From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.4 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED,USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 83118C65BAF for ; Mon, 10 Dec 2018 09:47:26 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 543FB20855 for ; Mon, 10 Dec 2018 09:47:26 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 543FB20855 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=suse.cz Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726706AbeLJJrY (ORCPT ); Mon, 10 Dec 2018 04:47:24 -0500 Received: from mx2.suse.de ([195.135.220.15]:36304 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726136AbeLJJrY (ORCPT ); Mon, 10 Dec 2018 04:47:24 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id A19FEAD6F; Mon, 10 Dec 2018 09:47:22 +0000 (UTC) Received: by quack2.suse.cz (Postfix, from userid 1000) id 2562D1E13F8; Mon, 10 Dec 2018 10:47:22 +0100 (CET) Date: Mon, 10 Dec 2018 10:47:22 +0100 From: Jan Kara To: Al Viro Cc: Alexander Lochmann , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Jan Kara , Horst Schirmeier Subject: Re: [PATCH] Fix sync. in blkdev_write_iter() acessing i_flags Message-ID: <20181210094722.GB29289@quack2.suse.cz> References: <4903939e-d3d6-b0c2-9c33-0fea0a61213c@tu-dortmund.de> <20181207175811.GZ2217@ZenIV.linux.org.uk> <5c86e85f-0ad4-935a-3021-7046551f361f@tu-dortmund.de> <20181208004944.GA2217@ZenIV.linux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181208004944.GA2217@ZenIV.linux.org.uk> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat 08-12-18 00:49:44, Al Viro wrote: > On Fri, Dec 07, 2018 at 08:49:16PM +0100, Alexander Lochmann wrote: > > > > _What_ SUID bit? We are talking about a write to block device, for fsck sake... > > > > > That's the way I understood Jan's explanation: > > " > > Thinking more about this I'm not sure if this is actually the right > > solution. Because for example the write(2) can set S_NOSEC flag wrongly > > when it would race with chmod adding SUID bit. So probably we rather need > > to acquire i_rwsem in blkdev_write_iter() if file does not have S_NOSEC set > > (we don't want to acquire it unconditionally as that would heavily impact > > scalability of block device writes). > > IDGI. We are talking about a block device here. What business could > file_remove_privs() have doing _anything_ to it? should_remove_suid() returns > to return 0 for those; what case do you have in mind? Somebody setting > security.capabilities on a block device inode? > > IMO the bug here is file_remove_privs() not buggering off immediately > after having observed that we are dealing with a block device. It really > has nothing useful to do. I didn't notice that S_ISREG() check in should_remove_suid(). My bad. And I wasn't quite sure whether some security module does not rely on inode_need_killpriv security hook. But now when I grep I see that cap_inode_need_killpriv() is really the only user and security.capabilities probably don't make sense for it since block devices cannot be executed anyway. So yes, the easiest fix is to just bail from file_remove_privs(). Probably for anything that is not a regular file, right? Directories cannot be written anyway and for pipes and character devices same logic applies as for block devices. Honza -- Jan Kara SUSE Labs, CR