xref: /openbmc/linux/fs/userfaultfd.c (revision 92a76f6d)
1 /*
2  *  fs/userfaultfd.c
3  *
4  *  Copyright (C) 2007  Davide Libenzi <davidel@xmailserver.org>
5  *  Copyright (C) 2008-2009 Red Hat, Inc.
6  *  Copyright (C) 2015  Red Hat, Inc.
7  *
8  *  This work is licensed under the terms of the GNU GPL, version 2. See
9  *  the COPYING file in the top-level directory.
10  *
11  *  Some part derived from fs/eventfd.c (anon inode setup) and
12  *  mm/ksm.c (mm hashing).
13  */
14 
15 #include <linux/hashtable.h>
16 #include <linux/sched.h>
17 #include <linux/mm.h>
18 #include <linux/poll.h>
19 #include <linux/slab.h>
20 #include <linux/seq_file.h>
21 #include <linux/file.h>
22 #include <linux/bug.h>
23 #include <linux/anon_inodes.h>
24 #include <linux/syscalls.h>
25 #include <linux/userfaultfd_k.h>
26 #include <linux/mempolicy.h>
27 #include <linux/ioctl.h>
28 #include <linux/security.h>
29 
30 static struct kmem_cache *userfaultfd_ctx_cachep __read_mostly;
31 
32 enum userfaultfd_state {
33 	UFFD_STATE_WAIT_API,
34 	UFFD_STATE_RUNNING,
35 };
36 
37 /*
38  * Start with fault_pending_wqh and fault_wqh so they're more likely
39  * to be in the same cacheline.
40  */
41 struct userfaultfd_ctx {
42 	/* waitqueue head for the pending (i.e. not read) userfaults */
43 	wait_queue_head_t fault_pending_wqh;
44 	/* waitqueue head for the userfaults */
45 	wait_queue_head_t fault_wqh;
46 	/* waitqueue head for the pseudo fd to wakeup poll/read */
47 	wait_queue_head_t fd_wqh;
48 	/* a refile sequence protected by fault_pending_wqh lock */
49 	struct seqcount refile_seq;
50 	/* pseudo fd refcounting */
51 	atomic_t refcount;
52 	/* userfaultfd syscall flags */
53 	unsigned int flags;
54 	/* state machine */
55 	enum userfaultfd_state state;
56 	/* released */
57 	bool released;
58 	/* mm with one ore more vmas attached to this userfaultfd_ctx */
59 	struct mm_struct *mm;
60 };
61 
62 struct userfaultfd_wait_queue {
63 	struct uffd_msg msg;
64 	wait_queue_t wq;
65 	struct userfaultfd_ctx *ctx;
66 };
67 
68 struct userfaultfd_wake_range {
69 	unsigned long start;
70 	unsigned long len;
71 };
72 
73 static int userfaultfd_wake_function(wait_queue_t *wq, unsigned mode,
74 				     int wake_flags, void *key)
75 {
76 	struct userfaultfd_wake_range *range = key;
77 	int ret;
78 	struct userfaultfd_wait_queue *uwq;
79 	unsigned long start, len;
80 
81 	uwq = container_of(wq, struct userfaultfd_wait_queue, wq);
82 	ret = 0;
83 	/* len == 0 means wake all */
84 	start = range->start;
85 	len = range->len;
86 	if (len && (start > uwq->msg.arg.pagefault.address ||
87 		    start + len <= uwq->msg.arg.pagefault.address))
88 		goto out;
89 	ret = wake_up_state(wq->private, mode);
90 	if (ret)
91 		/*
92 		 * Wake only once, autoremove behavior.
93 		 *
94 		 * After the effect of list_del_init is visible to the
95 		 * other CPUs, the waitqueue may disappear from under
96 		 * us, see the !list_empty_careful() in
97 		 * handle_userfault(). try_to_wake_up() has an
98 		 * implicit smp_mb__before_spinlock, and the
99 		 * wq->private is read before calling the extern
100 		 * function "wake_up_state" (which in turns calls
101 		 * try_to_wake_up). While the spin_lock;spin_unlock;
102 		 * wouldn't be enough, the smp_mb__before_spinlock is
103 		 * enough to avoid an explicit smp_mb() here.
104 		 */
105 		list_del_init(&wq->task_list);
106 out:
107 	return ret;
108 }
109 
110 /**
111  * userfaultfd_ctx_get - Acquires a reference to the internal userfaultfd
112  * context.
113  * @ctx: [in] Pointer to the userfaultfd context.
114  *
115  * Returns: In case of success, returns not zero.
116  */
117 static void userfaultfd_ctx_get(struct userfaultfd_ctx *ctx)
118 {
119 	if (!atomic_inc_not_zero(&ctx->refcount))
120 		BUG();
121 }
122 
123 /**
124  * userfaultfd_ctx_put - Releases a reference to the internal userfaultfd
125  * context.
126  * @ctx: [in] Pointer to userfaultfd context.
127  *
128  * The userfaultfd context reference must have been previously acquired either
129  * with userfaultfd_ctx_get() or userfaultfd_ctx_fdget().
130  */
131 static void userfaultfd_ctx_put(struct userfaultfd_ctx *ctx)
132 {
133 	if (atomic_dec_and_test(&ctx->refcount)) {
134 		VM_BUG_ON(spin_is_locked(&ctx->fault_pending_wqh.lock));
135 		VM_BUG_ON(waitqueue_active(&ctx->fault_pending_wqh));
136 		VM_BUG_ON(spin_is_locked(&ctx->fault_wqh.lock));
137 		VM_BUG_ON(waitqueue_active(&ctx->fault_wqh));
138 		VM_BUG_ON(spin_is_locked(&ctx->fd_wqh.lock));
139 		VM_BUG_ON(waitqueue_active(&ctx->fd_wqh));
140 		mmput(ctx->mm);
141 		kmem_cache_free(userfaultfd_ctx_cachep, ctx);
142 	}
143 }
144 
145 static inline void msg_init(struct uffd_msg *msg)
146 {
147 	BUILD_BUG_ON(sizeof(struct uffd_msg) != 32);
148 	/*
149 	 * Must use memset to zero out the paddings or kernel data is
150 	 * leaked to userland.
151 	 */
152 	memset(msg, 0, sizeof(struct uffd_msg));
153 }
154 
155 static inline struct uffd_msg userfault_msg(unsigned long address,
156 					    unsigned int flags,
157 					    unsigned long reason)
158 {
159 	struct uffd_msg msg;
160 	msg_init(&msg);
161 	msg.event = UFFD_EVENT_PAGEFAULT;
162 	msg.arg.pagefault.address = address;
163 	if (flags & FAULT_FLAG_WRITE)
164 		/*
165 		 * If UFFD_FEATURE_PAGEFAULT_FLAG_WRITE was set in the
166 		 * uffdio_api.features and UFFD_PAGEFAULT_FLAG_WRITE
167 		 * was not set in a UFFD_EVENT_PAGEFAULT, it means it
168 		 * was a read fault, otherwise if set it means it's
169 		 * a write fault.
