ovl: handle ATTR_KILL*

commit b99c2d91 upstream.

Before 4bacc9c9 ("overlayfs: Make f_path...") file->f_path pointed to
the underlying file, hence suid/sgid removal on write worked fine.

After that patch file->f_path pointed to the overlay file, and the file
mode bits weren't copied to overlay_inode->i_mode.  So the suid/sgid
removal simply stopped working.

The fix is to copy the mode bits, but then ovl_setattr() needs to clear
ATTR_MODE to avoid the BUG() in notify_change().  So do this first, then in
the next patch copy the mode.
Reported-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Fixes: 4bacc9c9 ("overlayfs: Make f_path always point to the overlay and f_inode to the underlay")
Cc: Eric Schultz <eric@startuperic.com>
Cc: Eric Hameleers <alien@slackware.com>
[backported by Eric Hameleers as seen in https://bugzilla.kernel.org/show_bug.cgi?id=150711]
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/overlayfs/inode.c

@@ -63,6 +63,9 @@ int ovl_setattr(struct dentry *dentry, struct iattr *attr)
 	if (!err) {
 		upperdentry = ovl_dentry_upper(dentry);
 
+		if (attr->ia_valid & (ATTR_KILL_SUID|ATTR_KILL_SGID))
+			attr->ia_valid &= ~ATTR_MODE;
+
 		mutex_lock(&upperdentry->d_inode->i_mutex);
 		err = notify_change(upperdentry, attr, NULL);
 		if (!err)