1 // SPDX-License-Identifier: GPL-2.0-or-later
2
3 #include <linux/plist.h>
4 #include <linux/sched/task.h>
5 #include <linux/sched/signal.h>
6 #include <linux/freezer.h>
7
8 #include "futex.h"
9
10 /*
11 * READ this before attempting to hack on futexes!
12 *
13 * Basic futex operation and ordering guarantees
14 * =============================================
15 *
16 * The waiter reads the futex value in user space and calls
17 * futex_wait(). This function computes the hash bucket and acquires
18 * the hash bucket lock. After that it reads the futex user space value
19 * again and verifies that the data has not changed. If it has not changed
20 * it enqueues itself into the hash bucket, releases the hash bucket lock
21 * and schedules.
22 *
23 * The waker side modifies the user space value of the futex and calls
24 * futex_wake(). This function computes the hash bucket and acquires the
25 * hash bucket lock. Then it looks for waiters on that futex in the hash
26 * bucket and wakes them.
27 *
28 * In futex wake up scenarios where no tasks are blocked on a futex, taking
29 * the hb spinlock can be avoided and simply return. In order for this
30 * optimization to work, ordering guarantees must exist so that the waiter
31 * being added to the list is acknowledged when the list is concurrently being
32 * checked by the waker, avoiding scenarios like the following:
33 *
34 * CPU 0 CPU 1
35 * val = *futex;
36 * sys_futex(WAIT, futex, val);
37 * futex_wait(futex, val);
38 * uval = *futex;
39 * *futex = newval;
40 * sys_futex(WAKE, futex);
41 * futex_wake(futex);
42 * if (queue_empty())
43 * return;
44 * if (uval == val)
45 * lock(hash_bucket(futex));
46 * queue();
47 * unlock(hash_bucket(futex));
48 * schedule();
49 *
50 * This would cause the waiter on CPU 0 to wait forever because it
51 * missed the transition of the user space value from val to newval
52 * and the waker did not find the waiter in the hash bucket queue.
53 *
54 * The correct serialization ensures that a waiter either observes
55 * the changed user space value before blocking or is woken by a
56 * concurrent waker:
57 *
58 * CPU 0 CPU 1
59 * val = *futex;
60 * sys_futex(WAIT, futex, val);
61 * futex_wait(futex, val);
62 *
63 * waiters++; (a)
64 * smp_mb(); (A) <-- paired with -.
65 * |
66 * lock(hash_bucket(futex)); |
67 * |
68 * uval = *futex; |
69 * | *futex = newval;
70 * | sys_futex(WAKE, futex);
71 * | futex_wake(futex);
72 * |
73 * `--------> smp_mb(); (B)
74 * if (uval == val)
75 * queue();
76 * unlock(hash_bucket(futex));
77 * schedule(); if (waiters)
78 * lock(hash_bucket(futex));
79 * else wake_waiters(futex);
80 * waiters--; (b) unlock(hash_bucket(futex));
81 *
82 * Where (A) orders the waiters increment and the futex value read through
83 * atomic operations (see futex_hb_waiters_inc) and where (B) orders the write
84 * to futex and the waiters read (see futex_hb_waiters_pending()).
85 *
86 * This yields the following case (where X:=waiters, Y:=futex):
87 *
88 * X = Y = 0
89 *
90 * w[X]=1 w[Y]=1
91 * MB MB
92 * r[Y]=y r[X]=x
93 *
94 * Which guarantees that x==0 && y==0 is impossible; which translates back into
95 * the guarantee that we cannot both miss the futex variable change and the
96 * enqueue.
97 *
98 * Note that a new waiter is accounted for in (a) even when it is possible that
99 * the wait call can return error, in which case we backtrack from it in (b).
100 * Refer to the comment in futex_q_lock().
101 *
102 * Similarly, in order to account for waiters being requeued on another
103 * address we always increment the waiters for the destination bucket before
104 * acquiring the lock. It then decrements them again after releasing it -
105 * the code that actually moves the futex(es) between hash buckets (requeue_futex)
106 * will do the additional required waiter count housekeeping. This is done for
107 * double_lock_hb() and double_unlock_hb(), respectively.
