Merge "futex: Handle faults correctly for PI futexes"

3 files changed, 168 insertions, 93 deletions

diff --git a/kernel/futex.c b/kernel/futex.c
index 6dba4ebfdcfd..acabbfd3d66f 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -809,6 +809,29 @@ static struct futex_pi_state * alloc_pi_state(void)
 	return pi_state;
 }
 
+static void pi_state_update_owner(struct futex_pi_state *pi_state,
+				  struct task_struct *new_owner)
+{
+	struct task_struct *old_owner = pi_state->owner;
+
+	lockdep_assert_held(&pi_state->pi_mutex.wait_lock);
+
+	if (old_owner) {
+		raw_spin_lock(&old_owner->pi_lock);
+		WARN_ON(list_empty(&pi_state->list));
+		list_del_init(&pi_state->list);
+		raw_spin_unlock(&old_owner->pi_lock);
+	}
+
+	if (new_owner) {
+		raw_spin_lock(&new_owner->pi_lock);
+		WARN_ON(!list_empty(&pi_state->list));
+		list_add(&pi_state->list, &new_owner->pi_state_list);
+		pi_state->owner = new_owner;
+		raw_spin_unlock(&new_owner->pi_lock);
+	}
+}
+
 /*
  * Must be called with the hb lock held.
  */
@@ -825,11 +848,8 @@ static void free_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);
-
-		rt_mutex_proxy_unlock(&pi_state->pi_mutex, pi_state->owner);
+		pi_state_update_owner(pi_state, NULL);
+		rt_mutex_proxy_unlock(&pi_state->pi_mutex);
 	}
 
 	if (current->pi_state_cache)
@@ -966,7 +986,8 @@ void exit_pi_state_list(struct task_struct *curr)
  *	FUTEX_OWNER_DIED bit. See [4]
  *
  * [10] There is no transient state which leaves owner and user space
- *	TID out of sync.
+ *	TID out of sync. Except one error case where the kernel is denied
+ *	write access to the user address, see fixup_pi_state_owner().
  */
 
 /*
@@ -1350,26 +1371,16 @@ static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q *this,
 		else
 			ret = -EINVAL;
 	}
-	if (ret) {
-		raw_spin_unlock(&pi_state->pi_mutex.wait_lock);
-		return ret;
-	}
-
-	raw_spin_lock_irq(&pi_state->owner->pi_lock);
-	WARN_ON(list_empty(&pi_state->list));
-	list_del_init(&pi_state->list);
-	raw_spin_unlock_irq(&pi_state->owner->pi_lock);
-
-	raw_spin_lock_irq(&new_owner->pi_lock);
-	WARN_ON(!list_empty(&pi_state->list));
-	list_add(&pi_state->list, &new_owner->pi_state_list);
-	pi_state->owner = new_owner;
-	raw_spin_unlock_irq(&new_owner->pi_lock);
 
-	/*
-	 * We've updated the uservalue, this unlock cannot fail.
-	 */
-	deboost = __rt_mutex_futex_unlock(&pi_state->pi_mutex, &wake_q);
+	if (!ret) {
+		/*
+		 * This is a point of no return; once we modified the uval
+		 * there is no going back and subsequent operations must
+		 * not fail.
+		 */
+		pi_state_update_owner(pi_state, new_owner);
+		deboost = __rt_mutex_futex_unlock(&pi_state->pi_mutex, &wake_q);
+	}
 
 	raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
 	spin_unlock(&hb->lock);
@@ -2125,30 +2136,32 @@ 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)
+static int __fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q,
+				  struct task_struct *argowner)
 {
-	u32 newtid = task_pid_vnr(newowner) | FUTEX_WAITERS;
 	struct futex_pi_state *pi_state = q->pi_state;
-	struct task_struct *oldowner = pi_state->owner;
-	u32 uval, uninitialized_var(curval), newval;
-	int ret;
+	struct task_struct *oldowner, *newowner;
+	u32 uval, curval, newval, newtid;
+	int err = 0;
+
+	oldowner = pi_state->owner;
 
