Linux-libre 4.3.2-gnu

1 files changed, 1332 insertions, 0 deletions

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
new file mode 100644
index 000000000..503117031
--- /dev/null
+++ b/fs/userfaultfd.c
@@ -0,0 +1,1332 @@
+/*
+ *  fs/userfaultfd.c
+ *
+ *  Copyright (C) 2007  Davide Libenzi <davidel@xmailserver.org>
+ *  Copyright (C) 2008-2009 Red Hat, Inc.
+ *  Copyright (C) 2015  Red Hat, Inc.
+ *
+ *  This work is licensed under the terms of the GNU GPL, version 2. See
+ *  the COPYING file in the top-level directory.
+ *
+ *  Some part derived from fs/eventfd.c (anon inode setup) and
+ *  mm/ksm.c (mm hashing).
+ */
+
+#include <linux/hashtable.h>
+#include <linux/sched.h>
+#include <linux/mm.h>
+#include <linux/poll.h>
+#include <linux/slab.h>
+#include <linux/seq_file.h>
+#include <linux/file.h>
+#include <linux/bug.h>
+#include <linux/anon_inodes.h>
+#include <linux/syscalls.h>
+#include <linux/userfaultfd_k.h>
+#include <linux/mempolicy.h>
+#include <linux/ioctl.h>
+#include <linux/security.h>
+
+static struct kmem_cache *userfaultfd_ctx_cachep __read_mostly;
+
+enum userfaultfd_state {
+	UFFD_STATE_WAIT_API,
+	UFFD_STATE_RUNNING,
+};
+
+/*
+ * Start with fault_pending_wqh and fault_wqh so they're more likely
+ * to be in the same cacheline.
+ */
+struct userfaultfd_ctx {
+	/* waitqueue head for the pending (i.e. not read) userfaults */
+	wait_queue_head_t fault_pending_wqh;
+	/* waitqueue head for the userfaults */
+	wait_queue_head_t fault_wqh;
+	/* waitqueue head for the pseudo fd to wakeup poll/read */
+	wait_queue_head_t fd_wqh;
+	/* a refile sequence protected by fault_pending_wqh lock */
+	struct seqcount refile_seq;
+	/* pseudo fd refcounting */
+	atomic_t refcount;
+	/* userfaultfd syscall flags */
+	unsigned int flags;
+	/* state machine */
+	enum userfaultfd_state state;
+	/* released */
+	bool released;
+	/* mm with one ore more vmas attached to this userfaultfd_ctx */
+	struct mm_struct *mm;
+};
+
+struct userfaultfd_wait_queue {
+	struct uffd_msg msg;
+	wait_queue_t wq;
+	struct userfaultfd_ctx *ctx;
+};
+
+struct userfaultfd_wake_range {
+	unsigned long start;
+	unsigned long len;
+};
+
+static int userfaultfd_wake_function(wait_queue_t *wq, unsigned mode,
+				     int wake_flags, void *key)
+{
+	struct userfaultfd_wake_range *range = key;
+	int ret;
+	struct userfaultfd_wait_queue *uwq;
+	unsigned long start, len;
+
+	uwq = container_of(wq, struct userfaultfd_wait_queue, wq);
+	ret = 0;
+	/* len == 0 means wake all */
+	start = range->start;
+	len = range->len;
+	if (len && (start > uwq->msg.arg.pagefault.address ||
+		    start + len <= uwq->msg.arg.pagefault.address))
+		goto out;
+	ret = wake_up_state(wq->private, mode);
+	if (ret)
+		/*
+		 * Wake only once, autoremove behavior.
+		 *
+		 * After the effect of list_del_init is visible to the
+		 * other CPUs, the waitqueue may disappear from under
+		 * us, see the !list_empty_careful() in
+		 * handle_userfault(). try_to_wake_up() has an
+		 * implicit smp_mb__before_spinlock, and the
+		 * wq->private is read before calling the extern
+		 * function "wake_up_state" (which in turns calls
+		 * try_to_wake_up). While the spin_lock;spin_unlock;
+		 * wouldn't be enough, the smp_mb__before_spinlock is
+		 * enough to avoid an explicit smp_mb() here.
+		 */
+		list_del_init(&wq->task_list);
+out:
+	return ret;
+}
+
+/**
+ * userfaultfd_ctx_get - Acquires a reference to the internal userfaultfd
+ * context.
+ * @ctx: [in] Pointer to the userfaultfd context.
+ *
+ * Returns: In case of success, returns not zero.
+ */
+static void userfaultfd_ctx_get(struct userfaultfd_ctx *ctx)
+{
+	if (!atomic_inc_not_zero(&ctx->refcount))
+		BUG();
+}
+
+/**
+ * userfaultfd_ctx_put - Releases a reference to the internal userfaultfd
+ * context.
+ * @ctx: [in] Pointer to userfaultfd context.
+ *
+ * The userfaultfd context reference must have been previously acquired either
+ * with userfaultfd_ctx_get() or userfaultfd_ctx_fdget().
+ */
+static void userfaultfd_ctx_put(struct userfaultfd_ctx *ctx)
+{
+	if (atomic_dec_and_test(&ctx->refcount)) {
+		VM_BUG_ON(spin_is_locked(&ctx->fault_pending_wqh.lock));
+		VM_BUG_ON(waitqueue_active(&ctx->fault_pending_wqh));
+		VM_BUG_ON(spin_is_locked(&ctx->fault_wqh.lock));
+		VM_BUG_ON(waitqueue_active(&ctx->fault_wqh));
+		VM_BUG_ON(spin_is_locked(&ctx->fd_wqh.lock));
+		VM_BUG_ON(waitqueue_active(&ctx->fd_wqh));
+		mmput(ctx->mm);
+		kmem_cache_free(userfaultfd_ctx_cachep, ctx);
+	}
+}
+
+static inline void msg_init(struct uffd_msg *msg)
+{
+	BUILD_BUG_ON(sizeof(struct uffd_msg) != 32);
+	/*
+	 * Must use memset to zero out the paddings or kernel data is
+	 * leaked to userland.
