From cd5ba5c89897fec633e8a8e75984fde6c5b5e600 Mon Sep 17 00:00:00 2001 From: Darren Hart Date: Fri, 9 Jul 2010 17:50:23 -0400 Subject: [PATCH] futex: Protect against pi_blocked_on corruption during requeue PI commit 19491de1201af8b1a506b32387a875dfb3bec2f9 in tip. The requeue_pi mechanism introduced proxy locking of the rtmutex. This creates a scenario where a task can wakeup, not knowing it has been enqueued on an rtmutex. Blocking on an hb->lock() can overwrite a valid value in current->pi_blocked_on, leading to an inconsistent state. Prevent overwriting pi_blocked_on by serializing on the waiter's pi_lock and using the new PI_WAKEUP_INPROGRESS state flag to indicate a waiter that has been woken by a timeout or signal. This prevents the rtmutex code from adding the waiter to the rtmutex wait list, returning EAGAIN to futex_requeue(), which will in turn ignore the waiter during a requeue. Care is taken to allow current to block on locks even if PI_WAKEUP_INPROGRESS is set. During normal wakeup, this results in one less hb->lock protected section. In the pre-requeue-timeout-or-signal wakeup, this removes the "greedy locking" behavior, no attempt will be made to acquire the lock. [ tglx: take pi_lock with lock_irq(), removed paranoid warning, plugged pi_state and pi_blocked_on leak, adjusted some comments ] Signed-off-by: Darren Hart Cc: Peter Zijlstra Cc: Eric Dumazet Cc: John Kacur Cc: Steven Rostedt Cc: Mike Galbraith LKML-Reference: <4C3C1DCF.9090509@us.ibm.com> Signed-off-by: Thomas Gleixner Signed-off-by: Paul Gortmaker --- kernel/futex.c | 76 +++++++++++++++++++++++++++++++++++++++-------- kernel/rtmutex.c | 33 +++++++++++++++++--- kernel/rtmutex_common.h | 3 ++ 3 files changed, 94 insertions(+), 18 deletions(-) diff --git a/kernel/futex.c b/kernel/futex.c index e7a35f1..6085332 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -1334,6 +1334,16 @@ retry_private: requeue_pi_wake_futex(this, &key2, hb2); drop_count++; continue; + } else if (ret == -EAGAIN) { + /* + * Waiter was woken by timeout or + * signal and has set pi_blocked_on to + * PI_WAKEUP_INPROGRESS before we + * tried to enqueue it on the rtmutex. + */ + this->pi_state = NULL; + free_pi_state(pi_state); + continue; } else if (ret) { /* -EDEADLK */ this->pi_state = NULL; @@ -2209,9 +2219,9 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, int fshared, int clockrt, u32 __user *uaddr2) { struct hrtimer_sleeper timeout, *to = NULL; + struct futex_hash_bucket *hb, *hb2; struct rt_mutex_waiter rt_waiter; struct rt_mutex *pi_mutex = NULL; - struct futex_hash_bucket *hb; union futex_key key2; struct futex_q q; int res, ret; @@ -2253,18 +2263,54 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, int fshared, /* Queue the futex_q, drop the hb lock, wait for wakeup. */ futex_wait_queue_me(hb, &q, to); - spin_lock(&hb->lock); - ret = handle_early_requeue_pi_wakeup(hb, &q, &key2, to); - spin_unlock(&hb->lock); - if (ret) - goto out_put_keys; + /* + * Avoid races with requeue and trying to block on two mutexes + * (hb->lock and uaddr2's rtmutex) by serializing access to + * pi_blocked_on with pi_lock. + */ + raw_spin_lock_irq(¤t->pi_lock); + if (current->pi_blocked_on) { + /* + * We have been requeued or are in the process of + * being requeued. + */ + raw_spin_unlock_irq(¤t->pi_lock); + } else { + /* + * Setting pi_blocked_on to PI_WAKEUP_INPROGRESS + * prevents a concurrent requeue from moving us to the + * uaddr2 rtmutex. After that we can safely acquire + * (and possibly block on) hb->lock. + */ + current->pi_blocked_on = PI_WAKEUP_INPROGRESS; + raw_spin_unlock_irq(¤t->pi_lock); + + spin_lock(&hb->lock); + + /* + * Clean up pi_blocked_on. We might leak it otherwise + * when we succeeded with the hb->lock in the fast + * path. + */ + raw_spin_lock_irq(¤t->pi_lock); + current->pi_blocked_on = NULL; + raw_spin_unlock_irq(¤t->pi_lock); + + ret = handle_early_requeue_pi_wakeup(hb, &q, &key2, to); + spin_unlock(&hb->lock); + if (ret) + goto out_put_keys; + } /* - * In order for us to be here, we know our q.key == key2, and since - * we took the hb->lock above, we also know that futex_requeue() has - * completed and we no longer have to concern ourselves with a wakeup - * race with the atomic proxy lock acquition by the requeue code. + * In order to be here, we have either been requeued, are