170 		 */
171 		msg.arg.pagefault.flags |= UFFD_PAGEFAULT_FLAG_WRITE;
172 	if (reason & VM_UFFD_WP)
173 		/*
174 		 * If UFFD_FEATURE_PAGEFAULT_FLAG_WP was set in the
175 		 * uffdio_api.features and UFFD_PAGEFAULT_FLAG_WP was
176 		 * not set in a UFFD_EVENT_PAGEFAULT, it means it was
177 		 * a missing fault, otherwise if set it means it's a
178 		 * write protect fault.
179 		 */
180 		msg.arg.pagefault.flags |= UFFD_PAGEFAULT_FLAG_WP;
181 	return msg;
182 }
183 
184 /*
185  * Verify the pagetables are still not ok after having reigstered into
186  * the fault_pending_wqh to avoid userland having to UFFDIO_WAKE any
187  * userfault that has already been resolved, if userfaultfd_read and
188  * UFFDIO_COPY|ZEROPAGE are being run simultaneously on two different
189  * threads.
190  */
191 static inline bool userfaultfd_must_wait(struct userfaultfd_ctx *ctx,
192 					 unsigned long address,
193 					 unsigned long flags,
194 					 unsigned long reason)
195 {
196 	struct mm_struct *mm = ctx->mm;
197 	pgd_t *pgd;
198 	pud_t *pud;
199 	pmd_t *pmd, _pmd;
200 	pte_t *pte;
201 	bool ret = true;
202 
203 	VM_BUG_ON(!rwsem_is_locked(&mm->mmap_sem));
204 
205 	pgd = pgd_offset(mm, address);
206 	if (!pgd_present(*pgd))
207 		goto out;
208 	pud = pud_offset(pgd, address);
209 	if (!pud_present(*pud))
210 		goto out;
211 	pmd = pmd_offset(pud, address);
212 	/*
213 	 * READ_ONCE must function as a barrier with narrower scope
214 	 * and it must be equivalent to:
215 	 *	_pmd = *pmd; barrier();
216 	 *
217 	 * This is to deal with the instability (as in
218 	 * pmd_trans_unstable) of the pmd.
219 	 */
220 	_pmd = READ_ONCE(*pmd);
221 	if (!pmd_present(_pmd))
222 		goto out;
223 
224 	ret = false;
225 	if (pmd_trans_huge(_pmd))
226 		goto out;
227 
228 	/*
229 	 * the pmd is stable (as in !pmd_trans_unstable) so we can re-read it
230 	 * and use the standard pte_offset_map() instead of parsing _pmd.
231 	 */
232 	pte = pte_offset_map(pmd, address);
233 	/*
234 	 * Lockless access: we're in a wait_event so it's ok if it
235 	 * changes under us.
236 	 */
237 	if (pte_none(*pte))
238 		ret = true;
239 	pte_unmap(pte);
240 
241 out:
242 	return ret;
243 }
244 
245 /*
246  * The locking rules involved in returning VM_FAULT_RETRY depending on
247  * FAULT_FLAG_ALLOW_RETRY, FAULT_FLAG_RETRY_NOWAIT and
248  * FAULT_FLAG_KILLABLE are not straightforward. The "Caution"
249  * recommendation in __lock_page_or_retry is not an understatement.
250  *
251  * If FAULT_FLAG_ALLOW_RETRY is set, the mmap_sem must be released
252  * before returning VM_FAULT_RETRY only if FAULT_FLAG_RETRY_NOWAIT is
253  * not set.
254  *
255  * If FAULT_FLAG_ALLOW_RETRY is set but FAULT_FLAG_KILLABLE is not
256  * set, VM_FAULT_RETRY can still be returned if and only if there are
257  * fatal_signal_pending()s, and the mmap_sem must be released before
258  * returning it.
259  */
260 int handle_userfault(struct vm_area_struct *vma, unsigned long address,
261 		     unsigned int flags, unsigned long reason)
262 {
263 	struct mm_struct *mm = vma->vm_mm;
264 	struct userfaultfd_ctx *ctx;
265 	struct userfaultfd_wait_queue uwq;
266 	int ret;
267 	bool must_wait, return_to_userland;
268 
269 	BUG_ON(!rwsem_is_locked(&mm->mmap_sem));
270 
271 	ret = VM_FAULT_SIGBUS;
272 	ctx = vma->vm_userfaultfd_ctx.ctx;
273 	if (!ctx)
274 		goto out;
275 
276 	BUG_ON(ctx->mm != mm);
277 
278 	VM_BUG_ON(reason & ~(VM_UFFD_MISSING|VM_UFFD_WP));
279 	VM_BUG_ON(!(reason & VM_UFFD_MISSING) ^ !!(reason & VM_UFFD_WP));
280 
281 	/*
282 	 * If it's already released don't get it. This avoids to loop
283 	 * in __get_user_pages if userfaultfd_release waits on the
284 	 * caller of handle_userfault to release the mmap_sem.
285 	 */
286 	if (unlikely(ACCESS_ONCE(ctx->released)))
287 		goto out;
288 
289 	/*
290 	 * We don't do userfault handling for the final child pid update.
291 	 */
292 	if (current->flags & PF_EXITING)
293 		goto out;
294 
295 	/*
296 	 * Check that we can return VM_FAULT_RETRY.
297 	 *
298 	 * NOTE: it should become possible to return VM_FAULT_RETRY
299 	 * even if FAULT_FLAG_TRIED is set without leading to gup()
300 	 * -EBUSY failures, if the userfaultfd is to be extended for
301 	 * VM_UFFD_WP tracking and we intend to arm the userfault
302 	 * without first stopping userland access to the memory. For
303 	 * VM_UFFD_MISSING userfaults this is enough for now.
304 	 */
305 	if (unlikely(!(flags & FAULT_FLAG_ALLOW_RETRY))) {
306 		/*
307 		 * Validate the invariant that nowait must allow retry
308 		 * to be sure not to return SIGBUS erroneously on
309 		 * nowait invocations.
310 		 */
311 		BUG_ON(flags & FAULT_FLAG_RETRY_NOWAIT);
312 #ifdef CONFIG_DEBUG_VM
313 		if (printk_ratelimit()) {
314 			printk(KERN_WARNING
315 			       "FAULT_FLAG_ALLOW_RETRY missing %x\n", flags);
316 			dump_stack();
317 		}
318 #endif
319 		goto out;
320 	}
321 
322 	/*
323 	 * Handle nowait, not much to do other than tell it to retry
324 	 * and wait.
