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1// SPDX-License-Identifier: GPL-2.0-or-later
2
3#include <linux/slab.h>
4#include <linux/sched/task.h>
5
6#include "futex.h"
7#include "../locking/rtmutex_common.h"
8
9/*
10 * PI code:
11 */
12int refill_pi_state_cache(void)
13{
14 struct futex_pi_state *pi_state;
15
16 if (likely(current->pi_state_cache))
17 return 0;
18
19 pi_state = kzalloc(sizeof(*pi_state), GFP_KERNEL);
20
21 if (!pi_state)
22 return -ENOMEM;
23
24 INIT_LIST_HEAD(&pi_state->list);
25 /* pi_mutex gets initialized later */
26 pi_state->owner = NULL;
27 refcount_set(&pi_state->refcount, 1);
28 pi_state->key = FUTEX_KEY_INIT;
29
30 current->pi_state_cache = pi_state;
31
32 return 0;
33}
34
35static struct futex_pi_state *alloc_pi_state(void)
36{
37 struct futex_pi_state *pi_state = current->pi_state_cache;
38
39 WARN_ON(!pi_state);
40 current->pi_state_cache = NULL;
41
42 return pi_state;
43}
44
45static void pi_state_update_owner(struct futex_pi_state *pi_state,
46 struct task_struct *new_owner)
47{
48 struct task_struct *old_owner = pi_state->owner;
49
50 lockdep_assert_held(&pi_state->pi_mutex.wait_lock);
51
52 if (old_owner) {
53 raw_spin_lock(&old_owner->pi_lock);
54 WARN_ON(list_empty(&pi_state->list));
55 list_del_init(&pi_state->list);
56 raw_spin_unlock(&old_owner->pi_lock);
57 }
58
59 if (new_owner) {
60 raw_spin_lock(&new_owner->pi_lock);
61 WARN_ON(!list_empty(&pi_state->list));
62 list_add(&pi_state->list, &new_owner->pi_state_list);
63 pi_state->owner = new_owner;
64 raw_spin_unlock(&new_owner->pi_lock);
65 }
66}
67
68void get_pi_state(struct futex_pi_state *pi_state)
69{
70 WARN_ON_ONCE(!refcount_inc_not_zero(&pi_state->refcount));
71}
72
73/*
74 * Drops a reference to the pi_state object and frees or caches it
75 * when the last reference is gone.
76 */
77void put_pi_state(struct futex_pi_state *pi_state)
78{
79 if (!pi_state)
80 return;
81
82 if (!refcount_dec_and_test(&pi_state->refcount))
83 return;
84
85 /*
86 * If pi_state->owner is NULL, the owner is most probably dying
87 * and has cleaned up the pi_state already
88 */
89 if (pi_state->owner) {
90 unsigned long flags;
91
92 raw_spin_lock_irqsave(&pi_state->pi_mutex.wait_lock, flags);
93 pi_state_update_owner(pi_state, NULL);
94 rt_mutex_proxy_unlock(&pi_state->pi_mutex);
95 raw_spin_unlock_irqrestore(&pi_state->pi_mutex.wait_lock, flags);
96 }
97
98 if (current->pi_state_cache) {
99 kfree(pi_state);
100 } else {
101 /*
102 * pi_state->list is already empty.
103 * clear pi_state->owner.
104 * refcount is at 0 - put it back to 1.
105 */
106 pi_state->owner = NULL;
107 refcount_set(&pi_state->refcount, 1);
108 current->pi_state_cache = pi_state;
109 }
110}
111
112/*
113 * We need to check the following states:
114 *
115 * Waiter | pi_state | pi->owner | uTID | uODIED | ?
116 *
117 * [1] NULL | --- | --- | 0 | 0/1 | Valid
118 * [2] NULL | --- | --- | >0 | 0/1 | Valid
119 *
120 * [3] Found | NULL | -- | Any | 0/1 | Invalid
121 *
122 * [4] Found | Found | NULL | 0 | 1 | Valid
123 * [5] Found | Found | NULL | >0 | 1 | Invalid
124 *
125 * [6] Found | Found | task | 0 | 1 | Valid
126 *
127 * [7] Found | Found | NULL | Any | 0 | Invalid
128 *
129 * [8] Found | Found | task | ==taskTID | 0/1 | Valid
130 * [9] Found | Found | task | 0 | 0 | Invalid
131 * [10] Found | Found | task | !=taskTID | 0/1 | Invalid
132 *
133 * [1] Indicates that the kernel can acquire the futex atomically. We
134 * came here due to a stale FUTEX_WAITERS/FUTEX_OWNER_DIED bit.
135 *
136 * [2] Valid, if TID does not belong to a kernel thread. If no matching
137 * thread is found then it indicates that the owner TID has died.
138 *
139 * [3] Invalid. The waiter is queued on a non PI futex
140 *
141 * [4] Valid state after exit_robust_list(), which sets the user space
142 * value to FUTEX_WAITERS | FUTEX_OWNER_DIED.
143 *
144 * [5] The user space value got manipulated between exit_robust_list()
145 * and exit_pi_state_list()
146 *
147 * [6] Valid state after exit_pi_state_list() which sets the new owner in
148 * the pi_state but cannot access the user space value.
