ovl: update S_ISGID when setting posix ACLs

This change fixes xfstest generic/375, which failed to clear the
setgid bit in the following test case on overlayfs:

  touch $testfile
  chown 100:100 $testfile
  chmod 2755 $testfile
  _runas -u 100 -g 101 -- setfacl -m u::rwx,g::rwx,o::rwx $testfile
Reported-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Tested-by: Amir Goldstein <amir73il@gmail.com>
Fixes: d837a49b ("ovl: fix POSIX ACL setting")
Cc: <stable@vger.kernel.org> # v4.8

fs/overlayfs/super.c

@@ -1037,6 +1037,21 @@ ovl_posix_acl_xattr_set(const struct xattr_handler *handler,
 
 	posix_acl_release(acl);
 
+	/*
+	 * Check if sgid bit needs to be cleared (actual setacl operation will
+	 * be done with mounter's capabilities and so that won't do it for us).
+	 */
+	if (unlikely(inode->i_mode & S_ISGID) &&
+	    handler->flags == ACL_TYPE_ACCESS &&
+	    !in_group_p(inode->i_gid) &&
+	    !capable_wrt_inode_uidgid(inode, CAP_FSETID)) {
+		struct iattr iattr = { .ia_valid = ATTR_KILL_SGID };
+
+		err = ovl_setattr(dentry, &iattr);
+		if (err)
+			return err;
+	}
+
 	err = ovl_xattr_set(dentry, handler->name, value, size, flags);
 	if (!err)
 		ovl_copyattr(ovl_inode_real(inode, NULL), inode);