in + * the process of being requeued, or requeue successfully + * acquired uaddr2 on our behalf. If pi_blocked_on was + * non-null above, we may be racing with a requeue. Do not + * rely on q->lock_ptr to be hb2->lock until after blocking on + * hb->lock or hb2->lock. */ + hb2 = hash_futex(&key2); /* Check if the requeue code acquired the second futex for us. */ if (!q.rt_waiter) { @@ -2273,10 +2319,12 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, int fshared, * did a lock-steal - fix up the PI-state in that case. */ if (q.pi_state && (q.pi_state->owner != current)) { - spin_lock(q.lock_ptr); + spin_lock(&hb2->lock); + BUG_ON(&hb2->lock != q.lock_ptr); + ret = fixup_pi_state_owner(uaddr2, &q, current, fshared); - spin_unlock(q.lock_ptr); + spin_unlock(&hb2->lock); } } else { /* @@ -2289,7 +2337,9 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, int fshared, ret = rt_mutex_finish_proxy_lock(pi_mutex, to, &rt_waiter, 1); debug_rt_mutex_free_waiter(&rt_waiter); - spin_lock(q.lock_ptr); + spin_lock(&hb2->lock); + BUG_ON(&hb2->lock != q.lock_ptr); + /* * Fixup the pi_state owner and possibly acquire the lock if we * haven't already. diff --git a/kernel/rtmutex.c b/kernel/rtmutex.c index 23dd443..348b1e7 100644 --- a/kernel/rtmutex.c +++ b/kernel/rtmutex.c @@ -82,6 +82,11 @@ static void fixup_rt_mutex_waiters(struct rt_mutex *lock) clear_rt_mutex_waiters(lock); } +static int rt_mutex_real_waiter(struct rt_mutex_waiter *waiter) +{ + return waiter && waiter != PI_WAKEUP_INPROGRESS; +} + /* * We can speed up the acquire/release, if the architecture * supports cmpxchg and if there's no debugging state to be set up @@ -227,7 +232,7 @@ static int rt_mutex_adjust_prio_chain(struct task_struct *task, * reached or the state of the chain has changed while we * dropped the locks. */ - if (!waiter || !waiter->task) + if (!rt_mutex_real_waiter(waiter) || !waiter->task) goto out_unlock_pi; /* @@ -448,6 +453,23 @@ static int task_blocks_on_rt_mutex(struct rt_mutex *lock, int chain_walk = 0, res; raw_spin_lock(&task->pi_lock); + + /* + * In the case of futex requeue PI, this will be a proxy + * lock. The task will wake unaware that it is enqueueed on + * this lock. Avoid blocking on two locks and corrupting + * pi_blocked_on via the PI_WAKEUP_INPROGRESS + * flag. futex_wait_requeue_pi() sets this when it wakes up + * before requeue (due to a signal or timeout). Do not enqueue + * the task if PI_WAKEUP_INPROGRESS is set. + */ + if (task != current && task->pi_blocked_on == PI_WAKEUP_INPROGRESS) { + raw_spin_unlock(&task->pi_lock); + return -EAGAIN; + } + + BUG_ON(rt_mutex_real_waiter(task->pi_blocked_on)); + __rt_mutex_adjust_prio(task); waiter->task = task; waiter->lock = lock; @@ -469,7 +491,7 @@ static int task_blocks_on_rt_mutex(struct rt_mutex *lock, plist_add(&waiter->pi_list_entry, &owner->pi_waiters); __rt_mutex_adjust_prio(owner); - if (owner->pi_blocked_on) + if (rt_mutex_real_waiter(owner->pi_blocked_on)) chain_walk = 1; raw_spin_unlock(&owner->pi_lock); } @@ -624,7 +646,7 @@ static void remove_waiter(struct rt_mutex *lock, } __rt_mutex_adjust_prio(owner); - if (owner->pi_blocked_on) + if (rt_mutex_real_waiter(owner->pi_blocked_on)) chain_walk = 1; raw_spin_unlock(&owner->pi_lock); @@ -658,7 +680,8 @@ void rt_mutex_adjust_pi(struct task_struct *task) raw_spin_lock_irqsave(&task->pi_lock, flags); waiter = task->pi_blocked_on; - if (!waiter || waiter->list_entry.prio == task->prio) { + if (!rt_mutex_real_waiter(waiter) || + waiter->list_entry.prio == task->prio) { raw_spin_unlock_irqrestore(&task->pi_lock, flags); return; } @@ -1527,7 +1550,7 @@ int rt_mutex_start_proxy_lock(struct rt_mutex *lock, ret = task_blocks_on_rt_mutex(lock, waiter, task, detect_deadlock, flags); - if (ret && !waiter->task) { + if (ret == -EDEADLK && !waiter->task) { /* * Reset the return value. We might have * returned with -EDEADLK and the owner diff --git a/kernel/rtmutex_common.h b/kernel/rtmutex_common.h index 4df690c..97fc68c 100644 --- a/kernel/rtmutex_common.h +++ b/kernel/rtmutex_common.h @@ -115,6 +115,9 @@ static inline unsigned long rt_mutex_owner_pending(struct rt_mutex *lock) /* * PI-futex support (proxy locking functions, etc.): */ + +#define PI_WAKEUP_INPROGRESS ((struct rt_mutex_waiter *) 1) + 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); -- 1.7.0.4