149 *
150 * [7] pi_state->owner can only be NULL when the OWNER_DIED bit is set.
151 *
152 * [8] Owner and user space value match
153 *
154 * [9] There is no transient state which sets the user space TID to 0
155 * except exit_robust_list(), but this is indicated by the
156 * FUTEX_OWNER_DIED bit. See [4]
157 *
158 * [10] There is no transient state which leaves owner and user space
159 * TID out of sync. Except one error case where the kernel is denied
160 * write access to the user address, see fixup_pi_state_owner().
161 *
162 *
163 * Serialization and lifetime rules:
164 *
165 * hb->lock:
166 *
167 * hb -> futex_q, relation
168 * futex_q -> pi_state, relation
169 *
170 * (cannot be raw because hb can contain arbitrary amount
171 * of futex_q's)
172 *
173 * pi_mutex->wait_lock:
174 *
175 * {uval, pi_state}
176 *
177 * (and pi_mutex 'obviously')
178 *
179 * p->pi_lock:
180 *
181 * p->pi_state_list -> pi_state->list, relation
182 * pi_mutex->owner -> pi_state->owner, relation
183 *
184 * pi_state->refcount:
185 *
186 * pi_state lifetime
187 *
188 *
189 * Lock order:
190 *
191 * hb->lock
192 * pi_mutex->wait_lock
193 * p->pi_lock
194 *
195 */
196
197/*
198 * Validate that the existing waiter has a pi_state and sanity check
199 * the pi_state against the user space value. If correct, attach to
200 * it.
201 */
202static int attach_to_pi_state(u32 __user *uaddr, u32 uval,
203 struct futex_pi_state *pi_state,
204 struct futex_pi_state **ps)
205{
206 pid_t pid = uval & FUTEX_TID_MASK;
207 u32 uval2;
208 int ret;
209
210 /*
211 * Userspace might have messed up non-PI and PI futexes [3]
212 */
213 if (unlikely(!pi_state))
214 return -EINVAL;
215
216 /*
217 * We get here with hb->lock held, and having found a
218 * futex_top_waiter(). This means that futex_lock_pi() of said futex_q
219 * has dropped the hb->lock in between futex_queue() and futex_unqueue_pi(),
220 * which in turn means that futex_lock_pi() still has a reference on
221 * our pi_state.
222 *
223 * The waiter holding a reference on @pi_state also protects against
224 * the unlocked put_pi_state() in futex_unlock_pi(), futex_lock_pi()
225 * and futex_wait_requeue_pi() as it cannot go to 0 and consequently
226 * free pi_state before we can take a reference ourselves.
227 */
228 WARN_ON(!refcount_read(&pi_state->refcount));
229
230 /*
231 * Now that we have a pi_state, we can acquire wait_lock
232 * and do the state validation.
233 */
234 raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
235
236 /*
237 * Since {uval, pi_state} is serialized by wait_lock, and our current
238 * uval was read without holding it, it can have changed. Verify it
239 * still is what we expect it to be, otherwise retry the entire
240 * operation.
241 */
242 if (futex_get_value_locked(&uval2, uaddr))
243 goto out_efault;
244
245 if (uval != uval2)
246 goto out_eagain;
247
248 /*
249 * Handle the owner died case:
250 */
251 if (uval & FUTEX_OWNER_DIED) {
252 /*
253 * exit_pi_state_list sets owner to NULL and wakes the
254 * topmost waiter. The task which acquires the
255 * pi_state->rt_mutex will fixup owner.
256 */
257 if (!pi_state->owner) {
258 /*
259 * No pi state owner, but the user space TID
260 * is not 0. Inconsistent state. [5]
261 */
262 if (pid)
263 goto out_einval;
264 /*
265 * Take a ref on the state and return success. [4]
266 */
267 goto out_attach;
268 }
269
270 /*
271 * If TID is 0, then either the dying owner has not
272 * yet executed exit_pi_state_list() or some waiter
273 * acquired the rtmutex in the pi state, but did not
274 * yet fixup the TID in user space.
275 *
276 * Take a ref on the state and return success. [6]
277 */
278 if (!pid)
279 goto out_attach;
280 } else {
281 /*
282 * If the owner died bit is not set, then the pi_state
283 * must have an owner. [7]
284 */
285 if (!pi_state->owner)
286 goto out_einval;
287 }
288
289 /*
290 * Bail out if user space manipulated the futex value. If pi
291 * state exists then the owner TID must be the same as the
292 * user space TID. [9/10]
293 */
294 if (pid != task_pid_vnr(pi_state->owner))
295 goto out_einval;
296
297out_attach:
298 get_pi_state(pi_state);
299 raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
300 *ps = pi_state;
301 return 0;
302
303out_einval:
304 ret = -EINVAL;
305 goto out_error;
306
307out_eagain:
308 ret = -EAGAIN;
309 goto out_error;
310
311out_efault:
312 ret = -EFAULT;
313 goto out_error;
314
315out_error:
316 raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
317 return ret;
318}
319
320static int handle_exit_race(u32 __user *uaddr, u32 uval,
321 struct task_struct *tsk)
322{
323 u32 uval2;
324
325 /*
326 * If the futex exit state is not yet FUTEX_STATE_DEAD, tell the
327 * caller that the alleged owner is busy.
