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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 0BAD0C43331 for ; Fri, 3 Apr 2020 00:58:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C62CA206F5 for ; Fri, 3 Apr 2020 00:58:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2390430AbgDCA6x (ORCPT ); Thu, 2 Apr 2020 20:58:53 -0400 Received: from zeniv.linux.org.uk ([195.92.253.2]:54420 "EHLO ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2389574AbgDCA6w (ORCPT ); Thu, 2 Apr 2020 20:58:52 -0400 Received: from viro by ZenIV.linux.org.uk with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1jKAfH-009AzZ-B4; Fri, 03 Apr 2020 00:58:31 +0000 Date: Fri, 3 Apr 2020 01:58:31 +0100 From: Al Viro To: Kees Cook Cc: Christophe Leroy , Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , airlied@linux.ie, daniel@ffwll.ch, torvalds@linux-foundation.org, akpm@linux-foundation.org, hpa@zytor.com, linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-mm@kvack.org, linux-arch@vger.kernel.org, Russell King , Christian Borntraeger Subject: Re: [PATCH RESEND 1/4] uaccess: Add user_read_access_begin/end and user_write_access_begin/end Message-ID: <20200403005831.GI23230@ZenIV.linux.org.uk> References: <27106d62fdbd4ffb47796236050e418131cb837f.1585811416.git.christophe.leroy@c-s.fr> <20200402162942.GG23230@ZenIV.linux.org.uk> <67e21b65-0e2d-7ca5-7518-cec1b7abc46c@c-s.fr> <20200402175032.GH23230@ZenIV.linux.org.uk> <202004021132.813F8E88@keescook> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <202004021132.813F8E88@keescook> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Apr 02, 2020 at 11:35:57AM -0700, Kees Cook wrote: > Yup, I think it's a weakness of the ARM implementation and I'd like to > not extend it further. AFAIK we should never nest, but I would not be > surprised at all if we did. > > If we were looking at a design goal for all architectures, I'd like > to be doing what the public PaX patchset did for their memory access > switching, which is to alarm if calling into "enable" found the access > already enabled, etc. Such a condition would show an unexpected nesting > (like we've seen with similar constructs with set_fs() not getting reset > during an exception handler, etc etc). FWIW, maybe I'm misreading the ARM uaccess logics, but... it smells like KERNEL_DS is somewhat more dangerous there than on e.g. x86. Look: with CONFIG_CPU_DOMAINS, set_fs(KERNEL_DS) tells MMU to ignore per-page permission bits in DOMAIN_KERNEL (i.e. for kernel address ranges), allowing them even if they would normally be denied. We need that for actual uaccess loads/stores, since those use insns that pretend to be done in user mode and we want them to access the kernel pages. But that affects the normal loads/stores as well; unless I'm misreading that code, it will ignore (supervisor) r/o on a page. And that's not just for the code inside the uaccess blocks; *everything* done under KERNEL_DS is subject to that. Why do we do that (modify_domain(), that is) inside set_fs() and not in uaccess_enable() et.al.?