108 */
109
110 bool __futex_wake_mark(struct futex_q *q)
111 {
112 if (WARN(q->pi_state || q->rt_waiter, "refusing to wake PI futex\n"))
113 return false;
114
115 __futex_unqueue(q);
116 /*
117 * The waiting task can free the futex_q as soon as q->lock_ptr = NULL
118 * is written, without taking any locks. This is possible in the event
119 * of a spurious wakeup, for example. A memory barrier is required here
120 * to prevent the following store to lock_ptr from getting ahead of the
121 * plist_del in __futex_unqueue().
122 */
123 smp_store_release(&q->lock_ptr, NULL);
124
125 return true;
126 }
127
128 /*
129 * The hash bucket lock must be held when this is called.
130 * Afterwards, the futex_q must not be accessed. Callers
131 * must ensure to later call wake_up_q() for the actual
132 * wakeups to occur.
133 */
134 void futex_wake_mark(struct wake_q_head *wake_q, struct futex_q *q)
135 {
136 struct task_struct *p = q->task;
137
138 get_task_struct(p);
139
140 if (!__futex_wake_mark(q)) {
141 put_task_struct(p);
142 return;
143 }
144
145 /*
146 * Queue the task for later wakeup for after we've released
147 * the hb->lock.
148 */
149 wake_q_add_safe(wake_q, p);
150 }
151
152 /*
153 * Wake up waiters matching bitset queued on this futex (uaddr).
154 */
155 int futex_wake(u32 __user *uaddr, unsigned int flags, int nr_wake, u32 bitset)
156 {
157 struct futex_hash_bucket *hb;
158 struct futex_q *this, *next;
159 union futex_key key = FUTEX_KEY_INIT;
160 DEFINE_WAKE_Q(wake_q);
161 int ret;
162
163 if (!bitset)
164 return -EINVAL;
165
166 ret = get_futex_key(uaddr, flags, &key, FUTEX_READ);
167 if (unlikely(ret != 0))
168 return ret;
169
170 if ((flags & FLAGS_STRICT) && !nr_wake)
171 return 0;
172
173 hb = futex_hash(&key);
174
175 /* Make sure we really have tasks to wakeup */
176 if (!futex_hb_waiters_pending(hb))
177 return ret;
178
179 spin_lock(&hb->lock);
180
181 plist_for_each_entry_safe(this, next, &hb->chain, list) {
182 if (futex_match (&this->key, &key)) {
183 if (this->pi_state || this->rt_waiter) {
184 ret = -EINVAL;
185 break;
186 }
187
188 /* Check if one of the bits is set in both bitsets */
189 if (!(this->bitset & bitset))
190 continue;
191
192 this->wake(&wake_q, this);
193 if (++ret >= nr_wake)
194 break;
195 }
196 }
197
198 spin_unlock(&hb->lock);
199 wake_up_q(&wake_q);
200 return ret;
201 }
202
203 static int futex_atomic_op_inuser(unsigned int encoded_op, u32 __user *uaddr)
204 {
205 unsigned int op = (encoded_op & 0x70000000) >> 28;
206 unsigned int cmp = (encoded_op & 0x0f000000) >> 24;
207 int oparg = sign_extend32((encoded_op & 0x00fff000) >> 12, 11);
208 int cmparg = sign_extend32(encoded_op & 0x00000fff, 11);
209 int oldval, ret;
210
211 if (encoded_op & (FUTEX_OP_OPARG_SHIFT << 28)) {
212 if (oparg < 0 || oparg > 31) {
213 char comm[sizeof(current->comm)];
214 /*
215 * kill this print and return -EINVAL when userspace
216 * is sane again
217 */
218 pr_info_ratelimited("futex_wake_op: %s tries to shift op by %d; fix this program\n",
219 get_task_comm(comm, current), oparg);
220 oparg &= 31;
221 }
222 oparg = 1 << oparg;
223 }
224
225 pagefault_disable();
226 ret = arch_futex_atomic_op_inuser(op, oparg, &oldval, uaddr);
227 pagefault_enable();
228 if (ret)