328 */
329 if (tsk && tsk->futex_state != FUTEX_STATE_DEAD)
330 return -EBUSY;
331
332 /*
333 * Reread the user space value to handle the following situation:
334 *
335 * CPU0 CPU1
336 *
337 * sys_exit() sys_futex()
338 * do_exit() futex_lock_pi()
339 * futex_lock_pi_atomic()
340 * exit_signals(tsk) No waiters:
341 * tsk->flags |= PF_EXITING; *uaddr == 0x00000PID
342 * mm_release(tsk) Set waiter bit
343 * exit_robust_list(tsk) { *uaddr = 0x80000PID;
344 * Set owner died attach_to_pi_owner() {
345 * *uaddr = 0xC0000000; tsk = get_task(PID);
346 * } if (!tsk->flags & PF_EXITING) {
347 * ... attach();
348 * tsk->futex_state = } else {
349 * FUTEX_STATE_DEAD; if (tsk->futex_state !=
350 * FUTEX_STATE_DEAD)
351 * return -EAGAIN;
352 * return -ESRCH; <--- FAIL
353 * }
354 *
355 * Returning ESRCH unconditionally is wrong here because the
356 * user space value has been changed by the exiting task.
357 *
358 * The same logic applies to the case where the exiting task is
359 * already gone.
360 */
361 if (futex_get_value_locked(&uval2, uaddr))
362 return -EFAULT;
363
364 /* If the user space value has changed, try again. */
365 if (uval2 != uval)
366 return -EAGAIN;
367
368 /*
369 * The exiting task did not have a robust list, the robust list was
370 * corrupted or the user space value in *uaddr is simply bogus.
371 * Give up and tell user space.
372 */
373 return -ESRCH;
374}
375
376static void __attach_to_pi_owner(struct task_struct *p, union futex_key *key,
377 struct futex_pi_state **ps)
378{
379 /*
380 * No existing pi state. First waiter. [2]
381 *
382 * This creates pi_state, we have hb->lock held, this means nothing can
383 * observe this state, wait_lock is irrelevant.
384 */
385 struct futex_pi_state *pi_state = alloc_pi_state();
386
387 /*
388 * Initialize the pi_mutex in locked state and make @p
389 * the owner of it:
390 */
391 rt_mutex_init_proxy_locked(&pi_state->pi_mutex, p);
392
393 /* Store the key for possible exit cleanups: */
394 pi_state->key = *key;
395
396 WARN_ON(!list_empty(&pi_state->list));
397 list_add(&pi_state->list, &p->pi_state_list);
398 /*
399 * Assignment without holding pi_state->pi_mutex.wait_lock is safe
400 * because there is no concurrency as the object is not published yet.
401 */
402 pi_state->owner = p;
403
404 *ps = pi_state;
405}
406/*
407 * Lookup the task for the TID provided from user space and attach to
408 * it after doing proper sanity checks.
409 */
410static int attach_to_pi_owner(u32 __user *uaddr, u32 uval, union futex_key *key,
411 struct futex_pi_state **ps,
412 struct task_struct **exiting)
413{
414 pid_t pid = uval & FUTEX_TID_MASK;
415 struct task_struct *p;
416
417 /*
418 * We are the first waiter - try to look up the real owner and attach
419 * the new pi_state to it, but bail out when TID = 0 [1]
420 *
421 * The !pid check is paranoid. None of the call sites should end up
422 * with pid == 0, but better safe than sorry. Let the caller retry
423 */
424 if (!pid)
425 return -EAGAIN;
426 p = find_get_task_by_vpid(pid);
427 if (!p)
428 return handle_exit_race(uaddr, uval, NULL);
429
430 if (unlikely(p->flags & PF_KTHREAD)) {
431 put_task_struct(p);
432 return -EPERM;
433 }
434
435 /*
436 * We need to look at the task state to figure out, whether the
437 * task is exiting. To protect against the change of the task state
438 * in futex_exit_release(), we do this protected by p->pi_lock:
439 */
440 raw_spin_lock_irq(&p->pi_lock);
441 if (unlikely(p->futex_state != FUTEX_STATE_OK)) {
442 /*
443 * The task is on the way out. When the futex state is
444 * FUTEX_STATE_DEAD, we know that the task has finished
445 * the cleanup:
446 */
447 int ret = handle_exit_race(uaddr, uval, p);
448
449 raw_spin_unlock_irq(&p->pi_lock);
450 /*
451 * If the owner task is between FUTEX_STATE_EXITING and
452 * FUTEX_STATE_DEAD then store the task pointer and keep
453 * the reference on the task struct. The calling code will
454 * drop all locks, wait for the task to reach
455 * FUTEX_STATE_DEAD and then drop the refcount. This is
456 * required to prevent a live lock when the current task
457 * preempted the exiting task between the two states.
