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>

1 files changed, 10 insertions, 4 deletions

diff --git a/fs/kernfs/inode.c b/fs/kernfs/inode.c
index abb0f1f53d93..985217626e66 100644
--- a/fs/kernfs/inode.c
+++ b/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)