229 return ret;
230
231 switch (cmp) {
232 case FUTEX_OP_CMP_EQ:
233 return oldval == cmparg;
234 case FUTEX_OP_CMP_NE:
235 return oldval != cmparg;
236 case FUTEX_OP_CMP_LT:
237 return oldval < cmparg;
238 case FUTEX_OP_CMP_GE:
239 return oldval >= cmparg;
240 case FUTEX_OP_CMP_LE:
241 return oldval <= cmparg;
242 case FUTEX_OP_CMP_GT:
243 return oldval > cmparg;
244 default:
245 return -ENOSYS;
246 }
247 }
248
249 /*
250 * Wake up all waiters hashed on the physical page that is mapped
251 * to this virtual address:
252 */
253 int futex_wake_op(u32 __user *uaddr1, unsigned int flags, u32 __user *uaddr2,
254 int nr_wake, int nr_wake2, int op)
255 {
256 union futex_key key1 = FUTEX_KEY_INIT, key2 = FUTEX_KEY_INIT;
257 struct futex_hash_bucket *hb1, *hb2;
258 struct futex_q *this, *next;
259 int ret, op_ret;
260 DEFINE_WAKE_Q(wake_q);
261
262 retry:
263 ret = get_futex_key(uaddr1, flags, &key1, FUTEX_READ);
264 if (unlikely(ret != 0))
265 return ret;
266 ret = get_futex_key(uaddr2, flags, &key2, FUTEX_WRITE);
267 if (unlikely(ret != 0))
268 return ret;
269
270 hb1 = futex_hash(&key1);
271 hb2 = futex_hash(&key2);
272
273 retry_private:
274 double_lock_hb(hb1, hb2);
275 op_ret = futex_atomic_op_inuser(op, uaddr2);
276 if (unlikely(op_ret < 0)) {
277 double_unlock_hb(hb1, hb2);
278
279 if (!IS_ENABLED(CONFIG_MMU) ||
280 unlikely(op_ret != -EFAULT && op_ret != -EAGAIN)) {
281 /*
282 * we don't get EFAULT from MMU faults if we don't have
283 * an MMU, but we might get them from range checking
284 */
285 ret = op_ret;
286 return ret;
287 }
288
289 if (op_ret == -EFAULT) {
290 ret = fault_in_user_writeable(uaddr2);
291 if (ret)
292 return ret;
293 }
294
295 cond_resched();
296 if (!(flags & FLAGS_SHARED))
297 goto retry_private;
298 goto retry;
299 }
300
301 plist_for_each_entry_safe(this, next, &hb1->chain, list) {
302 if (futex_match (&this->key, &key1)) {
303 if (this->pi_state || this->rt_waiter) {
304 ret = -EINVAL;
305 goto out_unlock;
306 }
307 this->wake(&wake_q, this);
308 if (++ret >= nr_wake)
309 break;
310 }
311 }
312
313 if (op_ret > 0) {
314 op_ret = 0;
315 plist_for_each_entry_safe(this, next, &hb2->chain, list) {
316 if (futex_match (&this->key, &key2)) {
317 if (this->pi_state || this->rt_waiter) {
318 ret = -EINVAL;
319 goto out_unlock;
320 }
321 this->wake(&wake_q, this);
322 if (++op_ret >= nr_wake2)
323 break;
324 }
325 }
326 ret += op_ret;
327 }
328
329 out_unlock:
330 double_unlock_hb(hb1, hb2);
331 wake_up_q(&wake_q);
332 return ret;
333 }
334
335 static long futex_wait_restart(struct restart_block *restart);
336
337 /**
338 * futex_wait_queue() - futex_queue() and wait for wakeup, timeout, or signal
339 * @hb: the futex hash bucket, must be locked by the caller
340 * @q: the futex_q to queue up on
341 * @timeout: the prepared hrtimer_sleeper, or null for no timeout
342 */
343 void futex_wait_queue(struct futex_hash_bucket *hb, struct futex_q *q,
344 struct hrtimer_sleeper *timeout)
345 {
346 /*
347 * The task state is guaranteed to be set before another task can
348 * wake it. set_current_state() is implemented using smp_store_mb() and
349 * futex_queue() calls spin_unlock() upon completion, both serializing
350 * access to the hash list and forcing another memory barrier.