458 */
459 if (ret == -EBUSY)
460 *exiting = p;
461 else
462 put_task_struct(p);
463 return ret;
464 }
465
466 __attach_to_pi_owner(p, key, ps);
467 raw_spin_unlock_irq(&p->pi_lock);
468
469 put_task_struct(p);
470
471 return 0;
472}
473
474static int lock_pi_update_atomic(u32 __user *uaddr, u32 uval, u32 newval)
475{
476 int err;
477 u32 curval;
478
479 if (unlikely(should_fail_futex(true)))
480 return -EFAULT;
481
482 err = futex_cmpxchg_value_locked(&curval, uaddr, uval, newval);
483 if (unlikely(err))
484 return err;
485
486 /* If user space value changed, let the caller retry */
487 return curval != uval ? -EAGAIN : 0;
488}
489
490/**
491 * futex_lock_pi_atomic() - Atomic work required to acquire a pi aware futex
492 * @uaddr: the pi futex user address
493 * @hb: the pi futex hash bucket
494 * @key: the futex key associated with uaddr and hb
495 * @ps: the pi_state pointer where we store the result of the
496 * lookup
497 * @task: the task to perform the atomic lock work for. This will
498 * be "current" except in the case of requeue pi.
499 * @exiting: Pointer to store the task pointer of the owner task
500 * which is in the middle of exiting
501 * @set_waiters: force setting the FUTEX_WAITERS bit (1) or not (0)
502 *
503 * Return:
504 * - 0 - ready to wait;
505 * - 1 - acquired the lock;
506 * - <0 - error
507 *
508 * The hb->lock must be held by the caller.
509 *
510 * @exiting is only set when the return value is -EBUSY. If so, this holds
511 * a refcount on the exiting task on return and the caller needs to drop it
512 * after waiting for the exit to complete.
513 */
514int futex_lock_pi_atomic(u32 __user *uaddr, struct futex_hash_bucket *hb,
515 union futex_key *key,
516 struct futex_pi_state **ps,
517 struct task_struct *task,
518 struct task_struct **exiting,
519 int set_waiters)
520{
521 u32 uval, newval, vpid = task_pid_vnr(task);
522 struct futex_q *top_waiter;
523 int ret;
524
525 /*
526 * Read the user space value first so we can validate a few
527 * things before proceeding further.
528 */
529 if (futex_get_value_locked(&uval, uaddr))
530 return -EFAULT;
531
532 if (unlikely(should_fail_futex(true)))
533 return -EFAULT;
534
535 /*
536 * Detect deadlocks.
537 */
538 if ((unlikely((uval & FUTEX_TID_MASK) == vpid)))
539 return -EDEADLK;
540
541 if ((unlikely(should_fail_futex(true))))
542 return -EDEADLK;
543
544 /*
545 * Lookup existing state first. If it exists, try to attach to
546 * its pi_state.
547 */
548 top_waiter = futex_top_waiter(hb, key);
549 if (top_waiter)
550 return attach_to_pi_state(uaddr, uval, top_waiter->pi_state, ps);
551
552 /*
553 * No waiter and user TID is 0. We are here because the
554 * waiters or the owner died bit is set or called from
555 * requeue_cmp_pi or for whatever reason something took the
556 * syscall.
557 */
558 if (!(uval & FUTEX_TID_MASK)) {
559 /*
560 * We take over the futex. No other waiters and the user space
561 * TID is 0. We preserve the owner died bit.
562 */
563 newval = uval & FUTEX_OWNER_DIED;
564 newval |= vpid;
565
566 /* The futex requeue_pi code can enforce the waiters bit */
567 if (set_waiters)
568 newval |= FUTEX_WAITERS;
569
570 ret = lock_pi_update_atomic(uaddr, uval, newval);
571 if (ret)
572 return ret;
573
574 /*
575 * If the waiter bit was requested the caller also needs PI
576 * state attached to the new owner of the user space futex.
577 *
578 * @task is guaranteed to be alive and it cannot be exiting
579 * because it is either sleeping or waiting in
580 * futex_requeue_pi_wakeup_sync().
581 *
582 * No need to do the full attach_to_pi_owner() exercise
583 * because @task is known and valid.
584 */
585 if (set_waiters) {
586 raw_spin_lock_irq(&task->pi_lock);
587 __attach_to_pi_owner(task, key, ps);
588 raw_spin_unlock_irq(&task->pi_lock);
589 }
590 return 1;
591 }
592
593 /*
594 * First waiter. Set the waiters bit before attaching ourself to
595 * the owner. If owner tries to unlock, it will be forced into
596 * the kernel and blocked on hb->lock.
597 */
598 newval = uval | FUTEX_WAITERS;
599 ret = lock_pi_update_atomic(uaddr, uval, newval);
600 if (ret)
601 return ret;
602 /*
603 * If the update of the user space value succeeded, we try to
604 * attach to the owner. If that fails, no harm done, we only
605 * set the FUTEX_WAITERS bit in the user space variable.
606 */
607 return attach_to_pi_owner(uaddr, newval, key, ps, exiting);
608}
609
610/*
611 * Caller must hold a reference on @pi_state.
612 */
613static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_pi_state *pi_state)
614{
615 struct rt_mutex_waiter *top_waiter;
616 struct task_struct *new_owner;
617 bool postunlock = false;
618 DEFINE_RT_WAKE_Q(wqh);
619 u32 curval, newval;
620 int ret = 0;
621
622 top_waiter = rt_mutex_top_waiter(&pi_state->pi_mutex);
623 if (WARN_ON_ONCE(!top_waiter)) {
624 /*
625 * As per the comment in futex_unlock_pi() this should not happen.