+	 */
+	memset(msg, 0, sizeof(struct uffd_msg));
+}
+
+static inline struct uffd_msg userfault_msg(unsigned long address,
+					    unsigned int flags,
+					    unsigned long reason)
+{
+	struct uffd_msg msg;
+	msg_init(&msg);
+	msg.event = UFFD_EVENT_PAGEFAULT;
+	msg.arg.pagefault.address = address;
+	if (flags & FAULT_FLAG_WRITE)
+		/*
+		 * If UFFD_FEATURE_PAGEFAULT_FLAG_WRITE was set in the
+		 * uffdio_api.features and UFFD_PAGEFAULT_FLAG_WRITE
+		 * was not set in a UFFD_EVENT_PAGEFAULT, it means it
+		 * was a read fault, otherwise if set it means it's
+		 * a write fault.
+		 */
+		msg.arg.pagefault.flags |= UFFD_PAGEFAULT_FLAG_WRITE;
+	if (reason & VM_UFFD_WP)
+		/*
+		 * If UFFD_FEATURE_PAGEFAULT_FLAG_WP was set in the
+		 * uffdio_api.features and UFFD_PAGEFAULT_FLAG_WP was
+		 * not set in a UFFD_EVENT_PAGEFAULT, it means it was
+		 * a missing fault, otherwise if set it means it's a
+		 * write protect fault.
+		 */
+		msg.arg.pagefault.flags |= UFFD_PAGEFAULT_FLAG_WP;
+	return msg;
+}
+
+/*
+ * Verify the pagetables are still not ok after having reigstered into
+ * the fault_pending_wqh to avoid userland having to UFFDIO_WAKE any
+ * userfault that has already been resolved, if userfaultfd_read and
+ * UFFDIO_COPY|ZEROPAGE are being run simultaneously on two different
+ * threads.
+ */
+static inline bool userfaultfd_must_wait(struct userfaultfd_ctx *ctx,
+					 unsigned long address,
+					 unsigned long flags,
+					 unsigned long reason)
+{
+	struct mm_struct *mm = ctx->mm;
+	pgd_t *pgd;
+	pud_t *pud;
+	pmd_t *pmd, _pmd;
+	pte_t *pte;
+	bool ret = true;
+
+	VM_BUG_ON(!rwsem_is_locked(&mm->mmap_sem));
+
+	pgd = pgd_offset(mm, address);
+	if (!pgd_present(*pgd))
+		goto out;
+	pud = pud_offset(pgd, address);
+	if (!pud_present(*pud))
+		goto out;
+	pmd = pmd_offset(pud, address);
+	/*
+	 * READ_ONCE must function as a barrier with narrower scope
+	 * and it must be equivalent to:
+	 *	_pmd = *pmd; barrier();
+	 *
+	 * This is to deal with the instability (as in
+	 * pmd_trans_unstable) of the pmd.
+	 */
+	_pmd = READ_ONCE(*pmd);
+	if (!pmd_present(_pmd))
+		goto out;
+
+	ret = false;
+	if (pmd_trans_huge(_pmd))
+		goto out;
+
+	/*
+	 * the pmd is stable (as in !pmd_trans_unstable) so we can re-read it
+	 * and use the standard pte_offset_map() instead of parsing _pmd.
+	 */
+	pte = pte_offset_map(pmd, address);
+	/*
+	 * Lockless access: we're in a wait_event so it's ok if it
+	 * changes under us.
+	 */
+	if (pte_none(*pte))
+		ret = true;
+	pte_unmap(pte);
+
+out:
+	return ret;
+}
+
+/*
+ * The locking rules involved in returning VM_FAULT_RETRY depending on
+ * FAULT_FLAG_ALLOW_RETRY, FAULT_FLAG_RETRY_NOWAIT and
+ * FAULT_FLAG_KILLABLE are not straightforward. The "Caution"
+ * recommendation in __lock_page_or_retry is not an understatement.
+ *
+ * If FAULT_FLAG_ALLOW_RETRY is set, the mmap_sem must be released
+ * before returning VM_FAULT_RETRY only if FAULT_FLAG_RETRY_NOWAIT is
+ * not set.
+ *
+ * If FAULT_FLAG_ALLOW_RETRY is set but FAULT_FLAG_KILLABLE is not
+ * set, VM_FAULT_RETRY can still be returned if and only if there are
+ * fatal_signal_pending()s, and the mmap_sem must be released before
+ * returning it.
+ */
+int handle_userfault(struct vm_area_struct *vma, unsigned long address,
+		     unsigned int flags, unsigned long reason)
+{
+	struct mm_struct *mm = vma->vm_mm;
+	struct userfaultfd_ctx *ctx;
+	struct userfaultfd_wait_queue uwq;
+	int ret;
+	bool must_wait, return_to_userland;
+
+	BUG_ON(!rwsem_is_locked(&mm->mmap_sem));
+
+	ret = VM_FAULT_SIGBUS;
+	ctx = vma->vm_userfaultfd_ctx.ctx;
+	if (!ctx)
+		goto out;
+
+	BUG_ON(ctx->mm != mm);
+
+	VM_BUG_ON(reason & ~(VM_UFFD_MISSING|VM_UFFD_WP));
+	VM_BUG_ON(!(reason & VM_UFFD_MISSING) ^ !!(reason & VM_UFFD_WP));
+
+	/*
+	 * If it's already released don't get it. This avoids to loop
+	 * in __get_user_pages if userfaultfd_release waits on the
+	 * caller of handle_userfault to release the mmap_sem.
+	 */
+	if (unlikely(ACCESS_ONCE(ctx->released)))
+		goto out;
+
+	/*
+	 * Check that we can return VM_FAULT_RETRY.
+	 *
+	 * NOTE: it should become possible to return VM_FAULT_RETRY
+	 * even if FAULT_FLAG_TRIED is set without leading to gup()
+	 * -EBUSY failures, if the userfaultfd is to be extended for
+	 * VM_UFFD_WP tracking and we intend to arm the userfault
+	 * without first stopping userland access to the memory. For
+	 * VM_UFFD_MISSING userfaults this is enough for now.
+	 */
+	if (unlikely(!(flags & FAULT_FLAG_ALLOW_RETRY))) {
+		/*
+		 * Validate the invariant that nowait must allow retry
+		 * to be sure not to return SIGBUS erroneously on
+		 * nowait invocations.