 	/* Owner died? */
 	if (!pi_state->owner)
 		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 failed to get the rtmutex the first time.
-	 * We have to replace the newowner TID in the user space variable.
+	 * We are here because either:
+	 *
+	 *  - we stole the lock and pi_state->owner needs updating to reflect
+	 *    that (@argowner == current),
+	 *
+	 * 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
@@ -2162,6 +2175,39 @@ static int fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q,
 	 * in lookup_pi_state.
 	 */
 retry:
+	if (!argowner) {
+		if (oldowner != current) {
+			/*
+			 * We raced against a concurrent self; things are
+			 * already fixed up. Nothing to do.
+			 */
+			return 0;
+		}
+
+		if (__rt_mutex_futex_trylock(&pi_state->pi_mutex)) {
+			/* We got the lock after all, nothing to fix. */
+			return 1;
+		}
+
+		/*
+		 * 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.
+			 */
+			return 1;
+		}
+		newowner = argowner;
+	}
+
+	newtid = task_pid_vnr(newowner) | FUTEX_WAITERS;
+
 	if (get_futex_value_locked(&uval, uaddr))
 		goto handle_fault;
 
@@ -2179,19 +2225,8 @@ retry:
 	 * We fixed up user space. Now we need to fix the pi_state
 	 * itself.
 	 */
-	if (pi_state->owner != NULL) {
-		raw_spin_lock_irq(&pi_state->owner->pi_lock);
-		WARN_ON(list_empty(&pi_state->list));
-		list_del_init(&pi_state->list);
-		raw_spin_unlock_irq(&pi_state->owner->pi_lock);
-	}
+	pi_state_update_owner(pi_state, newowner);
 
-	pi_state->owner = newowner;
-
-	raw_spin_lock_irq(&newowner->pi_lock);
-	WARN_ON(!list_empty(&pi_state->list));
-	list_add(&pi_state->list, &newowner->pi_state_list);
-	raw_spin_unlock_irq(&newowner->pi_lock);
 	return 0;
 
 	/*
@@ -2207,7 +2242,7 @@ retry:
 handle_fault:
 	spin_unlock(q->lock_ptr);
 
-	ret = fault_in_user_writeable(uaddr);
+	err = fault_in_user_writeable(uaddr);
 
 	spin_lock(q->lock_ptr);
 
@@ -2215,12 +2250,45 @@ handle_fault:
 	 * Check if someone else fixed it for us:
 	 */
 	if (pi_state->owner != oldowner)
-		return 0;
+		return argowner == current;
 
-	if (ret)
-		return ret;
+	/* Retry if err was -EAGAIN or the fault in succeeded */
+	if (!err)
+		goto retry;
 
-	goto retry;
+	/*
+	 * fault_in_user_writeable() failed so user state is immutable. At
+	 * best we can make the kernel state consistent but user state will
+	 * be most likely hosed and any subsequent unlock operation will be
+	 * rejected due to PI futex rule [10].
+	 *
+	 * Ensure that the rtmutex owner is also the pi_state owner despite
+	 * the user space value claiming something different. There is no
+	 * point in unlocking the rtmutex if current is the owner as it
+	 * would need to wait until the next waiter has taken the rtmutex
+	 * to guarantee consistent state. Keep it simple. Userspace asked
+	 * for this wreckaged state.
+	 *
+	 * The rtmutex has an owner - either current or some other
+	 * task. See the EAGAIN loop above.
+	 */
+	pi_state_update_owner(pi_state, rt_mutex_owner(&pi_state->pi_mutex));
+
+	return err;
+}
+
+static int fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q,
+				struct task_struct *argowner)
+{
+	struct futex_pi_state *pi_state = q->pi_state;
+	int ret;
+
+	lockdep_assert_held(q->lock_ptr);
+
+	raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
+	ret = __fixup_pi_state_owner(uaddr, q, argowner);
+	raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
+	return ret;
 }
 
 static long futex_wait_restart(struct restart_block *restart);
@@ -2248,6 +2316,10 @@ 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:
+		 *
+		 * 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);
@@ -2255,16 +2327,25 @@ static int fixup_owner(u32 __user *uaddr, struct futex_q *q, int locked)
 	}
 