325 	 */
326 	ret = VM_FAULT_RETRY;
327 	if (flags & FAULT_FLAG_RETRY_NOWAIT)
328 		goto out;
329 
330 	/* take the reference before dropping the mmap_sem */
331 	userfaultfd_ctx_get(ctx);
332 
333 	init_waitqueue_func_entry(&uwq.wq, userfaultfd_wake_function);
334 	uwq.wq.private = current;
335 	uwq.msg = userfault_msg(address, flags, reason);
336 	uwq.ctx = ctx;
337 
338 	return_to_userland = (flags & (FAULT_FLAG_USER|FAULT_FLAG_KILLABLE)) ==
339 		(FAULT_FLAG_USER|FAULT_FLAG_KILLABLE);
340 
341 	spin_lock(&ctx->fault_pending_wqh.lock);
342 	/*
343 	 * After the __add_wait_queue the uwq is visible to userland
344 	 * through poll/read().
345 	 */
346 	__add_wait_queue(&ctx->fault_pending_wqh, &uwq.wq);
347 	/*
348 	 * The smp_mb() after __set_current_state prevents the reads
349 	 * following the spin_unlock to happen before the list_add in
350 	 * __add_wait_queue.
351 	 */
352 	set_current_state(return_to_userland ? TASK_INTERRUPTIBLE :
353 			  TASK_KILLABLE);
354 	spin_unlock(&ctx->fault_pending_wqh.lock);
355 
356 	must_wait = userfaultfd_must_wait(ctx, address, flags, reason);
357 	up_read(&mm->mmap_sem);
358 
359 	if (likely(must_wait && !ACCESS_ONCE(ctx->released) &&
360 		   (return_to_userland ? !signal_pending(current) :
361 		    !fatal_signal_pending(current)))) {
362 		wake_up_poll(&ctx->fd_wqh, POLLIN);
363 		schedule();
364 		ret |= VM_FAULT_MAJOR;
365 	}
366 
367 	__set_current_state(TASK_RUNNING);
368 
369 	if (return_to_userland) {
370 		if (signal_pending(current) &&
371 		    !fatal_signal_pending(current)) {
372 			/*
373 			 * If we got a SIGSTOP or SIGCONT and this is
374 			 * a normal userland page fault, just let
375 			 * userland return so the signal will be
376 			 * handled and gdb debugging works.  The page
377 			 * fault code immediately after we return from
378 			 * this function is going to release the
379 			 * mmap_sem and it's not depending on it
380 			 * (unlike gup would if we were not to return
381 			 * VM_FAULT_RETRY).
382 			 *
383 			 * If a fatal signal is pending we still take
384 			 * the streamlined VM_FAULT_RETRY failure path
385 			 * and there's no need to retake the mmap_sem
386 			 * in such case.
387 			 */
388 			down_read(&mm->mmap_sem);
389 			ret = 0;
390 		}
391 	}
392 
393 	/*
394 	 * Here we race with the list_del; list_add in
395 	 * userfaultfd_ctx_read(), however because we don't ever run
396 	 * list_del_init() to refile across the two lists, the prev
397 	 * and next pointers will never point to self. list_add also
398 	 * would never let any of the two pointers to point to
399 	 * self. So list_empty_careful won't risk to see both pointers
400 	 * pointing to self at any time during the list refile. The
401 	 * only case where list_del_init() is called is the full
402 	 * removal in the wake function and there we don't re-list_add
403 	 * and it's fine not to block on the spinlock. The uwq on this
404 	 * kernel stack can be released after the list_del_init.
405 	 */
406 	if (!list_empty_careful(&uwq.wq.task_list)) {
407 		spin_lock(&ctx->fault_pending_wqh.lock);
408 		/*
409 		 * No need of list_del_init(), the uwq on the stack
410 		 * will be freed shortly anyway.
411 		 */
412 		list_del(&uwq.wq.task_list);
413 		spin_unlock(&ctx->fault_pending_wqh.lock);
414 	}
415 
416 	/*
417 	 * ctx may go away after this if the userfault pseudo fd is
418 	 * already released.
419 	 */
420 	userfaultfd_ctx_put(ctx);
421 
422 out:
423 	return ret;
424 }
425 
426 static int userfaultfd_release(struct inode *inode, struct file *file)
427 {
428 	struct userfaultfd_ctx *ctx = file->private_data;
429 	struct mm_struct *mm = ctx->mm;
430 	struct vm_area_struct *vma, *prev;
431 	/* len == 0 means wake all */
432 	struct userfaultfd_wake_range range = { .len = 0, };
433 	unsigned long new_flags;
434 
435 	ACCESS_ONCE(ctx->released) = true;
436 
437 	/*
438 	 * Flush page faults out of all CPUs. NOTE: all page faults
439 	 * must be retried without returning VM_FAULT_SIGBUS if
440 	 * userfaultfd_ctx_get() succeeds but vma->vma_userfault_ctx
441 	 * changes while handle_userfault released the mmap_sem. So
442 	 * it's critical that released is set to true (above), before
443 	 * taking the mmap_sem for writing.
444 	 */
445 	down_write(&mm->mmap_sem);
446 	prev = NULL;
447 	for (vma = mm->mmap; vma; vma = vma->vm_next) {
448 		cond_resched();
449 		BUG_ON(!!vma->vm_userfaultfd_ctx.ctx ^
450 		       !!(vma->vm_flags & (VM_UFFD_MISSING | VM_UFFD_WP)));
451 		if (vma->vm_userfaultfd_ctx.ctx != ctx) {
452 			prev = vma;
453 			continue;
454 		}
455 		new_flags = vma->vm_flags & ~(VM_UFFD_MISSING | VM_UFFD_WP);
456 		prev = vma_merge(mm, prev, vma->vm_start, vma->vm_end,
457 				 new_flags, vma->anon_vma,
458 				 vma->vm_file, vma->vm_pgoff,
459 				 vma_policy(vma),
460 				 NULL_VM_UFFD_CTX);
461 		if (prev)
462 			vma = prev;
463 		else
464 			prev = vma;
465 		vma->vm_flags = new_flags;
466 		vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
467 	}
468 	up_write(&mm->mmap_sem);
469 
470 	/*
471 	 * After no new page faults can wait on this fault_*wqh, flush
472 	 * the last page faults that may have been already waiting on
473 	 * the fault_*wqh.