+		 */
+		BUG_ON(flags & FAULT_FLAG_RETRY_NOWAIT);
+#ifdef CONFIG_DEBUG_VM
+		if (printk_ratelimit()) {
+			printk(KERN_WARNING
+			       "FAULT_FLAG_ALLOW_RETRY missing %x\n", flags);
+			dump_stack();
+		}
+#endif
+		goto out;
+	}
+
+	/*
+	 * Handle nowait, not much to do other than tell it to retry
+	 * and wait.
+	 */
+	ret = VM_FAULT_RETRY;
+	if (flags & FAULT_FLAG_RETRY_NOWAIT)
+		goto out;
+
+	/* take the reference before dropping the mmap_sem */
+	userfaultfd_ctx_get(ctx);
+
+	init_waitqueue_func_entry(&uwq.wq, userfaultfd_wake_function);
+	uwq.wq.private = current;
+	uwq.msg = userfault_msg(address, flags, reason);
+	uwq.ctx = ctx;
+
+	return_to_userland = (flags & (FAULT_FLAG_USER|FAULT_FLAG_KILLABLE)) ==
+		(FAULT_FLAG_USER|FAULT_FLAG_KILLABLE);
+
+	spin_lock(&ctx->fault_pending_wqh.lock);
+	/*
+	 * After the __add_wait_queue the uwq is visible to userland
+	 * through poll/read().
+	 */
+	__add_wait_queue(&ctx->fault_pending_wqh, &uwq.wq);
+	/*
+	 * The smp_mb() after __set_current_state prevents the reads
+	 * following the spin_unlock to happen before the list_add in
+	 * __add_wait_queue.
+	 */
+	set_current_state(return_to_userland ? TASK_INTERRUPTIBLE :
+			  TASK_KILLABLE);
+	spin_unlock(&ctx->fault_pending_wqh.lock);
+
+	must_wait = userfaultfd_must_wait(ctx, address, flags, reason);
+	up_read(&mm->mmap_sem);
+
+	if (likely(must_wait && !ACCESS_ONCE(ctx->released) &&
+		   (return_to_userland ? !signal_pending(current) :
+		    !fatal_signal_pending(current)))) {
+		wake_up_poll(&ctx->fd_wqh, POLLIN);
+		schedule();
+		ret |= VM_FAULT_MAJOR;
+	}
+
+	__set_current_state(TASK_RUNNING);
+
+	if (return_to_userland) {
+		if (signal_pending(current) &&
+		    !fatal_signal_pending(current)) {
+			/*
+			 * If we got a SIGSTOP or SIGCONT and this is
+			 * a normal userland page fault, just let
+			 * userland return so the signal will be
+			 * handled and gdb debugging works.  The page
+			 * fault code immediately after we return from
+			 * this function is going to release the
+			 * mmap_sem and it's not depending on it
+			 * (unlike gup would if we were not to return
+			 * VM_FAULT_RETRY).
+			 *
+			 * If a fatal signal is pending we still take
+			 * the streamlined VM_FAULT_RETRY failure path
+			 * and there's no need to retake the mmap_sem
+			 * in such case.
+			 */
+			down_read(&mm->mmap_sem);
+			ret = 0;
+		}
+	}
+
+	/*
+	 * Here we race with the list_del; list_add in
+	 * userfaultfd_ctx_read(), however because we don't ever run
+	 * list_del_init() to refile across the two lists, the prev
+	 * and next pointers will never point to self. list_add also
+	 * would never let any of the two pointers to point to
+	 * self. So list_empty_careful won't risk to see both pointers
+	 * pointing to self at any time during the list refile. The
+	 * only case where list_del_init() is called is the full
+	 * removal in the wake function and there we don't re-list_add
+	 * and it's fine not to block on the spinlock. The uwq on this
+	 * kernel stack can be released after the list_del_init.
+	 */
+	if (!list_empty_careful(&uwq.wq.task_list)) {
+		spin_lock(&ctx->fault_pending_wqh.lock);
+		/*
+		 * No need of list_del_init(), the uwq on the stack
+		 * will be freed shortly anyway.
+		 */
+		list_del(&uwq.wq.task_list);
+		spin_unlock(&ctx->fault_pending_wqh.lock);
+	}
+
+	/*
+	 * ctx may go away after this if the userfault pseudo fd is
+	 * already released.
+	 */
+	userfaultfd_ctx_put(ctx);
+
+out:
+	return ret;
+}
+
+static int userfaultfd_release(struct inode *inode, struct file *file)
+{
+	struct userfaultfd_ctx *ctx = file->private_data;
+	struct mm_struct *mm = ctx->mm;
+	struct vm_area_struct *vma, *prev;
+	/* len == 0 means wake all */
+	struct userfaultfd_wake_range range = { .len = 0, };
+	unsigned long new_flags;
+
+	ACCESS_ONCE(ctx->released) = true;
+
+	/*
+	 * Flush page faults out of all CPUs. NOTE: all page faults
+	 * must be retried without returning VM_FAULT_SIGBUS if
+	 * userfaultfd_ctx_get() succeeds but vma->vma_userfault_ctx
+	 * changes while handle_userfault released the mmap_sem. So
+	 * it's critical that released is set to true (above), before
+	 * taking the mmap_sem for writing.
+	 */
+	down_write(&mm->mmap_sem);
+	prev = NULL;
+	for (vma = mm->mmap; vma; vma = vma->vm_next) {
+		cond_resched();
+		BUG_ON(!!vma->vm_userfaultfd_ctx.ctx ^
+		       !!(vma->vm_flags & (VM_UFFD_MISSING | VM_UFFD_WP)));
+		if (vma->vm_userfaultfd_ctx.ctx != ctx) {
+			prev = vma;
+			continue;
+		}
+		new_flags = vma->vm_flags & ~(VM_UFFD_MISSING | VM_UFFD_WP);
+		prev = vma_merge(mm, prev, vma->vm_start, vma->vm_end,
+				 new_flags, vma->anon_vma,
+				 vma->vm_file, vma->vm_pgoff,
+				 vma_policy(vma),
+				 NULL_VM_UFFD_CTX);
+		if (prev)
+			vma = prev;
+		else
+			prev = vma;
+		vma->vm_flags = new_flags;
+		vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
+	}
+	up_write(&mm->mmap_sem);
+
+	/*
+	 * After no new page faults can wait on this fault_*wqh, flush
+	 * the last page faults that may have been already waiting on
+	 * the fault_*wqh.
