inotify: fix race

[Chris Wright]

-stable review patch.  If anyone has any objections, please let us know.
---------------------

From: Nick Piggin <npiggin@xxxxxxx>

upstream commit: d599e36a9ea85432587f4550acc113cd7549d12a

There is a race between setting an inode's children's "parent watched" flag
when placing the first watch on a parent, and instantiating new children of
that parent: a child could miss having its flags set by
set_dentry_child_flags, but then inotify_d_instantiate might still see
!inotify_inode_watched.

The solution is to set_dentry_child_flags after adding the watch.  Locking is
taken care of, because both set_dentry_child_flags and inotify_d_instantiate
hold dcache_lock and child->d_locks.

Signed-off-by: Nick Piggin <npiggin@xxxxxxx>
Cc: Robert Love <rlove@xxxxxxxxxx>
Cc: John McCutchan <ttb@xxxxxxxxxxxxxxxx>
Cc: Jan Kara <jack@xxxxxx>
Cc: Yan Zheng <yanzheng@xxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Christian Lamparter <chunkeey@xxxxxx>
Signed-off-by: Chris Wright <chrisw@xxxxxxxxxxxx>
---
 fs/inotify.c |   13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

--- a/fs/inotify.c
+++ b/fs/inotify.c
@@ -627,6 +627,7 @@ s32 inotify_add_watch(struct inotify_han
 		      struct inode *inode, u32 mask)
 {
 	int ret = 0;
+	int newly_watched;
 
 	/* don't allow invalid bits: we don't want flags set */
 	mask &= IN_ALL_EVENTS | IN_ONESHOT;
@@ -653,12 +654,18 @@ s32 inotify_add_watch(struct inotify_han
 	 */
 	watch->inode = igrab(inode);
 
-	if (!inotify_inode_watched(inode))
-		set_dentry_child_flags(inode, 1);
-
 	/* Add the watch to the handle's and the inode's list */
+	newly_watched = !inotify_inode_watched(inode);
 	list_add(&watch->h_list, &ih->watches);
 	list_add(&watch->i_list, &inode->inotify_watches);
+	/*
+	 * Set child flags _after_ adding the watch, so there is no race
+	 * windows where newly instantiated children could miss their parent's
+	 * watched flag.
+	 */
+	if (newly_watched)
+		set_dentry_child_flags(inode, 1);
+
 out:
 	mutex_unlock(&ih->mutex);
 	mutex_unlock(&inode->inotify_mutex);

-- 
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