474 	 */
475 	spin_lock(&ctx->fault_pending_wqh.lock);
476 	__wake_up_locked_key(&ctx->fault_pending_wqh, TASK_NORMAL, &range);
477 	__wake_up_locked_key(&ctx->fault_wqh, TASK_NORMAL, &range);
478 	spin_unlock(&ctx->fault_pending_wqh.lock);
479 
480 	wake_up_poll(&ctx->fd_wqh, POLLHUP);
481 	userfaultfd_ctx_put(ctx);
482 	return 0;
483 }
484 
485 /* fault_pending_wqh.lock must be hold by the caller */
486 static inline struct userfaultfd_wait_queue *find_userfault(
487 	struct userfaultfd_ctx *ctx)
488 {
489 	wait_queue_t *wq;
490 	struct userfaultfd_wait_queue *uwq;
491 
492 	VM_BUG_ON(!spin_is_locked(&ctx->fault_pending_wqh.lock));
493 
494 	uwq = NULL;
495 	if (!waitqueue_active(&ctx->fault_pending_wqh))
496 		goto out;
497 	/* walk in reverse to provide FIFO behavior to read userfaults */
498 	wq = list_last_entry(&ctx->fault_pending_wqh.task_list,
499 			     typeof(*wq), task_list);
500 	uwq = container_of(wq, struct userfaultfd_wait_queue, wq);
501 out:
502 	return uwq;
503 }
504 
505 static unsigned int userfaultfd_poll(struct file *file, poll_table *wait)
506 {
507 	struct userfaultfd_ctx *ctx = file->private_data;
508 	unsigned int ret;
509 
510 	poll_wait(file, &ctx->fd_wqh, wait);
511 
512 	switch (ctx->state) {
513 	case UFFD_STATE_WAIT_API:
514 		return POLLERR;
515 	case UFFD_STATE_RUNNING:
516 		/*
517 		 * poll() never guarantees that read won't block.
518 		 * userfaults can be waken before they're read().
519 		 */
520 		if (unlikely(!(file->f_flags & O_NONBLOCK)))
521 			return POLLERR;
522 		/*
523 		 * lockless access to see if there are pending faults
524 		 * __pollwait last action is the add_wait_queue but
525 		 * the spin_unlock would allow the waitqueue_active to
526 		 * pass above the actual list_add inside
527 		 * add_wait_queue critical section. So use a full
528 		 * memory barrier to serialize the list_add write of
529 		 * add_wait_queue() with the waitqueue_active read
530 		 * below.
531 		 */
532 		ret = 0;
533 		smp_mb();
534 		if (waitqueue_active(&ctx->fault_pending_wqh))
535 			ret = POLLIN;
536 		return ret;
537 	default:
538 		BUG();
539 	}
540 }
541 
542 static ssize_t userfaultfd_ctx_read(struct userfaultfd_ctx *ctx, int no_wait,
543 				    struct uffd_msg *msg)
544 {
545 	ssize_t ret;
546 	DECLARE_WAITQUEUE(wait, current);
547 	struct userfaultfd_wait_queue *uwq;
548 
549 	/* always take the fd_wqh lock before the fault_pending_wqh lock */
550 	spin_lock(&ctx->fd_wqh.lock);
551 	__add_wait_queue(&ctx->fd_wqh, &wait);
552 	for (;;) {
553 		set_current_state(TASK_INTERRUPTIBLE);
554 		spin_lock(&ctx->fault_pending_wqh.lock);
555 		uwq = find_userfault(ctx);
556 		if (uwq) {
557 			/*
558 			 * Use a seqcount to repeat the lockless check
559 			 * in wake_userfault() to avoid missing
560 			 * wakeups because during the refile both
561 			 * waitqueue could become empty if this is the
562 			 * only userfault.
563 			 */
564 			write_seqcount_begin(&ctx->refile_seq);
565 
566 			/*
567 			 * The fault_pending_wqh.lock prevents the uwq
568 			 * to disappear from under us.
569 			 *
570 			 * Refile this userfault from
571 			 * fault_pending_wqh to fault_wqh, it's not
572 			 * pending anymore after we read it.
573 			 *
574 			 * Use list_del() by hand (as
575 			 * userfaultfd_wake_function also uses
576 			 * list_del_init() by hand) to be sure nobody
577 			 * changes __remove_wait_queue() to use
578 			 * list_del_init() in turn breaking the
579 			 * !list_empty_careful() check in
580 			 * handle_userfault(). The uwq->wq.task_list
581 			 * must never be empty at any time during the
582 			 * refile, or the waitqueue could disappear
583 			 * from under us. The "wait_queue_head_t"
584 			 * parameter of __remove_wait_queue() is unused
585 			 * anyway.
586 			 */
587 			list_del(&uwq->wq.task_list);
588 			__add_wait_queue(&ctx->fault_wqh, &uwq->wq);
589 
590 			write_seqcount_end(&ctx->refile_seq);
591 
592 			/* careful to always initialize msg if ret == 0 */
593 			*msg = uwq->msg;
594 			spin_unlock(&ctx->fault_pending_wqh.lock);
595 			ret = 0;
596 			break;
597 		}
598 		spin_unlock(&ctx->fault_pending_wqh.lock);
599 		if (signal_pending(current)) {
600 			ret = -ERESTARTSYS;
601 			break;
602 		}
603 		if (no_wait) {
604 			ret = -EAGAIN;
605 			break;
606 		}
607 		spin_unlock(&ctx->fd_wqh.lock);
608 		schedule();
609 		spin_lock(&ctx->fd_wqh.lock);
610 	}
611 	__remove_wait_queue(&ctx->fd_wqh, &wait);
612 	__set_current_state(TASK_RUNNING);
613 	spin_unlock(&ctx->fd_wqh.lock);
614 
615 	return ret;
616 }
617 
618 static ssize_t userfaultfd_read(struct file *file, char __user *buf,
619 				size_t count, loff_t *ppos)
620 {
621 	struct userfaultfd_ctx *ctx = file->private_data;
622 	ssize_t _ret, ret = 0;
623 	struct uffd_msg msg;
624 	int no_wait = file->f_flags & O_NONBLOCK;
625 
626 	if (ctx->state == UFFD_STATE_WAIT_API)
627 		return -EINVAL;
628 
629 	for (;;) {
630 		if (count < sizeof(msg))
631 			return ret ? ret : -EINVAL;
632 		_ret = userfaultfd_ctx_read(ctx, no_wait, &msg);
633 		if (_ret < 0)
634 			return ret ? ret : _ret;
635 		if (copy_to_user((__u64 __user *) buf, &msg, sizeof(msg)))
636 			return ret ? ret : -EFAULT;
637 		ret += sizeof(msg);
638 		buf += sizeof(msg);
639 		count -= sizeof(msg);
640 		/*
641 		 * Allow to read more than one fault at time but only
642 		 * block if waiting for the very first one.