+	 */
+	spin_lock(&ctx->fault_pending_wqh.lock);
+	__wake_up_locked_key(&ctx->fault_pending_wqh, TASK_NORMAL, &range);
+	__wake_up_locked_key(&ctx->fault_wqh, TASK_NORMAL, &range);
+	spin_unlock(&ctx->fault_pending_wqh.lock);
+
+	wake_up_poll(&ctx->fd_wqh, POLLHUP);
+	userfaultfd_ctx_put(ctx);
+	return 0;
+}
+
+/* fault_pending_wqh.lock must be hold by the caller */
+static inline struct userfaultfd_wait_queue *find_userfault(
+	struct userfaultfd_ctx *ctx)
+{
+	wait_queue_t *wq;
+	struct userfaultfd_wait_queue *uwq;
+
+	VM_BUG_ON(!spin_is_locked(&ctx->fault_pending_wqh.lock));
+
+	uwq = NULL;
+	if (!waitqueue_active(&ctx->fault_pending_wqh))
+		goto out;
+	/* walk in reverse to provide FIFO behavior to read userfaults */
+	wq = list_last_entry(&ctx->fault_pending_wqh.task_list,
+			     typeof(*wq), task_list);
+	uwq = container_of(wq, struct userfaultfd_wait_queue, wq);
+out:
+	return uwq;
+}
+
+static unsigned int userfaultfd_poll(struct file *file, poll_table *wait)
+{
+	struct userfaultfd_ctx *ctx = file->private_data;
+	unsigned int ret;
+
+	poll_wait(file, &ctx->fd_wqh, wait);
+
+	switch (ctx->state) {
+	case UFFD_STATE_WAIT_API:
+		return POLLERR;
+	case UFFD_STATE_RUNNING:
+		/*
+		 * poll() never guarantees that read won't block.
+		 * userfaults can be waken before they're read().
+		 */
+		if (unlikely(!(file->f_flags & O_NONBLOCK)))
+			return POLLERR;
+		/*
+		 * lockless access to see if there are pending faults
+		 * __pollwait last action is the add_wait_queue but
+		 * the spin_unlock would allow the waitqueue_active to
+		 * pass above the actual list_add inside
+		 * add_wait_queue critical section. So use a full
+		 * memory barrier to serialize the list_add write of
+		 * add_wait_queue() with the waitqueue_active read
+		 * below.
+		 */
+		ret = 0;
+		smp_mb();
+		if (waitqueue_active(&ctx->fault_pending_wqh))
+			ret = POLLIN;
+		return ret;
+	default:
+		BUG();
+	}
+}
+
+static ssize_t userfaultfd_ctx_read(struct userfaultfd_ctx *ctx, int no_wait,
+				    struct uffd_msg *msg)
+{
+	ssize_t ret;
+	DECLARE_WAITQUEUE(wait, current);
+	struct userfaultfd_wait_queue *uwq;
+
+	/* always take the fd_wqh lock before the fault_pending_wqh lock */
+	spin_lock(&ctx->fd_wqh.lock);
+	__add_wait_queue(&ctx->fd_wqh, &wait);
+	for (;;) {
+		set_current_state(TASK_INTERRUPTIBLE);
+		spin_lock(&ctx->fault_pending_wqh.lock);
+		uwq = find_userfault(ctx);
+		if (uwq) {
+			/*
+			 * Use a seqcount to repeat the lockless check
+			 * in wake_userfault() to avoid missing
+			 * wakeups because during the refile both
+			 * waitqueue could become empty if this is the
+			 * only userfault.
+			 */
+			write_seqcount_begin(&ctx->refile_seq);
+
+			/*
+			 * The fault_pending_wqh.lock prevents the uwq
+			 * to disappear from under us.
+			 *
+			 * Refile this userfault from
+			 * fault_pending_wqh to fault_wqh, it's not
+			 * pending anymore after we read it.
+			 *
+			 * Use list_del() by hand (as
+			 * userfaultfd_wake_function also uses
+			 * list_del_init() by hand) to be sure nobody
+			 * changes __remove_wait_queue() to use
+			 * list_del_init() in turn breaking the
+			 * !list_empty_careful() check in
+			 * handle_userfault(). The uwq->wq.task_list
+			 * must never be empty at any time during the
+			 * refile, or the waitqueue could disappear
+			 * from under us. The "wait_queue_head_t"
+			 * parameter of __remove_wait_queue() is unused
+			 * anyway.
+			 */
+			list_del(&uwq->wq.task_list);
+			__add_wait_queue(&ctx->fault_wqh, &uwq->wq);
+
+			write_seqcount_end(&ctx->refile_seq);
+
+			/* careful to always initialize msg if ret == 0 */
+			*msg = uwq->msg;
+			spin_unlock(&ctx->fault_pending_wqh.lock);
+			ret = 0;
+			break;
+		}
+		spin_unlock(&ctx->fault_pending_wqh.lock);
+		if (signal_pending(current)) {
+			ret = -ERESTARTSYS;
+			break;
+		}
+		if (no_wait) {
+			ret = -EAGAIN;
+			break;
+		}
+		spin_unlock(&ctx->fd_wqh.lock);
+		schedule();
+		spin_lock(&ctx->fd_wqh.lock);
+	}
+	__remove_wait_queue(&ctx->fd_wqh, &wait);
+	__set_current_state(TASK_RUNNING);
+	spin_unlock(&ctx->fd_wqh.lock);
+
+	return ret;
+}
+
+static ssize_t userfaultfd_read(struct file *file, char __user *buf,
+				size_t count, loff_t *ppos)
+{
+	struct userfaultfd_ctx *ctx = file->private_data;
+	ssize_t _ret, ret = 0;
+	struct uffd_msg msg;
+	int no_wait = file->f_flags & O_NONBLOCK;
+
+	if (ctx->state == UFFD_STATE_WAIT_API)
+		return -EINVAL;
+
+	for (;;) {
+		if (count < sizeof(msg))
+			return ret ? ret : -EINVAL;
+		_ret = userfaultfd_ctx_read(ctx, no_wait, &msg);
+		if (_ret < 0)
+			return ret ? ret : _ret;
+		if (copy_to_user((__u64 __user *) buf, &msg, sizeof(msg)))
+			return ret ? ret : -EFAULT;
+		ret += sizeof(msg);
+		buf += sizeof(msg);
+		count -= sizeof(msg);
+		/*
+		 * Allow to read more than one fault at time but only
+		 * block if waiting for the very first one.