351 */
352 set_current_state(TASK_INTERRUPTIBLE|TASK_FREEZABLE);
353 futex_queue(q, hb);
354
355 /* Arm the timer */
356 if (timeout)
357 hrtimer_sleeper_start_expires(timeout, HRTIMER_MODE_ABS);
358
359 /*
360 * If we have been removed from the hash list, then another task
361 * has tried to wake us, and we can skip the call to schedule().
362 */
363 if (likely(!plist_node_empty(&q->list))) {
364 /*
365 * If the timer has already expired, current will already be
366 * flagged for rescheduling. Only call schedule if there
367 * is no timeout, or if it has yet to expire.
368 */
369 if (!timeout || timeout->task)
370 schedule();
371 }
372 __set_current_state(TASK_RUNNING);
373 }
374
375 /**
376 * futex_unqueue_multiple - Remove various futexes from their hash bucket
377 * @v: The list of futexes to unqueue
378 * @count: Number of futexes in the list
379 *
380 * Helper to unqueue a list of futexes. This can't fail.
381 *
382 * Return:
383 * - >=0 - Index of the last futex that was awoken;
384 * - -1 - No futex was awoken
385 */
386 int futex_unqueue_multiple(struct futex_vector *v, int count)
387 {
388 int ret = -1, i;
389
390 for (i = 0; i < count; i++) {
391 if (!futex_unqueue(&v[i].q))
392 ret = i;
393 }
394
395 return ret;
396 }
397
398 /**
399 * futex_wait_multiple_setup - Prepare to wait and enqueue multiple futexes
400 * @vs: The futex list to wait on
401 * @count: The size of the list
402 * @woken: Index of the last woken futex, if any. Used to notify the
403 * caller that it can return this index to userspace (return parameter)
404 *
405 * Prepare multiple futexes in a single step and enqueue them. This may fail if
406 * the futex list is invalid or if any futex was already awoken. On success the
407 * task is ready to interruptible sleep.
408 *
409 * Return:
410 * - 1 - One of the futexes was woken by another thread
411 * - 0 - Success
412 * - <0 - -EFAULT, -EWOULDBLOCK or -EINVAL
413 */
414 int futex_wait_multiple_setup(struct futex_vector *vs, int count, int *woken)
415 {
416 struct futex_hash_bucket *hb;
417 bool retry = false;
418 int ret, i;
419 u32 uval;
420
421 /*
422 * Enqueuing multiple futexes is tricky, because we need to enqueue
423 * each futex on the list before dealing with the next one to avoid
424 * deadlocking on the hash bucket. But, before enqueuing, we need to
425 * make sure that current->state is TASK_INTERRUPTIBLE, so we don't
426 * lose any wake events, which cannot be done before the get_futex_key
427 * of the next key, because it calls get_user_pages, which can sleep.
428 * Thus, we fetch the list of futexes keys in two steps, by first
429 * pinning all the memory keys in the futex key, and only then we read
430 * each key and queue the corresponding futex.
431 *
432 * Private futexes doesn't need to recalculate hash in retry, so skip
433 * get_futex_key() when retrying.
434 */
435 retry:
436 for (i = 0; i < count; i++) {
437 if (!(vs[i].w.flags & FLAGS_SHARED) && retry)
438 continue;
439
440 ret = get_futex_key(u64_to_user_ptr(vs[i].w.uaddr),
441 vs[i].w.flags,
442 &vs[i].q.key, FUTEX_READ);
443
444 if (unlikely(ret))
445 return ret;
446 }
447
448 set_current_state(TASK_INTERRUPTIBLE|TASK_FREEZABLE);
449
450 for (i = 0; i < count; i++) {
451 u32 __user *uaddr = (u32 __user *)(unsigned long)vs[i].w.uaddr;
452 struct futex_q *q = &vs[i].q;
453 u32 val = vs[i].w.val;
454
455 hb = futex_q_lock(q);
456 ret = futex_get_value_locked(&uval, uaddr);
457
458 if (!ret && uval == val) {
459 /*
460 * The bucket lock can't be held while dealing with the
461 * next futex. Queue each futex at this moment so hb can
462 * be unlocked.