643 		 */
644 		no_wait = O_NONBLOCK;
645 	}
646 }
647 
648 static void __wake_userfault(struct userfaultfd_ctx *ctx,
649 			     struct userfaultfd_wake_range *range)
650 {
651 	unsigned long start, end;
652 
653 	start = range->start;
654 	end = range->start + range->len;
655 
656 	spin_lock(&ctx->fault_pending_wqh.lock);
657 	/* wake all in the range and autoremove */
658 	if (waitqueue_active(&ctx->fault_pending_wqh))
659 		__wake_up_locked_key(&ctx->fault_pending_wqh, TASK_NORMAL,
660 				     range);
661 	if (waitqueue_active(&ctx->fault_wqh))
662 		__wake_up_locked_key(&ctx->fault_wqh, TASK_NORMAL, range);
663 	spin_unlock(&ctx->fault_pending_wqh.lock);
664 }
665 
666 static __always_inline void wake_userfault(struct userfaultfd_ctx *ctx,
667 					   struct userfaultfd_wake_range *range)
668 {
669 	unsigned seq;
670 	bool need_wakeup;
671 
672 	/*
673 	 * To be sure waitqueue_active() is not reordered by the CPU
674 	 * before the pagetable update, use an explicit SMP memory
675 	 * barrier here. PT lock release or up_read(mmap_sem) still
676 	 * have release semantics that can allow the
677 	 * waitqueue_active() to be reordered before the pte update.
678 	 */
679 	smp_mb();
680 
681 	/*
682 	 * Use waitqueue_active because it's very frequent to
683 	 * change the address space atomically even if there are no
684 	 * userfaults yet. So we take the spinlock only when we're
685 	 * sure we've userfaults to wake.
686 	 */
687 	do {
688 		seq = read_seqcount_begin(&ctx->refile_seq);
689 		need_wakeup = waitqueue_active(&ctx->fault_pending_wqh) ||
690 			waitqueue_active(&ctx->fault_wqh);
691 		cond_resched();
692 	} while (read_seqcount_retry(&ctx->refile_seq, seq));
693 	if (need_wakeup)
694 		__wake_userfault(ctx, range);
695 }
696 
697 static __always_inline int validate_range(struct mm_struct *mm,
698 					  __u64 start, __u64 len)
699 {
700 	__u64 task_size = mm->task_size;
701 
702 	if (start & ~PAGE_MASK)
703 		return -EINVAL;
704 	if (len & ~PAGE_MASK)
705 		return -EINVAL;
706 	if (!len)
707 		return -EINVAL;
708 	if (start < mmap_min_addr)
709 		return -EINVAL;
710 	if (start >= task_size)
711 		return -EINVAL;
712 	if (len > task_size - start)
713 		return -EINVAL;
714 	return 0;
715 }
716 
717 static int userfaultfd_register(struct userfaultfd_ctx *ctx,
718 				unsigned long arg)
719 {
720 	struct mm_struct *mm = ctx->mm;
721 	struct vm_area_struct *vma, *prev, *cur;
722 	int ret;
723 	struct uffdio_register uffdio_register;
724 	struct uffdio_register __user *user_uffdio_register;
725 	unsigned long vm_flags, new_flags;
726 	bool found;
727 	unsigned long start, end, vma_end;
728 
729 	user_uffdio_register = (struct uffdio_register __user *) arg;
730 
731 	ret = -EFAULT;
732 	if (copy_from_user(&uffdio_register, user_uffdio_register,
733 			   sizeof(uffdio_register)-sizeof(__u64)))
734 		goto out;
735 
736 	ret = -EINVAL;
737 	if (!uffdio_register.mode)
738 		goto out;
739 	if (uffdio_register.mode & ~(UFFDIO_REGISTER_MODE_MISSING|
740 				     UFFDIO_REGISTER_MODE_WP))
741 		goto out;
742 	vm_flags = 0;
743 	if (uffdio_register.mode & UFFDIO_REGISTER_MODE_MISSING)
744 		vm_flags |= VM_UFFD_MISSING;
745 	if (uffdio_register.mode & UFFDIO_REGISTER_MODE_WP) {
746 		vm_flags |= VM_UFFD_WP;
747 		/*
748 		 * FIXME: remove the below error constraint by
749 		 * implementing the wprotect tracking mode.
750 		 */
751 		ret = -EINVAL;
752 		goto out;
753 	}
754 
755 	ret = validate_range(mm, uffdio_register.range.start,
756 			     uffdio_register.range.len);
757 	if (ret)
758 		goto out;
759 
760 	start = uffdio_register.range.start;
761 	end = start + uffdio_register.range.len;
762 
763 	down_write(&mm->mmap_sem);
764 	vma = find_vma_prev(mm, start, &prev);
765 
766 	ret = -ENOMEM;
767 	if (!vma)
768 		goto out_unlock;
769 
770 	/* check that there's at least one vma in the range */
771 	ret = -EINVAL;
772 	if (vma->vm_start >= end)
773 		goto out_unlock;
774 
775 	/*
776 	 * Search for not compatible vmas.
777 	 *
778 	 * FIXME: this shall be relaxed later so that it doesn't fail
779 	 * on tmpfs backed vmas (in addition to the current allowance
780 	 * on anonymous vmas).
781 	 */
782 	found = false;
783 	for (cur = vma; cur && cur->vm_start < end; cur = cur->vm_next) {
784 		cond_resched();
785 
786 		BUG_ON(!!cur->vm_userfaultfd_ctx.ctx ^
787 		       !!(cur->vm_flags & (VM_UFFD_MISSING | VM_UFFD_WP)));
788 
789 		/* check not compatible vmas */
790 		ret = -EINVAL;
791 		if (cur->vm_ops)
792 			goto out_unlock;
793 
794 		/*
795 		 * Check that this vma isn't already owned by a
796 		 * different userfaultfd. We can't allow more than one
797 		 * userfaultfd to own a single vma simultaneously or we
798 		 * wouldn't know which one to deliver the userfaults to.
799 		 */
800 		ret = -EBUSY;
801 		if (cur->vm_userfaultfd_ctx.ctx &&
802 		    cur->vm_userfaultfd_ctx.ctx != ctx)
803 			goto out_unlock;
804 
805 		found = true;
806 	}
807 	BUG_ON(!found);
808 
809 	if (vma->vm_start < start)
810 		prev = vma;
811 
812 	ret = 0;
813 	do {
814 		cond_resched();
815 
816 		BUG_ON(vma->vm_ops);
817 		BUG_ON(vma->vm_userfaultfd_ctx.ctx &&
818 		       vma->vm_userfaultfd_ctx.ctx != ctx);
819 
820 		/*
821 		 * Nothing to do: this vma is already registered into this
822 		 * userfaultfd and with the right tracking mode too.
