1 files changed, 57 insertions, 59 deletions

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 5c2d806e6ae5..003f0d31743e 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -33,11 +33,6 @@ int sysctl_unprivileged_userfaultfd __read_mostly;
 
 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.
@@ -69,12 +64,10 @@ struct userfaultfd_ctx {
 	unsigned int flags;
 	/* features requested from the userspace */
 	unsigned int features;
-	/* state machine */
-	enum userfaultfd_state state;
 	/* released */
 	bool released;
 	/* memory mappings are changing because of non-cooperative event */
-	bool mmap_changing;
+	atomic_t mmap_changing;
 	/* mm with one ore more vmas attached to this userfaultfd_ctx */
 	struct mm_struct *mm;
 };
@@ -104,6 +97,14 @@ struct userfaultfd_wake_range {
 	unsigned long len;
 };
 
+/* internal indication that UFFD_API ioctl was successfully executed */
+#define UFFD_FEATURE_INITIALIZED		(1u << 31)
+
+static bool userfaultfd_is_initialized(struct userfaultfd_ctx *ctx)
+{
+	return ctx->features & UFFD_FEATURE_INITIALIZED;
+}
+
 static int userfaultfd_wake_function(wait_queue_entry_t *wq, unsigned mode,
 				     int wake_flags, void *key)
 {
@@ -623,7 +624,8 @@ static void userfaultfd_event_wait_completion(struct userfaultfd_ctx *ctx,
 	 * already released.
 	 */
 out:
-	WRITE_ONCE(ctx->mmap_changing, false);
+	atomic_dec(&ctx->mmap_changing);
+	VM_BUG_ON(atomic_read(&ctx->mmap_changing) < 0);
 	userfaultfd_ctx_put(ctx);
 }
 
@@ -666,15 +668,14 @@ int dup_userfaultfd(struct vm_area_struct *vma, struct list_head *fcs)
 
 		refcount_set(&ctx->refcount, 1);
 		ctx->flags = octx->flags;
-		ctx->state = UFFD_STATE_RUNNING;
 		ctx->features = octx->features;
 		ctx->released = false;
-		ctx->mmap_changing = false;
+		atomic_set(&ctx->mmap_changing, 0);
 		ctx->mm = vma->vm_mm;
 		mmgrab(ctx->mm);
 
 		userfaultfd_ctx_get(octx);
-		WRITE_ONCE(octx->mmap_changing, true);
+		atomic_inc(&octx->mmap_changing);
 		fctx->orig = octx;
 		fctx->new = ctx;
 		list_add_tail(&fctx->list, fcs);
@@ -721,7 +722,7 @@ void mremap_userfaultfd_prep(struct vm_area_struct *vma,
 	if (ctx->features & UFFD_FEATURE_EVENT_REMAP) {
 		vm_ctx->ctx = ctx;
 		userfaultfd_ctx_get(ctx);
-		WRITE_ONCE(ctx->mmap_changing, true);
+		atomic_inc(&ctx->mmap_changing);
 	} else {
 		/* Drop uffd context if remap feature not enabled */
 		vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
@@ -766,7 +767,7 @@ bool userfaultfd_remove(struct vm_area_struct *vma,
 		return true;
 
 	userfaultfd_ctx_get(ctx);
-	WRITE_ONCE(ctx->mmap_changing, true);
+	atomic_inc(&ctx->mmap_changing);
 	mmap_read_unlock(mm);
 
 	msg_init(&ewq.msg);
@@ -810,7 +811,7 @@ int userfaultfd_unmap_prep(struct vm_area_struct *vma,
 			return -ENOMEM;
 
 		userfaultfd_ctx_get(ctx);
-		WRITE_ONCE(ctx->mmap_changing, true);
+		atomic_inc(&ctx->mmap_changing);
 		unmap_ctx->ctx = ctx;
 		unmap_ctx->start = start;
 		unmap_ctx->end = end;
@@ -943,38 +944,33 @@ static __poll_t userfaultfd_poll(struct file *file, poll_table *wait)
 
 	poll_wait(file, &ctx->fd_wqh, wait);
 
-	switch (ctx->state) {
-	case UFFD_STATE_WAIT_API:
+	if (!userfaultfd_is_initialized(ctx))
 		return EPOLLERR;
-	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 EPOLLERR;
-		/*
-		 * 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 = EPOLLIN;
-		else if (waitqueue_active(&ctx->event_wqh))
-			ret = EPOLLIN;
 