463 */
464 futex_queue(q, hb);
465 continue;
466 }
467
468 futex_q_unlock(hb);
469 __set_current_state(TASK_RUNNING);
470
471 /*
472 * Even if something went wrong, if we find out that a futex
473 * was woken, we don't return error and return this index to
474 * userspace
475 */
476 *woken = futex_unqueue_multiple(vs, i);
477 if (*woken >= 0)
478 return 1;
479
480 if (ret) {
481 /*
482 * If we need to handle a page fault, we need to do so
483 * without any lock and any enqueued futex (otherwise
484 * we could lose some wakeup). So we do it here, after
485 * undoing all the work done so far. In success, we
486 * retry all the work.
487 */
488 if (get_user(uval, uaddr))
489 return -EFAULT;
490
491 retry = true;
492 goto retry;
493 }
494
495 if (uval != val)
496 return -EWOULDBLOCK;
497 }
498
499 return 0;
500 }
501
502 /**
503 * futex_sleep_multiple - Check sleeping conditions and sleep
504 * @vs: List of futexes to wait for
505 * @count: Length of vs
506 * @to: Timeout
507 *
508 * Sleep if and only if the timeout hasn't expired and no futex on the list has
509 * been woken up.
510 */
511 static void futex_sleep_multiple(struct futex_vector *vs, unsigned int count,
512 struct hrtimer_sleeper *to)
513 {
514 if (to && !to->task)
515 return;
516
517 for (; count; count--, vs++) {
518 if (!READ_ONCE(vs->q.lock_ptr))
519 return;
520 }
521
522 schedule();
523 }
524
525 /**
526 * futex_wait_multiple - Prepare to wait on and enqueue several futexes
527 * @vs: The list of futexes to wait on
528 * @count: The number of objects
529 * @to: Timeout before giving up and returning to userspace
530 *
531 * Entry point for the FUTEX_WAIT_MULTIPLE futex operation, this function
532 * sleeps on a group of futexes and returns on the first futex that is
533 * wake, or after the timeout has elapsed.
534 *
535 * Return:
536 * - >=0 - Hint to the futex that was awoken
537 * - <0 - On error
538 */
539 int futex_wait_multiple(struct futex_vector *vs, unsigned int count,
540 struct hrtimer_sleeper *to)
541 {
542 int ret, hint = 0;
543
544 if (to)
545 hrtimer_sleeper_start_expires(to, HRTIMER_MODE_ABS);
546
547 while (1) {
548 ret = futex_wait_multiple_setup(vs, count, &hint);
549 if (ret) {
550 if (ret > 0) {
551 /* A futex was woken during setup */
552 ret = hint;
553 }
554 return ret;
555 }
556
557 futex_sleep_multiple(vs, count, to);
558
559 __set_current_state(TASK_RUNNING);
560
561 ret = futex_unqueue_multiple(vs, count);
562 if (ret >= 0)
563 return ret;
564
565 if (to && !to->task)
566 return -ETIMEDOUT;
567 else if (signal_pending(current))
568 return -ERESTARTSYS;
569 /*
570 * The final case is a spurious wakeup, for
571 * which just retry.
572 */
573 }
574 }
575
576 /**
577 * futex_wait_setup() - Prepare to wait on a futex
578 * @uaddr: the futex userspace address
579 * @val: the expected value
580 * @flags: futex flags (FLAGS_SHARED, etc.)
581 * @q: the associated futex_q
582 * @hb: storage for hash_bucket pointer to be returned to caller
583 *
584 * Setup the futex_q and locate the hash_bucket. Get the futex value and
585 * compare it with the expected value. Handle atomic faults internally.
586 * Return with the hb lock held on success, and unlocked on failure.
587 *
588 * Return:
589 * - 0 - uaddr contains val and hb has been locked;
590 * - <1 - -EFAULT or -EWOULDBLOCK (uaddr does not contain val) and hb is unlocked
591 */
592 int futex_wait_setup(u32 __user *uaddr, u32 val, unsigned int flags,
593 struct futex_q *q, struct futex_hash_bucket **hb)
594 {
595 u32 uval;
596 int ret;
597
598 /*
599 * Access the page AFTER the hash-bucket is locked.
600 * Order is important:
601 *
602 * Userspace waiter: val = var; if (cond(val)) futex_wait(&var, val);
603 * Userspace waker: if (cond(var)) { var = new; futex_wake(&var); }
604 *
605 * The basic logical guarantee of a futex is that it blocks ONLY
606 * if cond(var) is known to be true at the time of blocking, for
607 * any cond. If we locked the hash-bucket after testing *uaddr, that
608 * would open a race condition where we could block indefinitely with
609 * cond(var) false, which would violate the guarantee.
