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=-5.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 2C55AC3F68F for ; Wed, 1 Jan 2020 00:55:01 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 01B7520718 for ; Wed, 1 Jan 2020 00:55:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727134AbgAAAy6 (ORCPT ); Tue, 31 Dec 2019 19:54:58 -0500 Received: from zeniv.linux.org.uk ([195.92.253.2]:40944 "EHLO ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727120AbgAAAy6 (ORCPT ); Tue, 31 Dec 2019 19:54:58 -0500 Received: from viro by ZenIV.linux.org.uk with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1imSHe-0004Zd-1d; Wed, 01 Jan 2020 00:54:46 +0000 Date: Wed, 1 Jan 2020 00:54:46 +0000 From: Al Viro To: Aleksa Sarai Cc: David Howells , Eric Biederman , Linus Torvalds , stable@vger.kernel.org, Christian Brauner , Serge Hallyn , dev@opencontainers.org, containers@lists.linux-foundation.org, linux-api@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH RFC 0/1] mount: universally disallow mounting over symlinks Message-ID: <20200101005446.GH4203@ZenIV.linux.org.uk> References: <20191230052036.8765-1-cyphar@cyphar.com> <20191230054413.GX4203@ZenIV.linux.org.uk> <20191230054913.c5avdjqbygtur2l7@yavin.dot.cyphar.com> <20191230072959.62kcojxpthhdwmfa@yavin.dot.cyphar.com> <20200101004324.GA11269@ZenIV.linux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200101004324.GA11269@ZenIV.linux.org.uk> User-Agent: Mutt/1.12.1 (2019-06-15) Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Wed, Jan 01, 2020 at 12:43:24AM +0000, Al Viro wrote: > I'm not sure if you have caught anything else, but we really, really should *NOT* > consider the LAST_BIND as "maybe we should follow the result" material. So > at least the following is needed; could you check if anything else remains > with that applied? > > diff --git a/fs/namei.c b/fs/namei.c > index d6c91d1e88cb..d4fbbda8a7ff 100644 > --- a/fs/namei.c > +++ b/fs/namei.c > @@ -2656,10 +2656,7 @@ mountpoint_last(struct nameidata *nd) > nd->flags &= ~LOOKUP_PARENT; > > if (unlikely(nd->last_type != LAST_NORM)) { > - error = handle_dots(nd, nd->last_type); > - if (error) > - return error; > - path.dentry = dget(nd->path.dentry); > + return handle_dots(nd, nd->last_type); > } else { > path.dentry = d_lookup(dir, &nd->last); > if (!path.dentry) { Note, BTW, that lookup_last() (aka walk_component()) does just that - we only hit step_into() on LAST_NORM. The same goes for do_last(). mountpoint_last() not doing the same is _not_ intentional - it's definitely a bug. Consider your testcase; link points to . here. So the only thing you could expect from trying to follow it would be the directory 'link' lives in. And you don't have it when you reach the fscker via /proc/self/fd/3; what happens instead is nd->path set to ./link (by nd_jump_link()) *AND* step_into() called, pushing the same ./link onto stack. It violates all kinds of assumptions made by fs/namei.c - when pushing a symlink onto stack nd->path is expected to contain the base directory for resolving it. I'm fairly sure that this is the cause of at least some of the insanity you've caught; there always could be something else, of course, but this hole needs to be closed in any case.