+		 */
+		no_wait = O_NONBLOCK;
+	}
+}
+
+static void __wake_userfault(struct userfaultfd_ctx *ctx,
+			     struct userfaultfd_wake_range *range)
+{
+	unsigned long start, end;
+
+	start = range->start;
+	end = range->start + range->len;
+
+	spin_lock(&ctx->fault_pending_wqh.lock);
+	/* wake all in the range and autoremove */
+	if (waitqueue_active(&ctx->fault_pending_wqh))
+		__wake_up_locked_key(&ctx->fault_pending_wqh, TASK_NORMAL,
+				     range);
+	if (waitqueue_active(&ctx->fault_wqh))
+		__wake_up_locked_key(&ctx->fault_wqh, TASK_NORMAL, range);
+	spin_unlock(&ctx->fault_pending_wqh.lock);
+}
+
+static __always_inline void wake_userfault(struct userfaultfd_ctx *ctx,
+					   struct userfaultfd_wake_range *range)
+{
+	unsigned seq;
+	bool need_wakeup;
+
+	/*
+	 * To be sure waitqueue_active() is not reordered by the CPU
+	 * before the pagetable update, use an explicit SMP memory
+	 * barrier here. PT lock release or up_read(mmap_sem) still
+	 * have release semantics that can allow the
+	 * waitqueue_active() to be reordered before the pte update.
+	 */
+	smp_mb();
+
+	/*
+	 * Use waitqueue_active because it's very frequent to
+	 * change the address space atomically even if there are no
+	 * userfaults yet. So we take the spinlock only when we're
+	 * sure we've userfaults to wake.
+	 */
+	do {
+		seq = read_seqcount_begin(&ctx->refile_seq);
+		need_wakeup = waitqueue_active(&ctx->fault_pending_wqh) ||
+			waitqueue_active(&ctx->fault_wqh);
+		cond_resched();
+	} while (read_seqcount_retry(&ctx->refile_seq, seq));
+	if (need_wakeup)
+		__wake_userfault(ctx, range);
+}
+
+static __always_inline int validate_range(struct mm_struct *mm,
+					  __u64 start, __u64 len)
+{
+	__u64 task_size = mm->task_size;
+
+	if (start & ~PAGE_MASK)
+		return -EINVAL;
+	if (len & ~PAGE_MASK)
+		return -EINVAL;
+	if (!len)
+		return -EINVAL;
+	if (start < mmap_min_addr)
+		return -EINVAL;
+	if (start >= task_size)
+		return -EINVAL;
+	if (len > task_size - start)
+		return -EINVAL;
+	return 0;
+}
+
+static int userfaultfd_register(struct userfaultfd_ctx *ctx,
+				unsigned long arg)
+{
+	struct mm_struct *mm = ctx->mm;
+	struct vm_area_struct *vma, *prev, *cur;
+	int ret;
+	struct uffdio_register uffdio_register;
+	struct uffdio_register __user *user_uffdio_register;
+	unsigned long vm_flags, new_flags;
+	bool found;
+	unsigned long start, end, vma_end;
+
+	user_uffdio_register = (struct uffdio_register __user *) arg;
+
+	ret = -EFAULT;
+	if (copy_from_user(&uffdio_register, user_uffdio_register,
+			   sizeof(uffdio_register)-sizeof(__u64)))
+		goto out;
+
+	ret = -EINVAL;
+	if (!uffdio_register.mode)
+		goto out;
+	if (uffdio_register.mode & ~(UFFDIO_REGISTER_MODE_MISSING|
+				     UFFDIO_REGISTER_MODE_WP))
+		goto out;
+	vm_flags = 0;
+	if (uffdio_register.mode & UFFDIO_REGISTER_MODE_MISSING)
+		vm_flags |= VM_UFFD_MISSING;
+	if (uffdio_register.mode & UFFDIO_REGISTER_MODE_WP) {
+		vm_flags |= VM_UFFD_WP;
+		/*
+		 * FIXME: remove the below error constraint by
+		 * implementing the wprotect tracking mode.
+		 */
+		ret = -EINVAL;
+		goto out;
+	}
+
+	ret = validate_range(mm, uffdio_register.range.start,
+			     uffdio_register.range.len);
+	if (ret)
+		goto out;
+
+	start = uffdio_register.range.start;
+	end = start + uffdio_register.range.len;
+
+	down_write(&mm->mmap_sem);
+	vma = find_vma_prev(mm, start, &prev);
+
+	ret = -ENOMEM;
+	if (!vma)
+		goto out_unlock;
+
+	/* check that there's at least one vma in the range */
+	ret = -EINVAL;
+	if (vma->vm_start >= end)
+		goto out_unlock;
+
+	/*
+	 * Search for not compatible vmas.
+	 *
+	 * FIXME: this shall be relaxed later so that it doesn't fail
+	 * on tmpfs backed vmas (in addition to the current allowance
+	 * on anonymous vmas).
+	 */
+	found = false;
+	for (cur = vma; cur && cur->vm_start < end; cur = cur->vm_next) {
+		cond_resched();
+
+		BUG_ON(!!cur->vm_userfaultfd_ctx.ctx ^
+		       !!(cur->vm_flags & (VM_UFFD_MISSING | VM_UFFD_WP)));
+
+		/* check not compatible vmas */
+		ret = -EINVAL;
+		if (cur->vm_ops)
+			goto out_unlock;
+
+		/*
+		 * Check that this vma isn't already owned by a
+		 * different userfaultfd. We can't allow more than one
+		 * userfaultfd to own a single vma simultaneously or we
+		 * wouldn't know which one to deliver the userfaults to.
+		 */
+		ret = -EBUSY;
+		if (cur->vm_userfaultfd_ctx.ctx &&
+		    cur->vm_userfaultfd_ctx.ctx != ctx)
+			goto out_unlock;
+
+		found = true;
+	}
+	BUG_ON(!found);
+
+	if (vma->vm_start < start)
+		prev = vma;
+
+	ret = 0;
+	do {
+		cond_resched();
+
+		BUG_ON(vma->vm_ops);
+		BUG_ON(vma->vm_userfaultfd_ctx.ctx &&
+		       vma->vm_userfaultfd_ctx.ctx != ctx);
+
+		/*
+		 * Nothing to do: this vma is already registered into this
+		 * userfaultfd and with the right tracking mode too.
