kernfs: protect lazy kernfs_iattrs allocation with mutex

kernfs_iattrs is allocated lazily when operations which require it
take place; unfortunately, the lazy allocation and returning weren't
properly synchronized and when there are multiple concurrent
operations, it might end up returning kernfs_iattrs which hasn't
finished initialization yet or different copies to different callers.

Fix it by synchronizing with a mutex.  This can be smarter with memory
barriers but let's go there if it actually turns out to be necessary.

Signed-off-by: Tejun Heo <tj@kernel.org>
Link: http://lkml.kernel.org/g/533ABA32.9080602@oracle.com
Reported-by: Sasha Levin <sasha.levin@oracle.com>
Cc: stable@vger.kernel.org # 3.14
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/kernfs/inode.c

@@ -48,14 +48,18 @@ void __init kernfs_inode_init(void)
 
 static struct kernfs_iattrs *kernfs_iattrs(struct kernfs_node *kn)
 {
+	static DEFINE_MUTEX(iattr_mutex);
+	struct kernfs_iattrs *ret;
 	struct iattr *iattrs;
 
+	mutex_lock(&iattr_mutex);
+
 	if (kn->iattr)
-		return kn->iattr;
+		goto out_unlock;
 
 	kn->iattr = kzalloc(sizeof(struct kernfs_iattrs), GFP_KERNEL);
 	if (!kn->iattr)
-		return NULL;
+		goto out_unlock;
 	iattrs = &kn->iattr->ia_iattr;
 
 	/* assign default attributes */
@@ -65,8 +69,10 @@ static struct kernfs_iattrs *kernfs_iattrs(struct kernfs_node *kn)
 	iattrs->ia_atime = iattrs->ia_mtime = iattrs->ia_ctime = CURRENT_TIME;
 
 	simple_xattrs_init(&kn->iattr->xattrs);
-
-	return kn->iattr;
+out_unlock:
+	ret = kn->iattr;
+	mutex_unlock(&iattr_mutex);
+	return ret;
 }
 
 static int __kernfs_setattr(struct kernfs_node *kn, const struct iattr *iattr)