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