+		 */
+		if (vma->vm_userfaultfd_ctx.ctx == ctx &&
+		    (vma->vm_flags & vm_flags) == vm_flags)
+			goto skip;
+
+		if (vma->vm_start > start)
+			start = vma->vm_start;
+		vma_end = min(end, vma->vm_end);
+
+		new_flags = (vma->vm_flags & ~vm_flags) | vm_flags;
+		prev = vma_merge(mm, prev, start, vma_end, new_flags,
+				 vma->anon_vma, vma->vm_file, vma->vm_pgoff,
+				 vma_policy(vma),
+				 ((struct vm_userfaultfd_ctx){ ctx }));
+		if (prev) {
+			vma = prev;
+			goto next;
+		}
+		if (vma->vm_start < start) {
+			ret = split_vma(mm, vma, start, 1);
+			if (ret)
+				break;
+		}
+		if (vma->vm_end > end) {
+			ret = split_vma(mm, vma, end, 0);
+			if (ret)
+				break;
+		}
+	next:
+		/*
+		 * In the vma_merge() successful mprotect-like case 8:
+		 * the next vma was merged into the current one and
+		 * the current one has not been updated yet.
+		 */
+		vma->vm_flags = new_flags;
+		vma->vm_userfaultfd_ctx.ctx = ctx;
+
+	skip:
+		prev = vma;
+		start = vma->vm_end;
+		vma = vma->vm_next;
+	} while (vma && vma->vm_start < end);
+out_unlock:
+	up_write(&mm->mmap_sem);
+	if (!ret) {
+		/*
+		 * Now that we scanned all vmas we can already tell
+		 * userland which ioctls methods are guaranteed to
+		 * succeed on this range.
+		 */
+		if (put_user(UFFD_API_RANGE_IOCTLS,
+			     &user_uffdio_register->ioctls))
+			ret = -EFAULT;
+	}
+out:
+	return ret;
+}
+
+static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
+				  unsigned long arg)
+{
+	struct mm_struct *mm = ctx->mm;
+	struct vm_area_struct *vma, *prev, *cur;
+	int ret;
+	struct uffdio_range uffdio_unregister;
+	unsigned long new_flags;
+	bool found;
+	unsigned long start, end, vma_end;
+	const void __user *buf = (void __user *)arg;
+
+	ret = -EFAULT;
+	if (copy_from_user(&uffdio_unregister, buf, sizeof(uffdio_unregister)))
+		goto out;
+
+	ret = validate_range(mm, uffdio_unregister.start,
+			     uffdio_unregister.len);
+	if (ret)
+		goto out;
+
+	start = uffdio_unregister.start;
+	end = start + uffdio_unregister.len;
+
+	down_write(&mm->mmap_sem);
+	vma = find_vma_prev(mm, start, &prev);
+
+	ret = -ENOMEM;
+	if (!vma)
+		goto out_unlock;
+
+	/* check that there's at least one vma in the range */
+	ret = -EINVAL;
+	if (vma->vm_start >= end)
+		goto out_unlock;
+
+	/*
+	 * Search for not compatible vmas.
+	 *
+	 * FIXME: this shall be relaxed later so that it doesn't fail
+	 * on tmpfs backed vmas (in addition to the current allowance
+	 * on anonymous vmas).
+	 */
+	found = false;
+	ret = -EINVAL;
+	for (cur = vma; cur && cur->vm_start < end; cur = cur->vm_next) {
+		cond_resched();
+
+		BUG_ON(!!cur->vm_userfaultfd_ctx.ctx ^
+		       !!(cur->vm_flags & (VM_UFFD_MISSING | VM_UFFD_WP)));
+
+		/*
+		 * Check not compatible vmas, not strictly required
+		 * here as not compatible vmas cannot have an
+		 * userfaultfd_ctx registered on them, but this
+		 * provides for more strict behavior to notice
+		 * unregistration errors.
+		 */
+		if (cur->vm_ops)
+			goto out_unlock;
+
+		found = true;
+	}
+	BUG_ON(!found);
+
+	if (vma->vm_start < start)
+		prev = vma;
+
+	ret = 0;
+	do {
+		cond_resched();
+
+		BUG_ON(vma->vm_ops);
+
+		/*
+		 * Nothing to do: this vma is already registered into this
+		 * userfaultfd and with the right tracking mode too.
+		 */
+		if (!vma->vm_userfaultfd_ctx.ctx)
+			goto skip;
+
+		if (vma->vm_start > start)
+			start = vma->vm_start;
+		vma_end = min(end, vma->vm_end);
+
+		new_flags = vma->vm_flags & ~(VM_UFFD_MISSING | VM_UFFD_WP);
+		prev = vma_merge(mm, prev, start, vma_end, new_flags,
+				 vma->anon_vma, vma->vm_file, vma->vm_pgoff,
+				 vma_policy(vma),
+				 NULL_VM_UFFD_CTX);
+		if (prev) {
+			vma = prev;
+			goto next;
+		}
+		if (vma->vm_start < start) {
+			ret = split_vma(mm, vma, start, 1);
+			if (ret)
+				break;
+		}
+		if (vma->vm_end > end) {
+			ret = split_vma(mm, vma, end, 0);
+			if (ret)
+				break;
+		}
+	next:
+		/*
+		 * In the vma_merge() successful mprotect-like case 8:
+		 * the next vma was merged into the current one and
+		 * the current one has not been updated yet.
+		 */
+		vma->vm_flags = new_flags;
+		vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
+
+	skip:
+		prev = vma;
+		start = vma->vm_end;
+		vma = vma->vm_next;
+	} while (vma && vma->vm_start < end);
+out_unlock:
+	up_write(&mm->mmap_sem);
+out:
+	return ret;
+}
+
+/*
+ * userfaultfd_wake may be used in combination with the
+ * UFFDIO_*_MODE_DONTWAKE to wakeup userfaults in batches.
