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 8F07AC43331 for ; Fri, 3 Apr 2020 00:58:57 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 1EC60206F5 for ; Fri, 3 Apr 2020 00:58:57 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 1EC60206F5 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=zeniv.linux.org.uk Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id B6BAA8E0008; Thu, 2 Apr 2020 20:58:56 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id B43218E0007; Thu, 2 Apr 2020 20:58:56 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id A7EF18E0008; Thu, 2 Apr 2020 20:58:56 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0018.hostedemail.com [216.40.44.18]) by kanga.kvack.org (Postfix) with ESMTP id 8DCB78E0007 for ; Thu, 2 Apr 2020 20:58:56 -0400 (EDT) Received: from smtpin23.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay02.hostedemail.com (Postfix) with ESMTP id 40E5B52C9 for ; Fri, 3 Apr 2020 00:58:56 +0000 (UTC) X-FDA: 76664734272.23.song37_4dfb8fe707744 X-HE-Tag: song37_4dfb8fe707744 X-Filterd-Recvd-Size: 3049 Received: from ZenIV.linux.org.uk (zeniv.linux.org.uk [195.92.253.2]) by imf08.hostedemail.com (Postfix) with ESMTP for ; Fri, 3 Apr 2020 00:58:55 +0000 (UTC) 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> X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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.?