610 *
611 * On the other hand, we insert q and release the hash-bucket only
612 * after testing *uaddr. This guarantees that futex_wait() will NOT
613 * absorb a wakeup if *uaddr does not match the desired values
614 * while the syscall executes.
615 */
616 retry:
617 ret = get_futex_key(uaddr, flags, &q->key, FUTEX_READ);
618 if (unlikely(ret != 0))
619 return ret;
620
621 retry_private:
622 *hb = futex_q_lock(q);
623
624 ret = futex_get_value_locked(&uval, uaddr);
625
626 if (ret) {
627 futex_q_unlock(*hb);
628
629 ret = get_user(uval, uaddr);
630 if (ret)
631 return ret;
632
633 if (!(flags & FLAGS_SHARED))
634 goto retry_private;
635
636 goto retry;
637 }
638
639 if (uval != val) {
640 futex_q_unlock(*hb);
641 ret = -EWOULDBLOCK;
642 }
643
644 return ret;
645 }
646
647 int __futex_wait(u32 __user *uaddr, unsigned int flags, u32 val,
648 struct hrtimer_sleeper *to, u32 bitset)
649 {
650 struct futex_q q = futex_q_init;
651 struct futex_hash_bucket *hb;
652 int ret;
653
654 if (!bitset)
655 return -EINVAL;
656
657 q.bitset = bitset;
658
659 retry:
660 /*
661 * Prepare to wait on uaddr. On success, it holds hb->lock and q
662 * is initialized.
663 */
664 ret = futex_wait_setup(uaddr, val, flags, &q, &hb);
665 if (ret)
666 return ret;
667
668 /* futex_queue and wait for wakeup, timeout, or a signal. */
669 futex_wait_queue(hb, &q, to);
670
671 /* If we were woken (and unqueued), we succeeded, whatever. */
672 if (!futex_unqueue(&q))
673 return 0;
674
675 if (to && !to->task)
676 return -ETIMEDOUT;
677
678 /*
679 * We expect signal_pending(current), but we might be the
680 * victim of a spurious wakeup as well.
681 */
682 if (!signal_pending(current))
683 goto retry;
684
685 return -ERESTARTSYS;
686 }
687
688 int futex_wait(u32 __user *uaddr, unsigned int flags, u32 val, ktime_t *abs_time, u32 bitset)
689 {
690 struct hrtimer_sleeper timeout, *to;
691 struct restart_block *restart;
692 int ret;
693
694 to = futex_setup_timer(abs_time, &timeout, flags,
695 current->timer_slack_ns);
696
697 ret = __futex_wait(uaddr, flags, val, to, bitset);
698
699 /* No timeout, nothing to clean up. */
700 if (!to)
701 return ret;
702
703 hrtimer_cancel(&to->timer);
704 destroy_hrtimer_on_stack(&to->timer);
705
706 if (ret == -ERESTARTSYS) {
707 restart = ¤t->restart_block;
708 restart->futex.uaddr = uaddr;
709 restart->futex.val = val;
710 restart->futex.time = *abs_time;
711 restart->futex.bitset = bitset;
712 restart->futex.flags = flags | FLAGS_HAS_TIMEOUT;
713
714 return set_restart_fn(restart, futex_wait_restart);
715 }
716
717 return ret;
718 }
719
720 static long futex_wait_restart(struct restart_block *restart)
721 {
722 u32 __user *uaddr = restart->futex.uaddr;
723 ktime_t t, *tp = NULL;
724
725 if (restart->futex.flags & FLAGS_HAS_TIMEOUT) {
726 t = restart->futex.time;
727 tp = &t;
728 }
729 restart->fn = do_no_restart_syscall;
730
731 return (long)futex_wait(uaddr, restart->futex.flags,
732 restart->futex.val, tp, restart->futex.bitset);
733 }
734
735
Linux® is a registered trademark of Linus Torvalds in the United States and other countries.
TOMOYO® is a registered trademark of NTT DATA CORPORATION.