vfs: fix deadlock in file_remove_privs() on overlayfs

commit c1892c3 upstream.

file_remove_privs() is called with inode lock on file_inode(), which
proceeds to calling notify_change() on file->f_path.dentry.  Which triggers
the WARN_ON_ONCE(!inode_is_locked(inode)) in addition to deadlocking later
when ovl_setattr tries to lock the underlying inode again.

Fix this mess by not mixing the layers, but doing everything on underlying
dentry/inode.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Fixes: 07a2daa ("ovl: Copy up underlying inode's ->i_mode to overlay inode")
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/inode.c

@@ -1733,16 +1733,16 @@ static int __remove_privs(struct dentry *dentry, int kill)
  */
 int file_remove_privs(struct file *file)
 {
-	struct dentry *dentry = file->f_path.dentry;
-	struct inode *inode = d_inode(dentry);
+	struct dentry *dentry = file_dentry(file);
+	struct inode *inode = file_inode(file);
 	int kill;
 	int error = 0;
 
 	/* Fast path for nothing security related */
 	if (IS_NOSEC(inode))
 		return 0;
 
-	kill = file_needs_remove_privs(file);
+	kill = dentry_needs_remove_privs(dentry);
 	if (kill < 0)
 		return kill;
 	if (kill)