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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 2A199CCA47B for ; Wed, 8 Jun 2022 12:48:25 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S239498AbiFHMsX (ORCPT ); Wed, 8 Jun 2022 08:48:23 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:34176 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S239514AbiFHMsT (ORCPT ); Wed, 8 Jun 2022 08:48:19 -0400 Received: from ams.source.kernel.org (ams.source.kernel.org [IPv6:2604:1380:4601:e00::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 2AB2692D0C; Wed, 8 Jun 2022 05:48:17 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id CF7B4B8276C; Wed, 8 Jun 2022 12:48:15 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5D66FC34116; Wed, 8 Jun 2022 12:48:12 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1654692494; bh=kL7aqTzSrEHcf3xZnL9DdRnwdBZsFUKtW/2ujP1VUbA=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=t+UPSSFw+vgYMzwwfmzMdDix74pc+PpAiHwFtFPbm0Rmj5nt+c93ofl1Absuy8yre 2rVzdZOxFIttdCCnGT4hEBVVtOVHP281fHSOQCtzoiy2GxMTKLGW9dqw9eUJ8DRfoI 59Xc/ltMZWoqXp5QV75udZRKq1gDkWO7ore0oaD9pXJ9cLK2oG4OzhE2jP4/CTyf1J BPJtzBgsFnYlWlZoKRPO3ZXpHq3PJ/Wn1kqdoqFnzSiQ/x0naJwOERiFExY+Eo/Hsb 0Z7orm0A5tvTn2jhiZUJusrIEYY21CSypQxJxVEtR60fAyBy4zofy73/IwZczAMKPK Tan82+dxllvFw== Date: Wed, 8 Jun 2022 14:48:08 +0200 From: Christian Brauner To: Amir Goldstein Cc: Christian =?utf-8?B?R8O2dHRzY2hl?= , selinux@vger.kernel.org, Miklos Szeredi , Linux API , linux-man , Alexander Viro , linux-fsdevel , linux-kernel Subject: Re: [RFC PATCH] f*xattr: allow O_PATH descriptors Message-ID: <20220608124808.uylo5lntzfgxxmns@wittgenstein> References: <20220607153139.35588-1-cgzones@googlemail.com> <20220608112728.b4xrdppxqmyqmtwf@wittgenstein> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-api@vger.kernel.org On Wed, Jun 08, 2022 at 03:28:52PM +0300, Amir Goldstein wrote: > On Wed, Jun 8, 2022 at 2:57 PM Christian Brauner wrote: > > > > On Tue, Jun 07, 2022 at 05:31:39PM +0200, Christian Göttsche wrote: > > > From: Miklos Szeredi > > > > > > Support file descriptors obtained via O_PATH for extended attribute > > > operations. > > > > > > Extended attributes are for example used by SELinux for the security > > > context of file objects. To avoid time-of-check-time-of-use issues while > > > setting those contexts it is advisable to pin the file in question and > > > operate on a file descriptor instead of the path name. This can be > > > emulated in userspace via /proc/self/fd/NN [1] but requires a procfs, > > > which might not be mounted e.g. inside of chroots, see[2]. > > > > > > [1]: https://github.com/SELinuxProject/selinux/commit/7e979b56fd2cee28f647376a7233d2ac2d12ca50 > > > [2]: https://github.com/SELinuxProject/selinux/commit/de285252a1801397306032e070793889c9466845 > > > > > > Original patch by Miklos Szeredi > > > https://patchwork.kernel.org/project/linux-fsdevel/patch/20200505095915.11275-6-mszeredi@redhat.com/ > > > > > > > While this carries a minute risk of someone relying on the property of > > > > xattr syscalls rejecting O_PATH descriptors, it saves the trouble of > > > > introducing another set of syscalls. > > > > > > > > Only file->f_path and file->f_inode are accessed in these functions. > > > > > > > > Current versions return EBADF, hence easy to detect the presense of > > > > this feature and fall back in case it's missing. > > > > > > CC: linux-api@vger.kernel.org > > > CC: linux-man@vger.kernel.org > > > Signed-off-by: Christian Göttsche > > > --- > > > > I'd be somewhat fine with getxattr and listxattr but I'm worried that > > setxattr/removexattr waters down O_PATH semantics even more. I don't > > want O_PATH fds to be useable for operations which are semantically > > equivalent to a write. > > It is not really semantically equivalent to a write if it works on a > O_RDONLY fd already. The fact that it works on a O_RDONLY fd has always been weird. And is probably a bug. If you look at xattr_permission() you can see that it checks for MAY_WRITE for set operations... setxattr() writes to disk for real filesystems. I don't know how much closer to a write this can get. In general, one semantic aberration doesn't justify piling another one on top. (And one thing that speaks for O_RDONLY is at least that it actually opens the file wheres O_PATH doesn't.) > > > > > In sensitive environments such as service management/container runtimes > > we often send O_PATH fds around precisely because it is restricted what > > they can be used for. I'd prefer to not to plug at this string. > > But unless I am mistaken, path_setxattr() and syscall_fsetxattr() > are almost identical w.r.t permission checks and everything else. > > So this change introduces nothing new that a user in said environment > cannot already accomplish with setxattr(). > > Besides, as the commit message said, doing setxattr() on an O_PATH > fd is already possible with setxattr("/proc/self/$fd"), so whatever security > hole you are trying to prevent is already wide open. That is very much a something that we're trying to restrict for this exact reason and is one of the main motivator for upgrade mask in openat2(). If I want to send a O_PATH around I want it to not be upgradable. Aleksa is working on upgrade masks with openat2() (see [1] and part of the original patchset in [2]. O_PATH semantics don't need to become weird. [1]: https://lore.kernel.org/all/20220526130355.fo6gzbst455fxywy@senku [2]: https://patchwork.ozlabs.org/project/linuxppc-dev/patch/20190728010207.9781-8-cyphar@cyphar.com