626 *
627 * When this happens, give up our locks and try again, giving
628 * the futex_lock_pi() instance time to complete, either by
629 * waiting on the rtmutex or removing itself from the futex
630 * queue.
631 */
632 ret = -EAGAIN;
633 goto out_unlock;
634 }
635
636 new_owner = top_waiter->task;
637
638 /*
639 * We pass it to the next owner. The WAITERS bit is always kept
640 * enabled while there is PI state around. We cleanup the owner
641 * died bit, because we are the owner.
642 */
643 newval = FUTEX_WAITERS | task_pid_vnr(new_owner);
644
645 if (unlikely(should_fail_futex(true))) {
646 ret = -EFAULT;
647 goto out_unlock;
648 }
649
650 ret = futex_cmpxchg_value_locked(&curval, uaddr, uval, newval);
651 if (!ret && (curval != uval)) {
652 /*
653 * If a unconditional UNLOCK_PI operation (user space did not
654 * try the TID->0 transition) raced with a waiter setting the
655 * FUTEX_WAITERS flag between get_user() and locking the hash
656 * bucket lock, retry the operation.
657 */
658 if ((FUTEX_TID_MASK & curval) == uval)
659 ret = -EAGAIN;
660 else
661 ret = -EINVAL;
662 }
663
664 if (!ret) {
665 /*
666 * This is a point of no return; once we modified the uval
667 * there is no going back and subsequent operations must
668 * not fail.
669 */
670 pi_state_update_owner(pi_state, new_owner);
671 postunlock = __rt_mutex_futex_unlock(&pi_state->pi_mutex, &wqh);
672 }
673
674out_unlock:
675 raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
676
677 if (postunlock)
678 rt_mutex_postunlock(&wqh);
679
680 return ret;
681}
682
683static int __fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q,
684 struct task_struct *argowner)
685{
686 struct futex_pi_state *pi_state = q->pi_state;
687 struct task_struct *oldowner, *newowner;
688 u32 uval, curval, newval, newtid;
689 int err = 0;
690
691 oldowner = pi_state->owner;
692
693 /*
694 * We are here because either:
695 *
696 * - we stole the lock and pi_state->owner needs updating to reflect
697 * that (@argowner == current),
698 *
699 * or:
700 *
701 * - someone stole our lock and we need to fix things to point to the
702 * new owner (@argowner == NULL).
703 *
704 * Either way, we have to replace the TID in the user space variable.
705 * This must be atomic as we have to preserve the owner died bit here.
706 *
707 * Note: We write the user space value _before_ changing the pi_state
708 * because we can fault here. Imagine swapped out pages or a fork
709 * that marked all the anonymous memory readonly for cow.
710 *
711 * Modifying pi_state _before_ the user space value would leave the
712 * pi_state in an inconsistent state when we fault here, because we
713 * need to drop the locks to handle the fault. This might be observed
714 * in the PID checks when attaching to PI state .
715 */
716retry:
717 if (!argowner) {
718 if (oldowner != current) {
719 /*
720 * We raced against a concurrent self; things are
721 * already fixed up. Nothing to do.
722 */
723 return 0;
724 }
725
726 if (__rt_mutex_futex_trylock(&pi_state->pi_mutex)) {
727 /* We got the lock. pi_state is correct. Tell caller. */
728 return 1;
729 }
730
731 /*
732 * The trylock just failed, so either there is an owner or
733 * there is a higher priority waiter than this one.
734 */
735 newowner = rt_mutex_owner(&pi_state->pi_mutex);
736 /*
737 * If the higher priority waiter has not yet taken over the
738 * rtmutex then newowner is NULL. We can't return here with
739 * that state because it's inconsistent vs. the user space
740 * state. So drop the locks and try again. It's a valid
741 * situation and not any different from the other retry
742 * conditions.
743 */
744 if (unlikely(!newowner)) {
745 err = -EAGAIN;
746 goto handle_err;
747 }
748 } else {
749 WARN_ON_ONCE(argowner != current);
750 if (oldowner == current) {
751 /*
752 * We raced against a concurrent self; things are
753 * already fixed up. Nothing to do.
754 */
755 return 1;
756 }
757 newowner = argowner;
758 }
759
760 newtid = task_pid_vnr(newowner) | FUTEX_WAITERS;
761 /* Owner died? */
762 if (!pi_state->owner)
763 newtid |= FUTEX_OWNER_DIED;
764
765 err = futex_get_value_locked(&uval, uaddr);
766 if (err)
767 goto handle_err;
768
769 for (;;) {
770 newval = (uval & FUTEX_OWNER_DIED) | newtid;
771
772 err = futex_cmpxchg_value_locked(&curval, uaddr, uval, newval);
773 if (err)
774 goto handle_err;
775
776 if (curval == uval)
777 break;
778 uval = curval;
779 }
780
781 /*
782 * We fixed up user space. Now we need to fix the pi_state
783 * itself.