 	/*
-	 * Paranoia check. If we did not take the lock, then we should not be
-	 * the owner of the rt_mutex.
+	 * 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 (rt_mutex_owner(&q->pi_state->pi_mutex) == current) {
-		printk(KERN_ERR "fixup_owner: ret = %d pi-mutex: %p "
-				"pi-state %p\n", ret,
-				q->pi_state->pi_mutex.owner,
-				q->pi_state->owner);
+	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. Warn and establish consistent state.
+	 */
+	if (WARN_ON_ONCE(rt_mutex_owner(&q->pi_state->pi_mutex) == current))
+		return fixup_pi_state_owner(uaddr, q, current);
+
 out:
 	return ret ? ret : locked;
 }
@@ -2569,13 +2650,6 @@ retry_private:
 	if (res)
 		ret = (res < 0) ? res : 0;
 
-	/*
-	 * If fixup_owner() faulted and was unable to handle the fault, unlock
-	 * it and return the fault to userspace.
-	 */
-	if (ret && (rt_mutex_owner(&q.pi_state->pi_mutex) == current))
-		rt_mutex_futex_unlock(&q.pi_state->pi_mutex);
-
 	/* Unqueue and drop the lock */
 	unqueue_me_pi(&q);
 
@@ -2880,8 +2954,6 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
 		if (q.pi_state && (q.pi_state->owner != current)) {
 			spin_lock(q.lock_ptr);
 			ret = fixup_pi_state_owner(uaddr2, &q, current);
-			if (ret && rt_mutex_owner(&q.pi_state->pi_mutex) == current)
-				rt_mutex_futex_unlock(&q.pi_state->pi_mutex);
 			/*
 			 * Drop the reference to the pi state which
 			 * the requeue_pi() code acquired for us.
@@ -2918,14 +2990,6 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
 		if (res)
 			ret = (res < 0) ? res : 0;
 
-		/*
-		 * If fixup_pi_state_owner() faulted and was unable to handle
-		 * the fault, unlock the rt_mutex and return the fault to
-		 * userspace.
-		 */
-		if (ret && rt_mutex_owner(pi_mutex) == current)
-			rt_mutex_futex_unlock(pi_mutex);
-
 		/* Unqueue and drop the lock. */
 		unqueue_me_pi(&q);
 	}
diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index e3dd1642423f..1c0cb5c3c6ad 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -1284,6 +1284,19 @@ 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:
  */
@@ -1305,13 +1318,7 @@ static inline int rt_mutex_slowtrylock(struct rt_mutex *lock)
 	 */
 	raw_spin_lock(&lock->wait_lock);
 
-	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(&lock->wait_lock);
 
@@ -1496,6 +1503,11 @@ 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
@@ -1650,8 +1662,7 @@ void rt_mutex_init_proxy_locked(struct rt_mutex *lock,
  * No locking. Caller has to do serializing itself
  * Special API call for PI-futex support
  */
-void rt_mutex_proxy_unlock(struct rt_mutex *lock,
-			   struct task_struct *proxy_owner)
+void rt_mutex_proxy_unlock(struct rt_mutex *lock)
 {
 	debug_rt_mutex_proxy_unlock(lock);
 	rt_mutex_set_owner(lock, NULL);
diff --git a/kernel/locking/rtmutex_common.h b/kernel/locking/rtmutex_common.h
index cdcaccfb7443..4584db96265d 100644
--- a/kernel/locking/rtmutex_common.h
+++ b/kernel/locking/rtmutex_common.h
@@ -101,8 +101,7 @@ enum rtmutex_chainwalk {
 extern struct task_struct *rt_mutex_next_owner(struct rt_mutex *lock);
 extern void rt_mutex_init_proxy_locked(struct rt_mutex *lock,
 				       struct task_struct *proxy_owner);
-extern void rt_mutex_proxy_unlock(struct rt_mutex *lock,
-				  struct task_struct *proxy_owner);
+extern void rt_mutex_proxy_unlock(struct rt_mutex *lock);
 extern int rt_mutex_start_proxy_lock(struct rt_mutex *lock,
 				     struct rt_mutex_waiter *waiter,
 				     struct task_struct *task);
@@ -113,6 +112,7 @@ extern bool rt_mutex_cleanup_proxy_lock(struct rt_mutex *lock,
 				 struct rt_mutex_waiter *waiter);
 extern int rt_mutex_timed_futex_lock(struct rt_mutex *l, struct hrtimer_sleeper *to);
 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,