823 		 */
824 		if (vma->vm_userfaultfd_ctx.ctx == ctx &&
825 		    (vma->vm_flags & vm_flags) == vm_flags)
826 			goto skip;
827 
828 		if (vma->vm_start > start)
829 			start = vma->vm_start;
830 		vma_end = min(end, vma->vm_end);
831 
832 		new_flags = (vma->vm_flags & ~vm_flags) | vm_flags;
833 		prev = vma_merge(mm, prev, start, vma_end, new_flags,
834 				 vma->anon_vma, vma->vm_file, vma->vm_pgoff,
835 				 vma_policy(vma),
836 				 ((struct vm_userfaultfd_ctx){ ctx }));
837 		if (prev) {
838 			vma = prev;
839 			goto next;
840 		}
841 		if (vma->vm_start < start) {
842 			ret = split_vma(mm, vma, start, 1);
843 			if (ret)
844 				break;
845 		}
846 		if (vma->vm_end > end) {
847 			ret = split_vma(mm, vma, end, 0);
848 			if (ret)
849 				break;
850 		}
851 	next:
852 		/*
853 		 * In the vma_merge() successful mprotect-like case 8:
854 		 * the next vma was merged into the current one and
855 		 * the current one has not been updated yet.
856 		 */
857 		vma->vm_flags = new_flags;
858 		vma->vm_userfaultfd_ctx.ctx = ctx;
859 
860 	skip:
861 		prev = vma;
862 		start = vma->vm_end;
863 		vma = vma->vm_next;
864 	} while (vma && vma->vm_start < end);
865 out_unlock:
866 	up_write(&mm->mmap_sem);
867 	if (!ret) {
868 		/*
869 		 * Now that we scanned all vmas we can already tell
870 		 * userland which ioctls methods are guaranteed to
871 		 * succeed on this range.
872 		 */
873 		if (put_user(UFFD_API_RANGE_IOCTLS,
874 			     &user_uffdio_register->ioctls))
875 			ret = -EFAULT;
876 	}
877 out:
878 	return ret;
879 }
880 
881 static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
882 				  unsigned long arg)
883 {
884 	struct mm_struct *mm = ctx->mm;
885 	struct vm_area_struct *vma, *prev, *cur;
886 	int ret;
887 	struct uffdio_range uffdio_unregister;
888 	unsigned long new_flags;
889 	bool found;
890 	unsigned long start, end, vma_end;
891 	const void __user *buf = (void __user *)arg;
892 
893 	ret = -EFAULT;
894 	if (copy_from_user(&uffdio_unregister, buf, sizeof(uffdio_unregister)))
895 		goto out;
896 
897 	ret = validate_range(mm, uffdio_unregister.start,
898 			     uffdio_unregister.len);
899 	if (ret)
900 		goto out;
901 
902 	start = uffdio_unregister.start;
903 	end = start + uffdio_unregister.len;
904 
905 	down_write(&mm->mmap_sem);
906 	vma = find_vma_prev(mm, start, &prev);
907 
908 	ret = -ENOMEM;
909 	if (!vma)
910 		goto out_unlock;
911 
912 	/* check that there's at least one vma in the range */
913 	ret = -EINVAL;
914 	if (vma->vm_start >= end)
915 		goto out_unlock;
916 
917 	/*
918 	 * Search for not compatible vmas.
919 	 *
920 	 * FIXME: this shall be relaxed later so that it doesn't fail
921 	 * on tmpfs backed vmas (in addition to the current allowance
922 	 * on anonymous vmas).
923 	 */
924 	found = false;
925 	ret = -EINVAL;
926 	for (cur = vma; cur && cur->vm_start < end; cur = cur->vm_next) {
927 		cond_resched();
928 
929 		BUG_ON(!!cur->vm_userfaultfd_ctx.ctx ^
930 		       !!(cur->vm_flags & (VM_UFFD_MISSING | VM_UFFD_WP)));
931 
932 		/*
933 		 * Check not compatible vmas, not strictly required
934 		 * here as not compatible vmas cannot have an
935 		 * userfaultfd_ctx registered on them, but this
936 		 * provides for more strict behavior to notice
937 		 * unregistration errors.
938 		 */
939 		if (cur->vm_ops)
940 			goto out_unlock;
941 
942 		found = true;
943 	}
944 	BUG_ON(!found);
945 
946 	if (vma->vm_start < start)
947 		prev = vma;
948 
949 	ret = 0;
950 	do {
951 		cond_resched();
952 
953 		BUG_ON(vma->vm_ops);
954 
955 		/*
956 		 * Nothing to do: this vma is already registered into this
957 		 * userfaultfd and with the right tracking mode too.
958 		 */
959 		if (!vma->vm_userfaultfd_ctx.ctx)
960 			goto skip;
961 
962 		if (vma->vm_start > start)
963 			start = vma->vm_start;
964 		vma_end = min(end, vma->vm_end);
965 
966 		new_flags = vma->vm_flags & ~(VM_UFFD_MISSING | VM_UFFD_WP);
967 		prev = vma_merge(mm, prev, start, vma_end, new_flags,
968 				 vma->anon_vma, vma->vm_file, vma->vm_pgoff,
969 				 vma_policy(vma),
970 				 NULL_VM_UFFD_CTX);
971 		if (prev) {
972 			vma = prev;
973 			goto next;
974 		}
975 		if (vma->vm_start < start) {
976 			ret = split_vma(mm, vma, start, 1);
977 			if (ret)
978 				break;
979 		}
980 		if (vma->vm_end > end) {
981 			ret = split_vma(mm, vma, end, 0);
982 			if (ret)
983 				break;
984 		}
985 	next:
986 		/*
987 		 * In the vma_merge() successful mprotect-like case 8:
988 		 * the next vma was merged into the current one and
989 		 * the current one has not been updated yet.
990 		 */
991 		vma->vm_flags = new_flags;
992 		vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
993 
994 	skip:
995 		prev = vma;
996 		start = vma->vm_end;
997 		vma = vma->vm_next;
998 	} while (vma && vma->vm_start < end);
999 out_unlock:
1000 	up_write(&mm->mmap_sem);
1001 out:
1002 	return ret;
1003 }
1004 
1005 /*
1006  * userfaultfd_wake may be used in combination with the
1007  * UFFDIO_*_MODE_DONTWAKE to wakeup userfaults in batches.
1008  */
1009 static int userfaultfd_wake(struct userfaultfd_ctx *ctx,
1010 			    unsigned long arg)
1011 {
1012 	int ret;
1013 	struct uffdio_range uffdio_wake;
1014 	struct userfaultfd_wake_range range;
1015 	const void __user *buf = (void __user *)arg;
1016 
1017 	ret = -EFAULT;
1018 	if (copy_from_user(&uffdio_wake, buf, sizeof(uffdio_wake)))
1019 		goto out;
1020 
1021 	ret = validate_range(ctx->mm, uffdio_wake.start, uffdio_wake.len);
1022 	if (ret)
1023 		goto out;
1024 
1025 	range.start = uffdio_wake.start;
1026 	range.len = uffdio_wake.len;
1027 
1028 	/*
1029 	 * len == 0 means wake all and we don't want to wake all here,
1030 	 * so check it again to be sure.