784 */
785 pi_state_update_owner(pi_state, newowner);
786
787 return argowner == current;
788
789 /*
790 * In order to reschedule or handle a page fault, we need to drop the
791 * locks here. In the case of a fault, this gives the other task
792 * (either the highest priority waiter itself or the task which stole
793 * the rtmutex) the chance to try the fixup of the pi_state. So once we
794 * are back from handling the fault we need to check the pi_state after
795 * reacquiring the locks and before trying to do another fixup. When
796 * the fixup has been done already we simply return.
797 *
798 * Note: we hold both hb->lock and pi_mutex->wait_lock. We can safely
799 * drop hb->lock since the caller owns the hb -> futex_q relation.
800 * Dropping the pi_mutex->wait_lock requires the state revalidate.
801 */
802handle_err:
803 raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
804 spin_unlock(q->lock_ptr);
805
806 switch (err) {
807 case -EFAULT:
808 err = fault_in_user_writeable(uaddr);
809 break;
810
811 case -EAGAIN:
812 cond_resched();
813 err = 0;
814 break;
815
816 default:
817 WARN_ON_ONCE(1);
818 break;
819 }
820
821 spin_lock(q->lock_ptr);
822 raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
823
824 /*
825 * Check if someone else fixed it for us:
826 */
827 if (pi_state->owner != oldowner)
828 return argowner == current;
829
830 /* Retry if err was -EAGAIN or the fault in succeeded */
831 if (!err)
832 goto retry;
833
834 /*
835 * fault_in_user_writeable() failed so user state is immutable. At
836 * best we can make the kernel state consistent but user state will
837 * be most likely hosed and any subsequent unlock operation will be
838 * rejected due to PI futex rule [10].
839 *
840 * Ensure that the rtmutex owner is also the pi_state owner despite
841 * the user space value claiming something different. There is no
842 * point in unlocking the rtmutex if current is the owner as it
843 * would need to wait until the next waiter has taken the rtmutex
844 * to guarantee consistent state. Keep it simple. Userspace asked
845 * for this wreckaged state.
846 *
847 * The rtmutex has an owner - either current or some other
848 * task. See the EAGAIN loop above.
849 */
850 pi_state_update_owner(pi_state, rt_mutex_owner(&pi_state->pi_mutex));
851
852 return err;
853}
854
855static int fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q,
856 struct task_struct *argowner)
857{
858 struct futex_pi_state *pi_state = q->pi_state;
859 int ret;
860
861 lockdep_assert_held(q->lock_ptr);
862
863 raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
864 ret = __fixup_pi_state_owner(uaddr, q, argowner);
865 raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
866 return ret;
867}
868
869/**
870 * fixup_pi_owner() - Post lock pi_state and corner case management
871 * @uaddr: user address of the futex
872 * @q: futex_q (contains pi_state and access to the rt_mutex)
873 * @locked: if the attempt to take the rt_mutex succeeded (1) or not (0)
874 *
875 * After attempting to lock an rt_mutex, this function is called to cleanup
876 * the pi_state owner as well as handle race conditions that may allow us to
877 * acquire the lock. Must be called with the hb lock held.
878 *
879 * Return:
880 * - 1 - success, lock taken;
881 * - 0 - success, lock not taken;
882 * - <0 - on error (-EFAULT)
883 */
884int fixup_pi_owner(u32 __user *uaddr, struct futex_q *q, int locked)
885{
886 if (locked) {
887 /*
888 * Got the lock. We might not be the anticipated owner if we
889 * did a lock-steal - fix up the PI-state in that case:
890 *
891 * Speculative pi_state->owner read (we don't hold wait_lock);
892 * since we own the lock pi_state->owner == current is the
893 * stable state, anything else needs more attention.
894 */
895 if (q->pi_state->owner != current)
896 return fixup_pi_state_owner(uaddr, q, current);
897 return 1;
898 }
899
900 /*
901 * If we didn't get the lock; check if anybody stole it from us. In
902 * that case, we need to fix up the uval to point to them instead of
903 * us, otherwise bad things happen. [10]
904 *
905 * Another speculative read; pi_state->owner == current is unstable
906 * but needs our attention.
907 */
908 if (q->pi_state->owner == current)
909 return fixup_pi_state_owner(uaddr, q, NULL);
910
911 /*
912 * Paranoia check. If we did not take the lock, then we should not be
913 * the owner of the rt_mutex. Warn and establish consistent state.
914 */
915 if (WARN_ON_ONCE(rt_mutex_owner(&q->pi_state->pi_mutex) == current))
916 return fixup_pi_state_owner(uaddr, q, current);
917
918 return 0;
919}
920
921/*
922 * Userspace tried a 0 -> TID atomic transition of the futex value
923 * and failed. The kernel side here does the whole locking operation:
924 * if there are waiters then it will block as a consequence of relying
925 * on rt-mutexes, it does PI, etc. (Due to races the kernel might see
926 * a 0 value of the futex too.).
927 *
928 * Also serves as futex trylock_pi()'ing, and due semantics.
