         */
 retry:
+       if (!argowner) {
+               if (oldowner != current) {
+                       /*
+                        * We raced against a concurrent self; things are
+                        * already fixed up. Nothing to do.
+                        */
+                       ret = 0;
+                       goto out_unlock;
+               }
+
+               if (__rt_mutex_futex_trylock(&pi_state->pi_mutex)) {
+                       /* We got the lock after all, nothing to fix. */
+                       ret = 0;
+                       goto out_unlock;
+               }
+
+               /*
+                * Since we just failed the trylock; there must be an owner.
+                */
+               newowner = rt_mutex_owner(&pi_state->pi_mutex);
+               BUG_ON(!newowner);
+       } else {
+               WARN_ON_ONCE(argowner != current);
+               if (oldowner == current) {
+                       /*
+                        * We raced against a concurrent self; things are
+                        * already fixed up. Nothing to do.
+                        */
+                       ret = 0;
+                       goto out_unlock;
+               }
+               newowner = argowner;
+       }
+
+       newtid = task_pid_vnr(newowner) | FUTEX_WAITERS;
+
        if (get_futex_value_locked(&uval, uaddr))
                goto handle_fault;
 
@@ -2434,9 +2472,9 @@ static int fixup_owner(u32 __user *uaddr, struct futex_q *q, int locked)
                 * Got the lock. We might not be the anticipated owner if we
                 * did a lock-steal - fix up the PI-state in that case:
                 *
-                * We can safely read pi_state->owner without holding wait_lock
-                * because we now own the rt_mutex, only the owner will attempt
-                * to change it.
+                * Speculative pi_state->owner read (we don't hold wait_lock);
+                * since we own the lock pi_state->owner == current is the
+                * stable state, anything else needs more attention.
                 */
                if (q->pi_state->owner != current)
                        ret = fixup_pi_state_owner(uaddr, q, current);
@@ -2444,6 +2482,19 @@ static int fixup_owner(u32 __user *uaddr, struct futex_q *q, int locked)
        }
 
        /*
+        * If we didn't get the lock; check if anybody stole it from us. In
+        * that case, we need to fix up the uval to point to them instead of
+        * us, otherwise bad things happen. [10]
+        *
+        * Another speculative read; pi_state->owner == current is unstable
+        * but needs our attention.
+        */
+       if (q->pi_state->owner == current) {
+               ret = fixup_pi_state_owner(uaddr, q, NULL);
+               goto out;
+       }
+
+       /*
         * Paranoia check. If we did not take the lock, then we should not be
         * the owner of the rt_mutex.
         */

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 6f3dba6..65cc0cb 100644 (file)
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -1290,6 +1290,19 @@ rt_mutex_slowlock(struct rt_mutex *lock, int state,
        return ret;
 }
 
+static inline int __rt_mutex_slowtrylock(struct rt_mutex *lock)
+{
+       int ret = try_to_take_rt_mutex(lock, current, NULL);
+
+       /*
+        * try_to_take_rt_mutex() sets the lock waiters bit
+        * unconditionally. Clean this up.
+        */
+       fixup_rt_mutex_waiters(lock);
+
+       return ret;
+}
+
 /*
  * Slow path try-lock function:
  */
@@ -1312,13 +1325,7 @@ static inline int rt_mutex_slowtrylock(struct rt_mutex *lock)
         */
        raw_spin_lock_irqsave(&lock->wait_lock, flags);
 
-       ret = try_to_take_rt_mutex(lock, current, NULL);
-
-       /*
-        * try_to_take_rt_mutex() sets the lock waiters bit
-        * unconditionally. Clean this up.
-        */
-       fixup_rt_mutex_waiters(lock);
+       ret = __rt_mutex_slowtrylock(lock);
 
        raw_spin_unlock_irqrestore(&lock->wait_lock, flags);
 
@@ -1505,6 +1512,11 @@ int __sched rt_mutex_futex_trylock(struct rt_mutex *lock)
        return rt_mutex_slowtrylock(lock);
 }
 
+int __sched __rt_mutex_futex_trylock(struct rt_mutex *lock)
+{
+       return __rt_mutex_slowtrylock(lock);
+}
+
 /**
  * rt_mutex_timed_lock - lock a rt_mutex interruptible
  *                     the timeout structure is provided

diff --git a/kernel/locking/rtmutex_common.h b/kernel/locking/rtmutex_common.h
index 124e98c..68686b3 100644 (file)
--- a/kernel/locking/rtmutex_common.h
+++ b/kernel/locking/rtmutex_common.h
@@ -148,6 +148,7 @@ extern bool rt_mutex_cleanup_proxy_lock(struct rt_mutex *lock,
                                 struct rt_mutex_waiter *waiter);
 
 extern int rt_mutex_futex_trylock(struct rt_mutex *l);
+extern int __rt_mutex_futex_trylock(struct rt_mutex *l);
 
 extern void rt_mutex_futex_unlock(struct rt_mutex *lock);
 extern bool __rt_mutex_futex_unlock(struct rt_mutex *lock,