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=-3.7 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 78C57C433DF for ; Wed, 13 May 2020 16:11:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id AD9FA204EC for ; Wed, 13 May 2020 16:11:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2389327AbgEMQL1 (ORCPT ); Wed, 13 May 2020 12:11:27 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52438 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728692AbgEMQL1 (ORCPT ); Wed, 13 May 2020 12:11:27 -0400 Received: from ZenIV.linux.org.uk (zeniv.linux.org.uk [IPv6:2002:c35c:fd02::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id EAF32C061A0C for ; Wed, 13 May 2020 09:11:26 -0700 (PDT) Received: from viro by ZenIV.linux.org.uk with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1jYtyT-007b0k-8h; Wed, 13 May 2020 16:11:13 +0000 Date: Wed, 13 May 2020 17:11:13 +0100 From: Al Viro To: Patrick Donnelly Cc: open list Subject: Re: file system permissions regression affecting root Message-ID: <20200513161113.GU23230@ZenIV.linux.org.uk> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, May 13, 2020 at 08:00:28AM -0700, Patrick Donnelly wrote: > In newer kernels (at least 5.6), it appears root is not able to write > to files owned by other users in a sticky directory: Yes. Controlled by /proc/sys/fs/protected_regular, which systemd crowd has decided to enable in commit 2732587540035227fe59e4b64b60127352611b35 Author: Lucas Werkmeister Date: Wed Jan 16 00:16:10 2019 +0100 Enable regular file and FIFO protection These sysctls were added in Linux 4.19 (torvalds/linux@30aba6656f), and we should enable them just like we enable the older hardlink/symlink protection since v199. Implements #11414. in their tree. The relevant part is this: diff --git a/sysctl.d/50-default.conf b/sysctl.d/50-default.conf index b0645f33e7..27084f6242 100644 --- a/sysctl.d/50-default.conf +++ b/sysctl.d/50-default.conf @@ -36,3 +36,7 @@ net.core.default_qdisc = fq_codel # Enable hard and soft link protection fs.protected_hardlinks = 1 fs.protected_symlinks = 1 + +# Enable regular file and FIFO protection +fs.protected_regular = 1 +fs.protected_fifos = 1 so if you want the normal behaviour (and I certainly agree that the value of that "protection" is not terribly high - I don't enable it on any of my boxen and I don't use systemd, so they can't make those decisions for me), I would suggest going into /etc/sysctl.d/ and telling the damn thing _not_ to enable that. >From the same commit: + * The fs.protected_regular and fs.protected_fifos sysctls, which were + added in Linux 4.19 to make some data spoofing attacks harder, are + now enabled by default. While this will hopefully improve the + security of most installations, it is technically a backwards + incompatible change; to disable these sysctls again, place the + following lines in /etc/sysctl.d/60-protected.conf or a similar file: + + fs.protected_regular = 0 + fs.protected_fifos = 0 + + Note that the similar hardlink and symlink protection has been + enabled since v199, and may be disabled likewise.