929 */
930int futex_lock_pi(u32 __user *uaddr, unsigned int flags, ktime_t *time, int trylock)
931{
932 struct hrtimer_sleeper timeout, *to;
933 struct task_struct *exiting = NULL;
934 struct rt_mutex_waiter rt_waiter;
935 struct futex_hash_bucket *hb;
936 struct futex_q q = futex_q_init;
937 int res, ret;
938
939 if (!IS_ENABLED(CONFIG_FUTEX_PI))
940 return -ENOSYS;
941
942 if (refill_pi_state_cache())
943 return -ENOMEM;
944
945 to = futex_setup_timer(time, &timeout, flags, 0);
946
947retry:
948 ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &q.key, FUTEX_WRITE);
949 if (unlikely(ret != 0))
950 goto out;
951
952retry_private:
953 hb = futex_q_lock(&q);
954
955 ret = futex_lock_pi_atomic(uaddr, hb, &q.key, &q.pi_state, current,
956 &exiting, 0);
957 if (unlikely(ret)) {
958 /*
959 * Atomic work succeeded and we got the lock,
960 * or failed. Either way, we do _not_ block.
961 */
962 switch (ret) {
963 case 1:
964 /* We got the lock. */
965 ret = 0;
966 goto out_unlock_put_key;
967 case -EFAULT:
968 goto uaddr_faulted;
969 case -EBUSY:
970 case -EAGAIN:
971 /*
972 * Two reasons for this:
973 * - EBUSY: Task is exiting and we just wait for the
974 * exit to complete.
975 * - EAGAIN: The user space value changed.
976 */
977 futex_q_unlock(hb);
978 /*
979 * Handle the case where the owner is in the middle of
980 * exiting. Wait for the exit to complete otherwise
981 * this task might loop forever, aka. live lock.
982 */
983 wait_for_owner_exiting(ret, exiting);
984 cond_resched();
985 goto retry;
986 default:
987 goto out_unlock_put_key;
988 }
989 }
990
991 WARN_ON(!q.pi_state);
992
993 /*
994 * Only actually queue now that the atomic ops are done:
995 */
996 __futex_queue(&q, hb);
997
998 if (trylock) {
999 ret = rt_mutex_futex_trylock(&q.pi_state->pi_mutex);
1000 /* Fixup the trylock return value: */
1001 ret = ret ? 0 : -EWOULDBLOCK;
1002 goto no_block;
1003 }
1004
1005 rt_mutex_init_waiter(&rt_waiter);
1006
1007 /*
1008 * On PREEMPT_RT, when hb->lock becomes an rt_mutex, we must not
1009 * hold it while doing rt_mutex_start_proxy(), because then it will
1010 * include hb->lock in the blocking chain, even through we'll not in
1011 * fact hold it while blocking. This will lead it to report -EDEADLK
1012 * and BUG when futex_unlock_pi() interleaves with this.
1013 *
1014 * Therefore acquire wait_lock while holding hb->lock, but drop the
1015 * latter before calling __rt_mutex_start_proxy_lock(). This
1016 * interleaves with futex_unlock_pi() -- which does a similar lock
1017 * handoff -- such that the latter can observe the futex_q::pi_state
1018 * before __rt_mutex_start_proxy_lock() is done.
1019 */
1020 raw_spin_lock_irq(&q.pi_state->pi_mutex.wait_lock);
1021 spin_unlock(q.lock_ptr);
1022 /*
1023 * __rt_mutex_start_proxy_lock() unconditionally enqueues the @rt_waiter
1024 * such that futex_unlock_pi() is guaranteed to observe the waiter when
1025 * it sees the futex_q::pi_state.
1026 */
1027 ret = __rt_mutex_start_proxy_lock(&q.pi_state->pi_mutex, &rt_waiter, current);
1028 raw_spin_unlock_irq(&q.pi_state->pi_mutex.wait_lock);
1029
1030 if (ret) {
1031 if (ret == 1)
1032 ret = 0;
1033 goto cleanup;
1034 }
1035
1036 if (unlikely(to))
1037 hrtimer_sleeper_start_expires(to, HRTIMER_MODE_ABS);
1038
1039 ret = rt_mutex_wait_proxy_lock(&q.pi_state->pi_mutex, to, &rt_waiter);
1040
1041cleanup:
1042 spin_lock(q.lock_ptr);
1043 /*
1044 * If we failed to acquire the lock (deadlock/signal/timeout), we must
1045 * first acquire the hb->lock before removing the lock from the
1046 * rt_mutex waitqueue, such that we can keep the hb and rt_mutex wait
1047 * lists consistent.
1048 *
1049 * In particular; it is important that futex_unlock_pi() can not
1050 * observe this inconsistency.
1051 */
1052 if (ret && !rt_mutex_cleanup_proxy_lock(&q.pi_state->pi_mutex, &rt_waiter))
1053 ret = 0;
1054
1055no_block:
1056 /*
1057 * Fixup the pi_state owner and possibly acquire the lock if we
1058 * haven't already.
1059 */
1060 res = fixup_pi_owner(uaddr, &q, !ret);
1061 /*
1062 * If fixup_pi_owner() returned an error, propagate that. If it acquired
1063 * the lock, clear our -ETIMEDOUT or -EINTR.
