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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS 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 A5CAAC46460 for ; Thu, 9 Aug 2018 14:25:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5E0AC21E23 for ; Thu, 9 Aug 2018 14:25:00 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 5E0AC21E23 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com 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 S1732274AbeHIQuF (ORCPT ); Thu, 9 Aug 2018 12:50:05 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:59652 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1730839AbeHIQuF (ORCPT ); Thu, 9 Aug 2018 12:50:05 -0400 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.rdu2.redhat.com [10.11.54.5]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 3DB5F4023444; Thu, 9 Aug 2018 14:24:56 +0000 (UTC) Received: from warthog.procyon.org.uk (ovpn-120-78.rdu2.redhat.com [10.10.120.78]) by smtp.corp.redhat.com (Postfix) with ESMTP id 5EA862314E; Thu, 9 Aug 2018 14:24:55 +0000 (UTC) Organization: Red Hat UK Ltd. Registered Address: Red Hat UK Ltd, Amberley Place, 107-111 Peascod Street, Windsor, Berkshire, SI4 1TE, United Kingdom. Registered in England and Wales under Company Registration No. 3798903 From: David Howells In-Reply-To: <87in4n9zg0.fsf@xmission.com> References: <87in4n9zg0.fsf@xmission.com> <153313703562.13253.5766498657900728120.stgit@warthog.procyon.org.uk> <153313723557.13253.9055982745313603422.stgit@warthog.procyon.org.uk> To: ebiederm@xmission.com (Eric W. Biederman) Cc: dhowells@redhat.com, viro@zeniv.linux.org.uk, linux-api@vger.kernel.org, torvalds@linux-foundation.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 28/33] vfs: syscall: Add fsconfig() for configuring and managing a context [ver #11] MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-ID: <27373.1533824694.1@warthog.procyon.org.uk> Date: Thu, 09 Aug 2018 15:24:54 +0100 Message-ID: <27374.1533824694@warthog.procyon.org.uk> X-Scanned-By: MIMEDefang 2.79 on 10.11.54.5 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.6]); Thu, 09 Aug 2018 14:24:56 +0000 (UTC) X-Greylist: inspected by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.6]); Thu, 09 Aug 2018 14:24:56 +0000 (UTC) for IP:'10.11.54.5' DOMAIN:'int-mx05.intmail.prod.int.rdu2.redhat.com' HELO:'smtp.corp.redhat.com' FROM:'dhowells@redhat.com' RCPT:'' Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Eric W. Biederman wrote: > First let me thank you for adding both FSCONFIG_CMD_CREATE and > FSCONFIG_CMD_RECONFIGURE. Unfortunately the implementation is currently > broken. So this patch gets my: > > This is broken in two specific ways. > ... > 2) FSCONFIG_CMD_CREATE will succeed even if the superblock already > exists and it can not use all of the superblock parameters. > > This happens because vfs_get_super will only call fill_super > if the super block is created. Which is reasonable on the face > of it. But it in practice this introduces security problems. > > a) Either through reconfiguring a shared super block you did not > realize was shared (as we saw with devpts). > > b) Mounting a super block and not honoring it's mount options > because something has already mounted it. As we see today > with proc. Leaving userspace to think the filesystem will behave > one way when in fact it behaves another. > > I have already explained this several times, and apparently I have been > ignored. This fundamental usability issue that leads to security > problems. I've also explained why you're wrong or at least only partially right. I *do* *not* want to implement sget() in userspace with the ability for userspace to lock out other mount requests - which is what it appears that you've been asking for. However, as I have said, I *am* willing to add one of more flags to help with this, but I can't make any "legacy" fs honour them as this requires the fs_context to be passed down to sget_fc() and the filesystem - which is why I was considering leaving it for later. (1) An FSOPEN_EXCL flag to tell sget_fc() to fail if the superblock already exists at all. (2) An FSOPEN_FAIL_ON_MISMATCH flag to explicitly say that we *don't* want a superblock with different parameters. The implication of providing neither flag is that we are happy to accept a superblock from the same source but with different parameters. But it doesn't seem to be an absolute imperative to roll this out immediately, since what I have exactly mirrors what the kernel currently does - and forcing a change in that behaviour risks breaking userspace. If it keeps you happy, however, I can try and work one up. David