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=-13.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,MAILING_LIST_MULTI, MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS autolearn=unavailable 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 54FE4C43460 for ; Thu, 8 Apr 2021 15:41:27 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 282EF610A3 for ; Thu, 8 Apr 2021 15:41:27 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231975AbhDHPlf (ORCPT ); Thu, 8 Apr 2021 11:41:35 -0400 Received: from mail.kernel.org ([198.145.29.99]:50224 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231480AbhDHPle (ORCPT ); Thu, 8 Apr 2021 11:41:34 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 94BD761107; Thu, 8 Apr 2021 15:41:21 +0000 (UTC) Date: Thu, 8 Apr 2021 17:41:16 +0200 From: Christian Brauner To: Amir Goldstein Cc: Jan Kara , linux-fsdevel , Linux API , "J. Bruce Fields" Subject: Re: open_by_handle_at() in userns Message-ID: <20210408154116.ngzkmn3xjd7i5cl2@wittgenstein> References: <20210328155624.930558-1-amir73il@gmail.com> <20210330073101.5pqvw72fxvyp5kvf@wittgenstein> <20210331100854.sdgtzma6ifj7w5yn@wittgenstein> <20210408125530.gnv5hqcmgewklypn@wittgenstein> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Thu, Apr 08, 2021 at 06:34:01PM +0300, Amir Goldstein wrote: > On Thu, Apr 8, 2021 at 3:55 PM Christian Brauner > wrote: > > > > On Thu, Apr 08, 2021 at 02:44:47PM +0300, Amir Goldstein wrote: > > > > One thing your patch > > > > > > > > commit ea31e84fda83c17b88851de399f76f5d9fc1abf4 > > > > Author: Amir Goldstein > > > > Date: Sat Mar 20 12:58:12 2021 +0200 > > > > > > > > fs: allow open by file handle inside userns > > > > > > > > open_by_handle_at(2) requires CAP_DAC_READ_SEARCH in init userns, > > > > where most filesystems are mounted. > > > > > > > > Relax the requirement to allow a user with CAP_DAC_READ_SEARCH > > > > inside userns to open by file handle in filesystems that were > > > > mounted inside that userns. > > > > > > > > In addition, also allow open by handle in an idmapped mount, which is > > > > mapped to the userns while verifying that the returned open file path > > > > is under the root of the idmapped mount. > > > > > > > > This is going to be needed for setting an fanotify mark on a filesystem > > > > and watching events inside userns. > > > > > > > > Signed-off-by: Amir Goldstein > > > > > > > > Requires fs/exportfs/expfs.c to be made idmapped mounts aware. > > > > open_by_handle_at() uses exportfs_decode_fh() which e.g. has the > > > > following and other callchains: > > > > > > > > exportfs_decode_fh() > > > > -> exportfs_decode_fh_raw() > > > > -> lookup_one_len() > > > > -> inode_permission(mnt_userns, ...) > > > > > > > > That's not a huge problem though I did all these changes for the > > > > overlayfs support for idmapped mounts I have in a branch from an earlier > > > > version of the idmapped mounts patchset. Basically lookup_one_len(), > > > > lookup_one_len_unlocked(), and lookup_positive_unlocked() need to take > > > > the mnt_userns into account. I can rebase my change and send it for > > > > consideration next cycle. If you can live without the > > > > open_by_handle_at() support for now in this patchset (Which I think you > > > > said you could.) then it's not a blocker either. Sorry for the > > > > inconvenience. > > > > > > > > > > Christian, > > > > > > I think making exportfs_decode_fh() idmapped mount aware is not > > > enough, because when a dentry alias is found in dcache, none of > > > those lookup functions are called. > > > > > > I think we will also need something like this: > > > https://github.com/amir73il/linux/commits/fhandle_userns > > > > > > I factored-out a helper from nfsd_apcceptable() which implements > > > the "subtree_check" nfsd logic and uses it for open_by_handle_at(). > > > > > > I've also added a small patch to name_to_handle_at() with a UAPI > > > change that could make these changes usable by userspace nfs > > > server inside userns, but I have no demo nor tests for that and frankly, > > > I have little incentive to try and promote this UAPI change without > > > anybody asking for it... > > > > Ah, at first I was confused about why this would matter but it matters > > because nfsd already implements a check of that sort directly in nfsd > > independent of idmapped mounts: > > https://github.com/amir73il/linux/commit/4bef9ff1718935b7b42afbae71cfaab7770e8436 > > > > The check is needed for slightly different reasons. > nfsd "subtree_check" feature explicitly meant to forbid access in case > file was moved "out of reach", for example, out of the export path. > Note the nfsd "subtree_check" affects both file handle encoding > (i.e. "connectable") and file handle decoding (i.e. nfsd_acceptable()). > > open_by_handle_at() in idmapped mount needs to verify that the ancestry > inode owners can be mapped to the userns, because we already checked > that user has CAP_DAC_READ_SEARCH in userns, but it's nicer to do > the full inode_permission() check IMO. > > > Afaict, an nfs server can't be mounted inside of userns right now. That > > is something that folks from Netflix and from Kinvolk have been > > interested in enabling. They also want the ability to use idmapped > > mounts + nfs. Understandable that you don't want to drive this of > > course. I'll sync with them about this. > > > > Independent of that, I thought our last understanding was that you > > wouldn't need to handle open_by_handle_at() for now. > > > > I don't need it. But I realized that the fanotify_userns demo branch > I provided you is buggy in terms of security, so I wanted to give you > (or whoever wants to pursue this) a better reference. > It was one of those things that are easier to code than to explain ;-) And I highly appreciate it! :) Please keep those branches for reference somewhere. I've noted them down to make sure to look at them before I do anything! Christian