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=-7.3 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 150ACC07E9A for ; Mon, 12 Jul 2021 19:31:44 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id EE62A61006 for ; Mon, 12 Jul 2021 19:31:43 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236440AbhGLTeb (ORCPT ); Mon, 12 Jul 2021 15:34:31 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42158 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S236190AbhGLTe2 (ORCPT ); Mon, 12 Jul 2021 15:34:28 -0400 Received: from fieldses.org (fieldses.org [IPv6:2600:3c00:e000:2f7::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 56C29C0613DD; Mon, 12 Jul 2021 12:31:40 -0700 (PDT) Received: by fieldses.org (Postfix, from userid 2815) id 485F269D6; Mon, 12 Jul 2021 15:31:39 -0400 (EDT) DKIM-Filter: OpenDKIM Filter v2.11.0 fieldses.org 485F269D6 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=fieldses.org; s=default; t=1626118299; bh=Zn3SvcMF3FFO49+WTI8pt7ML3V5uhGmeYXXeqsXQrcY=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=v8DlyIayyg8yDjwJfuxsFYTJvh+t9cDW+Uz+VYzSDSXEbEke0mosrcMLlzmpjVmDX 06wu4GYGB/ZF34UAZkd7LRQK1LLMfmlqQkWptRk7aDVwFxxNO8dF2uGtyK7jm+7Tbu RIR8atVX3reEl7AA0HxjsvEmxSoIIB5kZMNxDEwg= Date: Mon, 12 Jul 2021 15:31:39 -0400 From: "J. Bruce Fields" To: Vivek Goyal Cc: Bruce Fields , Casey Schaufler , Christian Brauner , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, viro@zeniv.linux.org.uk, virtio-fs@redhat.com, dwalsh@redhat.com, dgilbert@redhat.com, casey.schaufler@intel.com, linux-security-module@vger.kernel.org, selinux@vger.kernel.org, tytso@mit.edu, miklos@szeredi.hu, gscrivan@redhat.com, jack@suse.cz, Christoph Hellwig Subject: Re: [PATCH v2 1/1] xattr: Allow user.* xattr on symlink and special files Message-ID: <20210712193139.GA22997@fieldses.org> References: <20210708175738.360757-1-vgoyal@redhat.com> <20210708175738.360757-2-vgoyal@redhat.com> <20210709091915.2bd4snyfjndexw2b@wittgenstein> <20210709152737.GA398382@redhat.com> <710d1c6f-d477-384f-0cc1-8914258f1fb1@schaufler-ca.com> <20210709175947.GB398382@redhat.com> <20210712140247.GA486376@redhat.com> <20210712154106.GB18679@fieldses.org> <20210712174759.GA502004@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210712174759.GA502004@redhat.com> User-Agent: Mutt/1.5.21 (2010-09-15) Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Mon, Jul 12, 2021 at 01:47:59PM -0400, Vivek Goyal wrote: > On Mon, Jul 12, 2021 at 11:41:06AM -0400, J. Bruce Fields wrote: > > Looks like 0xd is what the server returns to access on a device node > > with mode bits rw- for the caller. > > > > Commit c11d7fd1b317 "nfsd: take xattr bits into account for permission > > checks" added the ACCESS_X* bits for regular files and directories but > > not others. > > > > But you don't want to determine permission from the mode bits anyway, > > you want it to depend on the owner, > > Thinking more about this part. Current implementation of my patch is > effectively doing both the checks. It checks that you are owner or > have CAP_FOWNER in xattr_permission() and then goes on to call > inode_permission(). And that means file mode bits will also play a > role. If caller does not have write permission on the file, it will > be denied setxattr(). > > If I don't call inode_permission(), and just return 0 right away for > file owner (for symlinks and special files), then just being owner > is enough to write user.* xattr. And then even security modules will > not get a chance to block that operation. IOW, if you are owner of > a symlink or special file, you can write as many user.* xattr as you > like and except quota does not look like anything else can block > it. I am wondering if this approach is ok? Yeah, I'd expect security modules to get a say, and I wouldn't expect mode bits on device nodes to be useful for deciding whether it makes sense for xattrs to be readable or writeable. But, I don't really know. Do we have any other use cases besides this case of storing security labels in user xattrs? --b.