+ */
+static int userfaultfd_wake(struct userfaultfd_ctx *ctx,
+			    unsigned long arg)
+{
+	int ret;
+	struct uffdio_range uffdio_wake;
+	struct userfaultfd_wake_range range;
+	const void __user *buf = (void __user *)arg;
+
+	ret = -EFAULT;
+	if (copy_from_user(&uffdio_wake, buf, sizeof(uffdio_wake)))
+		goto out;
+
+	ret = validate_range(ctx->mm, uffdio_wake.start, uffdio_wake.len);
+	if (ret)
+		goto out;
+
+	range.start = uffdio_wake.start;
+	range.len = uffdio_wake.len;
+
+	/*
+	 * len == 0 means wake all and we don't want to wake all here,
+	 * so check it again to be sure.
+	 */
+	VM_BUG_ON(!range.len);
+
+	wake_userfault(ctx, &range);
+	ret = 0;
+
+out:
+	return ret;
+}
+
+static int userfaultfd_copy(struct userfaultfd_ctx *ctx,
+			    unsigned long arg)
+{
+	__s64 ret;
+	struct uffdio_copy uffdio_copy;
+	struct uffdio_copy __user *user_uffdio_copy;
+	struct userfaultfd_wake_range range;
+
+	user_uffdio_copy = (struct uffdio_copy __user *) arg;
+
+	ret = -EFAULT;
+	if (copy_from_user(&uffdio_copy, user_uffdio_copy,
+			   /* don't copy "copy" last field */
+			   sizeof(uffdio_copy)-sizeof(__s64)))
+		goto out;
+
+	ret = validate_range(ctx->mm, uffdio_copy.dst, uffdio_copy.len);
+	if (ret)
+		goto out;
+	/*
+	 * double check for wraparound just in case. copy_from_user()
+	 * will later check uffdio_copy.src + uffdio_copy.len to fit
+	 * in the userland range.
+	 */
+	ret = -EINVAL;
+	if (uffdio_copy.src + uffdio_copy.len <= uffdio_copy.src)
+		goto out;
+	if (uffdio_copy.mode & ~UFFDIO_COPY_MODE_DONTWAKE)
+		goto out;
+
+	ret = mcopy_atomic(ctx->mm, uffdio_copy.dst, uffdio_copy.src,
+			   uffdio_copy.len);
+	if (unlikely(put_user(ret, &user_uffdio_copy->copy)))
+		return -EFAULT;
+	if (ret < 0)
+		goto out;
+	BUG_ON(!ret);
+	/* len == 0 would wake all */
+	range.len = ret;
+	if (!(uffdio_copy.mode & UFFDIO_COPY_MODE_DONTWAKE)) {
+		range.start = uffdio_copy.dst;
+		wake_userfault(ctx, &range);
+	}
+	ret = range.len == uffdio_copy.len ? 0 : -EAGAIN;
+out:
+	return ret;
+}
+
+static int userfaultfd_zeropage(struct userfaultfd_ctx *ctx,
+				unsigned long arg)
+{
+	__s64 ret;
+	struct uffdio_zeropage uffdio_zeropage;
+	struct uffdio_zeropage __user *user_uffdio_zeropage;
+	struct userfaultfd_wake_range range;
+
+	user_uffdio_zeropage = (struct uffdio_zeropage __user *) arg;
+
+	ret = -EFAULT;
+	if (copy_from_user(&uffdio_zeropage, user_uffdio_zeropage,
+			   /* don't copy "zeropage" last field */
+			   sizeof(uffdio_zeropage)-sizeof(__s64)))
+		goto out;
+
+	ret = validate_range(ctx->mm, uffdio_zeropage.range.start,
+			     uffdio_zeropage.range.len);
+	if (ret)
+		goto out;
+	ret = -EINVAL;
+	if (uffdio_zeropage.mode & ~UFFDIO_ZEROPAGE_MODE_DONTWAKE)
+		goto out;
+
+	ret = mfill_zeropage(ctx->mm, uffdio_zeropage.range.start,
+			     uffdio_zeropage.range.len);
+	if (unlikely(put_user(ret, &user_uffdio_zeropage->zeropage)))
+		return -EFAULT;
+	if (ret < 0)
+		goto out;
+	/* len == 0 would wake all */
+	BUG_ON(!ret);
+	range.len = ret;
+	if (!(uffdio_zeropage.mode & UFFDIO_ZEROPAGE_MODE_DONTWAKE)) {
+		range.start = uffdio_zeropage.range.start;
+		wake_userfault(ctx, &range);
+	}
+	ret = range.len == uffdio_zeropage.range.len ? 0 : -EAGAIN;
+out:
+	return ret;
+}
+
+/*
+ * userland asks for a certain API version and we return which bits
+ * and ioctl commands are implemented in this kernel for such API
+ * version or -EINVAL if unknown.