1031 	 */
1032 	VM_BUG_ON(!range.len);
1033 
1034 	wake_userfault(ctx, &range);
1035 	ret = 0;
1036 
1037 out:
1038 	return ret;
1039 }
1040 
1041 static int userfaultfd_copy(struct userfaultfd_ctx *ctx,
1042 			    unsigned long arg)
1043 {
1044 	__s64 ret;
1045 	struct uffdio_copy uffdio_copy;
1046 	struct uffdio_copy __user *user_uffdio_copy;
1047 	struct userfaultfd_wake_range range;
1048 
1049 	user_uffdio_copy = (struct uffdio_copy __user *) arg;
1050 
1051 	ret = -EFAULT;
1052 	if (copy_from_user(&uffdio_copy, user_uffdio_copy,
1053 			   /* don't copy "copy" last field */
1054 			   sizeof(uffdio_copy)-sizeof(__s64)))
1055 		goto out;
1056 
1057 	ret = validate_range(ctx->mm, uffdio_copy.dst, uffdio_copy.len);
1058 	if (ret)
1059 		goto out;
1060 	/*
1061 	 * double check for wraparound just in case. copy_from_user()
1062 	 * will later check uffdio_copy.src + uffdio_copy.len to fit
1063 	 * in the userland range.
1064 	 */
1065 	ret = -EINVAL;
1066 	if (uffdio_copy.src + uffdio_copy.len <= uffdio_copy.src)
1067 		goto out;
1068 	if (uffdio_copy.mode & ~UFFDIO_COPY_MODE_DONTWAKE)
1069 		goto out;
1070 
1071 	ret = mcopy_atomic(ctx->mm, uffdio_copy.dst, uffdio_copy.src,
1072 			   uffdio_copy.len);
1073 	if (unlikely(put_user(ret, &user_uffdio_copy->copy)))
1074 		return -EFAULT;
1075 	if (ret < 0)
1076 		goto out;
1077 	BUG_ON(!ret);
1078 	/* len == 0 would wake all */
1079 	range.len = ret;
1080 	if (!(uffdio_copy.mode & UFFDIO_COPY_MODE_DONTWAKE)) {
1081 		range.start = uffdio_copy.dst;
1082 		wake_userfault(ctx, &range);
1083 	}
1084 	ret = range.len == uffdio_copy.len ? 0 : -EAGAIN;
1085 out:
1086 	return ret;
1087 }
1088 
1089 static int userfaultfd_zeropage(struct userfaultfd_ctx *ctx,
1090 				unsigned long arg)
1091 {
1092 	__s64 ret;
1093 	struct uffdio_zeropage uffdio_zeropage;
1094 	struct uffdio_zeropage __user *user_uffdio_zeropage;
1095 	struct userfaultfd_wake_range range;
1096 
1097 	user_uffdio_zeropage = (struct uffdio_zeropage __user *) arg;
1098 
1099 	ret = -EFAULT;
1100 	if (copy_from_user(&uffdio_zeropage, user_uffdio_zeropage,
1101 			   /* don't copy "zeropage" last field */
1102 			   sizeof(uffdio_zeropage)-sizeof(__s64)))
1103 		goto out;
1104 
1105 	ret = validate_range(ctx->mm, uffdio_zeropage.range.start,
1106 			     uffdio_zeropage.range.len);
1107 	if (ret)
1108 		goto out;
1109 	ret = -EINVAL;
1110 	if (uffdio_zeropage.mode & ~UFFDIO_ZEROPAGE_MODE_DONTWAKE)
1111 		goto out;
1112 
1113 	ret = mfill_zeropage(ctx->mm, uffdio_zeropage.range.start,
1114 			     uffdio_zeropage.range.len);
1115 	if (unlikely(put_user(ret, &user_uffdio_zeropage->zeropage)))
1116 		return -EFAULT;
1117 	if (ret < 0)
1118 		goto out;
1119 	/* len == 0 would wake all */
1120 	BUG_ON(!ret);
1121 	range.len = ret;
1122 	if (!(uffdio_zeropage.mode & UFFDIO_ZEROPAGE_MODE_DONTWAKE)) {
1123 		range.start = uffdio_zeropage.range.start;
1124 		wake_userfault(ctx, &range);
1125 	}
1126 	ret = range.len == uffdio_zeropage.range.len ? 0 : -EAGAIN;
1127 out:
1128 	return ret;
1129 }
1130 
1131 /*
1132  * userland asks for a certain API version and we return which bits
1133  * and ioctl commands are implemented in this kernel for such API
1134  * version or -EINVAL if unknown.
1135  */
1136 static int userfaultfd_api(struct userfaultfd_ctx *ctx,
1137 			   unsigned long arg)
1138 {
1139 	struct uffdio_api uffdio_api;
1140 	void __user *buf = (void __user *)arg;
1141 	int ret;
1142 
1143 	ret = -EINVAL;
1144 	if (ctx->state != UFFD_STATE_WAIT_API)
1145 		goto out;
1146 	ret = -EFAULT;
1147 	if (copy_from_user(&uffdio_api, buf, sizeof(uffdio_api)))
1148 		goto out;
1149 	if (uffdio_api.api != UFFD_API || uffdio_api.features) {
1150 		memset(&uffdio_api, 0, sizeof(uffdio_api));
1151 		if (copy_to_user(buf, &uffdio_api, sizeof(uffdio_api)))
1152 			goto out;
1153 		ret = -EINVAL;
1154 		goto out;
1155 	}
1156 	uffdio_api.features = UFFD_API_FEATURES;
1157 	uffdio_api.ioctls = UFFD_API_IOCTLS;
1158 	ret = -EFAULT;
1159 	if (copy_to_user(buf, &uffdio_api, sizeof(uffdio_api)))
1160 		goto out;
1161 	ctx->state = UFFD_STATE_RUNNING;
1162 	ret = 0;
1163 out:
1164 	return ret;
1165 }
1166 
1167 static long userfaultfd_ioctl(struct file *file, unsigned cmd,
1168 			      unsigned long arg)
1169 {
1170 	int ret = -EINVAL;
1171 	struct userfaultfd_ctx *ctx = file->private_data;
1172 
1173 	if (cmd != UFFDIO_API && ctx->state == UFFD_STATE_WAIT_API)
1174 		return -EINVAL;
1175 
1176 	switch(cmd) {
1177 	case UFFDIO_API:
1178 		ret = userfaultfd_api(ctx, arg);
1179 		break;
1180 	case UFFDIO_REGISTER:
1181 		ret = userfaultfd_register(ctx, arg);
1182 		break;
1183 	case UFFDIO_UNREGISTER:
1184 		ret = userfaultfd_unregister(ctx, arg);
1185 		break;
1186 	case UFFDIO_WAKE:
1187 		ret = userfaultfd_wake(ctx, arg);
1188 		break;
1189 	case UFFDIO_COPY:
1190 		ret = userfaultfd_copy(ctx, arg);
1191 		break;
1192 	case UFFDIO_ZEROPAGE:
1193 		ret = userfaultfd_zeropage(ctx, arg);
1194 		break;
1195 	}
1196 	return ret;
1197 }
1198 
1199 #ifdef CONFIG_PROC_FS
1200 static void userfaultfd_show_fdinfo(struct seq_file *m, struct file *f)
1201 {
1202 	struct userfaultfd_ctx *ctx = f->private_data;
1203 	wait_queue_t *wq;
1204 	struct userfaultfd_wait_queue *uwq;
1205 	unsigned long pending = 0, total = 0;
1206 
1207 	spin_lock(&ctx->fault_pending_wqh.lock);
1208 	list_for_each_entry(wq, &ctx->fault_pending_wqh.task_list, task_list) {
1209 		uwq = container_of(wq, struct userfaultfd_wait_queue, wq);
1210 		pending++;
1211 		total++;
1212 	}
1213 	list_for_each_entry(wq, &ctx->fault_wqh.task_list, task_list) {
1214 		uwq = container_of(wq, struct userfaultfd_wait_queue, wq);
1215 		total++;
1216 	}
1217 	spin_unlock(&ctx->fault_pending_wqh.lock);
1218 
1219 	/*
1220 	 * If more protocols will be added, there will be all shown
1221 	 * separated by a space. Like this:
1222 	 *	protocols: aa:... bb:...