-		return ret;
-	default:
-		WARN_ON_ONCE(1);
+	/*
+	 * poll() never guarantees that read won't block.
+	 * userfaults can be waken before they're read().
+	 */
+	if (unlikely(!(file->f_flags & O_NONBLOCK)))
 		return EPOLLERR;
-	}
+	/*
+	 * 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 = EPOLLIN;
+	else if (waitqueue_active(&ctx->event_wqh))
+		ret = EPOLLIN;
+
+	return ret;
 }
 
 static const struct file_operations userfaultfd_fops;
@@ -1169,7 +1165,7 @@ static ssize_t userfaultfd_read(struct file *file, char __user *buf,
 	int no_wait = file->f_flags & O_NONBLOCK;
 	struct inode *inode = file_inode(file);
 
-	if (ctx->state == UFFD_STATE_WAIT_API)
+	if (!userfaultfd_is_initialized(ctx))
 		return -EINVAL;
 
 	for (;;) {
@@ -1700,7 +1696,7 @@ static int userfaultfd_copy(struct userfaultfd_ctx *ctx,
 	user_uffdio_copy = (struct uffdio_copy __user *) arg;
 
 	ret = -EAGAIN;
-	if (READ_ONCE(ctx->mmap_changing))
+	if (atomic_read(&ctx->mmap_changing))
 		goto out;
 
 	ret = -EFAULT;
@@ -1757,7 +1753,7 @@ static int userfaultfd_zeropage(struct userfaultfd_ctx *ctx,
 	user_uffdio_zeropage = (struct uffdio_zeropage __user *) arg;
 
 	ret = -EAGAIN;
-	if (READ_ONCE(ctx->mmap_changing))
+	if (atomic_read(&ctx->mmap_changing))
 		goto out;
 
 	ret = -EFAULT;
@@ -1807,7 +1803,7 @@ static int userfaultfd_writeprotect(struct userfaultfd_ctx *ctx,
 	struct userfaultfd_wake_range range;
 	bool mode_wp, mode_dontwake;
 
-	if (READ_ONCE(ctx->mmap_changing))
+	if (atomic_read(&ctx->mmap_changing))
 		return -EAGAIN;
 
 	user_uffdio_wp = (struct uffdio_writeprotect __user *) arg;
@@ -1855,7 +1851,7 @@ static int userfaultfd_continue(struct userfaultfd_ctx *ctx, unsigned long arg)
 	user_uffdio_continue = (struct uffdio_continue __user *)arg;
 
 	ret = -EAGAIN;
-	if (READ_ONCE(ctx->mmap_changing))
+	if (atomic_read(&ctx->mmap_changing))
 		goto out;
 
 	ret = -EFAULT;
@@ -1908,9 +1904,10 @@ out:
 static inline unsigned int uffd_ctx_features(__u64 user_features)
 {
 	/*
-	 * For the current set of features the bits just coincide
+	 * For the current set of features the bits just coincide. Set
+	 * UFFD_FEATURE_INITIALIZED to mark the features as enabled.
 	 */
-	return (unsigned int)user_features;
+	return (unsigned int)user_features | UFFD_FEATURE_INITIALIZED;
 }
 
 /*
@@ -1923,12 +1920,10 @@ static int userfaultfd_api(struct userfaultfd_ctx *ctx,
 {
 	struct uffdio_api uffdio_api;
 	void __user *buf = (void __user *)arg;
+	unsigned int ctx_features;
 	int ret;
 	__u64 features;
 
-	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;
@@ -1952,9 +1947,13 @@ static int userfaultfd_api(struct userfaultfd_ctx *ctx,
 	ret = -EFAULT;
 	if (copy_to_user(buf, &uffdio_api, sizeof(uffdio_api)))
 		goto out;
-	ctx->state = UFFD_STATE_RUNNING;
+
 	/* only enable the requested features for this uffd context */
-	ctx->features = uffd_ctx_features(features);
+	ctx_features = uffd_ctx_features(features);
+	ret = -EINVAL;
+	if (cmpxchg(&ctx->features, 0, ctx_features) != 0)
+		goto err_out;
+
 	ret = 0;
 out:
 	return ret;
@@ -1971,7 +1970,7 @@ static long userfaultfd_ioctl(struct file *file, unsigned cmd,
 	int ret = -EINVAL;
 	struct userfaultfd_ctx *ctx = file->private_data;
 
-	if (cmd != UFFDIO_API && ctx->state == UFFD_STATE_WAIT_API)
+	if (cmd != UFFDIO_API && !userfaultfd_is_initialized(ctx))
 		return -EINVAL;
 
 	switch(cmd) {
@@ -2085,9 +2084,8 @@ SYSCALL_DEFINE1(userfaultfd, int, flags)
 	refcount_set(&ctx->refcount, 1);
 	ctx->flags = flags;
 	ctx->features = 0;
-	ctx->state = UFFD_STATE_WAIT_API;
 	ctx->released = false;
-	ctx->mmap_changing = false;
+	atomic_set(&ctx->mmap_changing, 0);
 	ctx->mm = current->mm;
 	/* prevent the mm struct to be freed */
 	mmgrab(ctx->mm);