+ */
+static int userfaultfd_api(struct userfaultfd_ctx *ctx,
+			   unsigned long arg)
+{
+	struct uffdio_api uffdio_api;
+	void __user *buf = (void __user *)arg;
+	int ret;
+
+	ret = -EINVAL;
+	if (ctx->state != UFFD_STATE_WAIT_API)
+		goto out;
+	ret = -EFAULT;
+	if (copy_from_user(&uffdio_api, buf, sizeof(uffdio_api)))
+		goto out;
+	if (uffdio_api.api != UFFD_API || uffdio_api.features) {
+		memset(&uffdio_api, 0, sizeof(uffdio_api));
+		if (copy_to_user(buf, &uffdio_api, sizeof(uffdio_api)))
+			goto out;
+		ret = -EINVAL;
+		goto out;
+	}
+	uffdio_api.features = UFFD_API_FEATURES;
+	uffdio_api.ioctls = UFFD_API_IOCTLS;
+	ret = -EFAULT;
+	if (copy_to_user(buf, &uffdio_api, sizeof(uffdio_api)))
+		goto out;
+	ctx->state = UFFD_STATE_RUNNING;
+	ret = 0;
+out:
+	return ret;
+}
+
+static long userfaultfd_ioctl(struct file *file, unsigned cmd,
+			      unsigned long arg)
+{
+	int ret = -EINVAL;
+	struct userfaultfd_ctx *ctx = file->private_data;
+
+	if (cmd != UFFDIO_API && ctx->state == UFFD_STATE_WAIT_API)
+		return -EINVAL;
+
+	switch(cmd) {
+	case UFFDIO_API:
+		ret = userfaultfd_api(ctx, arg);
+		break;
+	case UFFDIO_REGISTER:
+		ret = userfaultfd_register(ctx, arg);
+		break;
+	case UFFDIO_UNREGISTER:
+		ret = userfaultfd_unregister(ctx, arg);
+		break;
+	case UFFDIO_WAKE:
+		ret = userfaultfd_wake(ctx, arg);
+		break;
+	case UFFDIO_COPY:
+		ret = userfaultfd_copy(ctx, arg);
+		break;
+	case UFFDIO_ZEROPAGE:
+		ret = userfaultfd_zeropage(ctx, arg);
+		break;
+	}
+	return ret;
+}
+
+#ifdef CONFIG_PROC_FS
+static void userfaultfd_show_fdinfo(struct seq_file *m, struct file *f)
+{
+	struct userfaultfd_ctx *ctx = f->private_data;
+	wait_queue_t *wq;
+	struct userfaultfd_wait_queue *uwq;
+	unsigned long pending = 0, total = 0;
+
+	spin_lock(&ctx->fault_pending_wqh.lock);
+	list_for_each_entry(wq, &ctx->fault_pending_wqh.task_list, task_list) {
+		uwq = container_of(wq, struct userfaultfd_wait_queue, wq);
+		pending++;
+		total++;
+	}
+	list_for_each_entry(wq, &ctx->fault_wqh.task_list, task_list) {
+		uwq = container_of(wq, struct userfaultfd_wait_queue, wq);
+		total++;
+	}
+	spin_unlock(&ctx->fault_pending_wqh.lock);
+
+	/*
+	 * If more protocols will be added, there will be all shown
+	 * separated by a space. Like this:
+	 *	protocols: aa:... bb:...
+	 */
+	seq_printf(m, "pending:\t%lu\ntotal:\t%lu\nAPI:\t%Lx:%x:%Lx\n",
+		   pending, total, UFFD_API, UFFD_API_FEATURES,
+		   UFFD_API_IOCTLS|UFFD_API_RANGE_IOCTLS);
+}
+#endif
+
+static const struct file_operations userfaultfd_fops = {
+#ifdef CONFIG_PROC_FS
+	.show_fdinfo	= userfaultfd_show_fdinfo,
+#endif
+	.release	= userfaultfd_release,
+	.poll		= userfaultfd_poll,
+	.read		= userfaultfd_read,
+	.unlocked_ioctl = userfaultfd_ioctl,
+	.compat_ioctl	= userfaultfd_ioctl,
+	.llseek		= noop_llseek,
+};
+
+static void init_once_userfaultfd_ctx(void *mem)
+{
+	struct userfaultfd_ctx *ctx = (struct userfaultfd_ctx *) mem;
+
+	init_waitqueue_head(&ctx->fault_pending_wqh);
+	init_waitqueue_head(&ctx->fault_wqh);
+	init_waitqueue_head(&ctx->fd_wqh);
+	seqcount_init(&ctx->refile_seq);
+}
+
+/**
+ * userfaultfd_file_create - Creates an userfaultfd file pointer.
+ * @flags: Flags for the userfaultfd file.
+ *
+ * This function creates an userfaultfd file pointer, w/out installing
+ * it into the fd table. This is useful when the userfaultfd file is
+ * used during the initialization of data structures that require
+ * extra setup after the userfaultfd creation. So the userfaultfd
+ * creation is split into the file pointer creation phase, and the
+ * file descriptor installation phase.  In this way races with
+ * userspace closing the newly installed file descriptor can be
+ * avoided.  Returns an userfaultfd file pointer, or a proper error
+ * pointer.
+ */
+static struct file *userfaultfd_file_create(int flags)
+{
+	struct file *file;
+	struct userfaultfd_ctx *ctx;
+
+	BUG_ON(!current->mm);
+
+	/* Check the UFFD_* constants for consistency.  */
+	BUILD_BUG_ON(UFFD_CLOEXEC != O_CLOEXEC);
+	BUILD_BUG_ON(UFFD_NONBLOCK != O_NONBLOCK);
+
+	file = ERR_PTR(-EINVAL);
+	if (flags & ~UFFD_SHARED_FCNTL_FLAGS)
+		goto out;
+
+	file = ERR_PTR(-ENOMEM);
+	ctx = kmem_cache_alloc(userfaultfd_ctx_cachep, GFP_KERNEL);
+	if (!ctx)
+		goto out;
+
+	atomic_set(&ctx->refcount, 1);
+	ctx->flags = flags;
+	ctx->state = UFFD_STATE_WAIT_API;
+	ctx->released = false;
+	ctx->mm = current->mm;
+	/* prevent the mm struct to be freed */
+	atomic_inc(&ctx->mm->mm_users);
+
+	file = anon_inode_getfile("[userfaultfd]", &userfaultfd_fops, ctx,
+				  O_RDWR | (flags & UFFD_SHARED_FCNTL_FLAGS));
+	if (IS_ERR(file)) {
+		mmput(ctx->mm);
+		kmem_cache_free(userfaultfd_ctx_cachep, ctx);
+	}
+out:
+	return file;
+}
+
+SYSCALL_DEFINE1(userfaultfd, int, flags)
+{
+	int fd, error;
+	struct file *file;
+
+	error = get_unused_fd_flags(flags & UFFD_SHARED_FCNTL_FLAGS);
+	if (error < 0)
+		return error;
+	fd = error;
+
+	file = userfaultfd_file_create(flags);
+	if (IS_ERR(file)) {
+		error = PTR_ERR(file);
+		goto err_put_unused_fd;
+	}
+	fd_install(fd, file);
+
+	return fd;
+
+err_put_unused_fd:
+	put_unused_fd(fd);
+
+	return error;
+}
+
+static int __init userfaultfd_init(void)
+{
+	userfaultfd_ctx_cachep = kmem_cache_create("userfaultfd_ctx_cache",
+						sizeof(struct userfaultfd_ctx),
+						0,
+						SLAB_HWCACHE_ALIGN|SLAB_PANIC,
+						init_once_userfaultfd_ctx);
+	return 0;
+}
+__initcall(userfaultfd_init);