1064 */
1065 if (res)
1066 ret = (res < 0) ? res : 0;
1067
1068 futex_unqueue_pi(&q);
1069 spin_unlock(q.lock_ptr);
1070 goto out;
1071
1072out_unlock_put_key:
1073 futex_q_unlock(hb);
1074
1075out:
1076 if (to) {
1077 hrtimer_cancel(&to->timer);
1078 destroy_hrtimer_on_stack(&to->timer);
1079 }
1080 return ret != -EINTR ? ret : -ERESTARTNOINTR;
1081
1082uaddr_faulted:
1083 futex_q_unlock(hb);
1084
1085 ret = fault_in_user_writeable(uaddr);
1086 if (ret)
1087 goto out;
1088
1089 if (!(flags & FLAGS_SHARED))
1090 goto retry_private;
1091
1092 goto retry;
1093}
1094
1095/*
1096 * Userspace attempted a TID -> 0 atomic transition, and failed.
1097 * This is the in-kernel slowpath: we look up the PI state (if any),
1098 * and do the rt-mutex unlock.
1099 */
1100int futex_unlock_pi(u32 __user *uaddr, unsigned int flags)
1101{
1102 u32 curval, uval, vpid = task_pid_vnr(current);
1103 union futex_key key = FUTEX_KEY_INIT;
1104 struct futex_hash_bucket *hb;
1105 struct futex_q *top_waiter;
1106 int ret;
1107
1108 if (!IS_ENABLED(CONFIG_FUTEX_PI))
1109 return -ENOSYS;
1110
1111retry:
1112 if (get_user(uval, uaddr))
1113 return -EFAULT;
1114 /*
1115 * We release only a lock we actually own:
1116 */
1117 if ((uval & FUTEX_TID_MASK) != vpid)
1118 return -EPERM;
1119
1120 ret = get_futex_key(uaddr, flags & FLAGS_SHARED, &key, FUTEX_WRITE);
1121 if (ret)
1122 return ret;
1123
1124 hb = futex_hash(&key);
1125 spin_lock(&hb->lock);
1126
1127 /*
1128 * Check waiters first. We do not trust user space values at
1129 * all and we at least want to know if user space fiddled
1130 * with the futex value instead of blindly unlocking.
1131 */
1132 top_waiter = futex_top_waiter(hb, &key);
1133 if (top_waiter) {
1134 struct futex_pi_state *pi_state = top_waiter->pi_state;
1135
1136 ret = -EINVAL;
1137 if (!pi_state)
1138 goto out_unlock;
1139
1140 /*
1141 * If current does not own the pi_state then the futex is
1142 * inconsistent and user space fiddled with the futex value.
1143 */
1144 if (pi_state->owner != current)
1145 goto out_unlock;
1146
1147 get_pi_state(pi_state);
1148 /*
1149 * By taking wait_lock while still holding hb->lock, we ensure
1150 * there is no point where we hold neither; and therefore
1151 * wake_futex_p() must observe a state consistent with what we
1152 * observed.
1153 *
1154 * In particular; this forces __rt_mutex_start_proxy() to
1155 * complete such that we're guaranteed to observe the
1156 * rt_waiter. Also see the WARN in wake_futex_pi().
1157 */
1158 raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
1159 spin_unlock(&hb->lock);
1160
1161 /* drops pi_state->pi_mutex.wait_lock */
1162 ret = wake_futex_pi(uaddr, uval, pi_state);
1163
1164 put_pi_state(pi_state);
1165
1166 /*
1167 * Success, we're done! No tricky corner cases.
1168 */
1169 if (!ret)
1170 return ret;
1171 /*
1172 * The atomic access to the futex value generated a
1173 * pagefault, so retry the user-access and the wakeup:
1174 */
1175 if (ret == -EFAULT)
1176 goto pi_faulted;
1177 /*
1178 * A unconditional UNLOCK_PI op raced against a waiter
1179 * setting the FUTEX_WAITERS bit. Try again.
1180 */
1181 if (ret == -EAGAIN)
1182 goto pi_retry;
1183 /*
1184 * wake_futex_pi has detected invalid state. Tell user
1185 * space.
1186 */
1187 return ret;
1188 }
1189
1190 /*
1191 * We have no kernel internal state, i.e. no waiters in the
1192 * kernel. Waiters which are about to queue themselves are stuck
1193 * on hb->lock. So we can safely ignore them. We do neither
1194 * preserve the WAITERS bit not the OWNER_DIED one. We are the
1195 * owner.
1196 */
1197 if ((ret = futex_cmpxchg_value_locked(&curval, uaddr, uval, 0))) {
1198 spin_unlock(&hb->lock);
1199 switch (ret) {
1200 case -EFAULT:
1201 goto pi_faulted;
1202
1203 case -EAGAIN:
1204 goto pi_retry;
1205
1206 default:
1207 WARN_ON_ONCE(1);
1208 return ret;
1209 }
1210 }
1211
1212 /*
1213 * If uval has changed, let user space handle it.
1214 */
1215 ret = (curval == uval) ? 0 : -EAGAIN;
1216
1217out_unlock:
1218 spin_unlock(&hb->lock);
1219 return ret;
1220
1221pi_retry:
1222 cond_resched();
1223 goto retry;
1224
1225pi_faulted:
1226
1227 ret = fault_in_user_writeable(uaddr);
1228 if (!ret)
1229 goto retry;
1230
1231 return ret;
1232}
1233