1223 	 */
1224 	seq_printf(m, "pending:\t%lu\ntotal:\t%lu\nAPI:\t%Lx:%x:%Lx\n",
1225 		   pending, total, UFFD_API, UFFD_API_FEATURES,
1226 		   UFFD_API_IOCTLS|UFFD_API_RANGE_IOCTLS);
1227 }
1228 #endif
1229 
1230 static const struct file_operations userfaultfd_fops = {
1231 #ifdef CONFIG_PROC_FS
1232 	.show_fdinfo	= userfaultfd_show_fdinfo,
1233 #endif
1234 	.release	= userfaultfd_release,
1235 	.poll		= userfaultfd_poll,
1236 	.read		= userfaultfd_read,
1237 	.unlocked_ioctl = userfaultfd_ioctl,
1238 	.compat_ioctl	= userfaultfd_ioctl,
1239 	.llseek		= noop_llseek,
1240 };
1241 
1242 static void init_once_userfaultfd_ctx(void *mem)
1243 {
1244 	struct userfaultfd_ctx *ctx = (struct userfaultfd_ctx *) mem;
1245 
1246 	init_waitqueue_head(&ctx->fault_pending_wqh);
1247 	init_waitqueue_head(&ctx->fault_wqh);
1248 	init_waitqueue_head(&ctx->fd_wqh);
1249 	seqcount_init(&ctx->refile_seq);
1250 }
1251 
1252 /**
1253  * userfaultfd_file_create - Creates an userfaultfd file pointer.
1254  * @flags: Flags for the userfaultfd file.
1255  *
1256  * This function creates an userfaultfd file pointer, w/out installing
1257  * it into the fd table. This is useful when the userfaultfd file is
1258  * used during the initialization of data structures that require
1259  * extra setup after the userfaultfd creation. So the userfaultfd
1260  * creation is split into the file pointer creation phase, and the
1261  * file descriptor installation phase.  In this way races with
1262  * userspace closing the newly installed file descriptor can be
1263  * avoided.  Returns an userfaultfd file pointer, or a proper error
1264  * pointer.
1265  */
1266 static struct file *userfaultfd_file_create(int flags)
1267 {
1268 	struct file *file;
1269 	struct userfaultfd_ctx *ctx;
1270 
1271 	BUG_ON(!current->mm);
1272 
1273 	/* Check the UFFD_* constants for consistency.  */
1274 	BUILD_BUG_ON(UFFD_CLOEXEC != O_CLOEXEC);
1275 	BUILD_BUG_ON(UFFD_NONBLOCK != O_NONBLOCK);
1276 
1277 	file = ERR_PTR(-EINVAL);
1278 	if (flags & ~UFFD_SHARED_FCNTL_FLAGS)
1279 		goto out;
1280 
1281 	file = ERR_PTR(-ENOMEM);
1282 	ctx = kmem_cache_alloc(userfaultfd_ctx_cachep, GFP_KERNEL);
1283 	if (!ctx)
1284 		goto out;
1285 
1286 	atomic_set(&ctx->refcount, 1);
1287 	ctx->flags = flags;
1288 	ctx->state = UFFD_STATE_WAIT_API;
1289 	ctx->released = false;
1290 	ctx->mm = current->mm;
1291 	/* prevent the mm struct to be freed */
1292 	atomic_inc(&ctx->mm->mm_users);
1293 
1294 	file = anon_inode_getfile("[userfaultfd]", &userfaultfd_fops, ctx,
1295 				  O_RDWR | (flags & UFFD_SHARED_FCNTL_FLAGS));
1296 	if (IS_ERR(file)) {
1297 		mmput(ctx->mm);
1298 		kmem_cache_free(userfaultfd_ctx_cachep, ctx);
1299 	}
1300 out:
1301 	return file;
1302 }
1303 
1304 SYSCALL_DEFINE1(userfaultfd, int, flags)
1305 {
1306 	int fd, error;
1307 	struct file *file;
1308 
1309 	error = get_unused_fd_flags(flags & UFFD_SHARED_FCNTL_FLAGS);
1310 	if (error < 0)
1311 		return error;
1312 	fd = error;
1313 
1314 	file = userfaultfd_file_create(flags);
1315 	if (IS_ERR(file)) {
1316 		error = PTR_ERR(file);
1317 		goto err_put_unused_fd;
1318 	}
1319 	fd_install(fd, file);
1320 
1321 	return fd;
1322 
1323 err_put_unused_fd:
1324 	put_unused_fd(fd);
1325 
1326 	return error;
1327 }
1328 
1329 static int __init userfaultfd_init(void)
1330 {
1331 	userfaultfd_ctx_cachep = kmem_cache_create("userfaultfd_ctx_cache",
1332 						sizeof(struct userfaultfd_ctx),
1333 						0,
1334 						SLAB_HWCACHE_ALIGN|SLAB_PANIC,
1335 						init_once_userfaultfd_ctx);
1336 	return 0;
1337 }
1338 __initcall(userfaultfd_init);
1339