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=-2.0 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 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 83D17C43331 for ; Fri, 3 Apr 2020 10:03:27 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 4388520757 for ; Fri, 3 Apr 2020 10:03:27 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=armlinux.org.uk header.i=@armlinux.org.uk header.b="doolofrt" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 4388520757 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=armlinux.org.uk Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id E407E8E0008; Fri, 3 Apr 2020 06:03:26 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id DF1328E0007; Fri, 3 Apr 2020 06:03:26 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id CE0B08E0008; Fri, 3 Apr 2020 06:03:26 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0143.hostedemail.com [216.40.44.143]) by kanga.kvack.org (Postfix) with ESMTP id B643B8E0007 for ; Fri, 3 Apr 2020 06:03:26 -0400 (EDT) Received: from smtpin17.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with ESMTP id 60058180AD820 for ; Fri, 3 Apr 2020 10:03:26 +0000 (UTC) X-FDA: 76666106412.17.cave11_1d7989f3eb516 X-HE-Tag: cave11_1d7989f3eb516 X-Filterd-Recvd-Size: 6354 Received: from pandora.armlinux.org.uk (pandora.armlinux.org.uk [78.32.30.218]) by imf33.hostedemail.com (Postfix) with ESMTP for ; Fri, 3 Apr 2020 10:03:22 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=armlinux.org.uk; s=pandora-2019; h=Sender:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=yLhCR68Xx5hXytQdHGQlISjAF/TJxI6l40ZChrHYRsU=; b=doolofrtnnwkKzrDYH1z3uTz+ Ga0SYykJLMFHS/skroRuv5Zpte0rMgHTC22elbzbJJkid/eQs9ERNRlQkP/Wj8QnOgEalVGFKMHYQ eOtTWbNewf52qneI28UDohL2SMl2Ypnfv+U56rxIo+n3dKpT9Gw0rB4VJOLRjnl+dUr2EAqyGFi3K DqcDI9N8CsDBZ64l9527i11cqv9kX3rDys+AK9QcDbwsirH7c0YJc2NuY7Knc4D6lMxEkAapq9Lh5 JVJbgfHr4pFfxp5QtT4eXLJwY4ImacIlHVA1EHipja65a8hmeNELjKTyp1UKzpfZH7JCcFFHu3/BB PCWbI7QJA==; Received: from shell.armlinux.org.uk ([fd8f:7570:feb6:1:5054:ff:fe00:4ec]:45070) by pandora.armlinux.org.uk with esmtpsa (TLSv1.2:ECDHE-RSA-AES256-GCM-SHA384:256) (Exim 4.90_1) (envelope-from ) id 1jKJAA-0008Rg-To; Fri, 03 Apr 2020 11:03:03 +0100 Received: from linux by shell.armlinux.org.uk with local (Exim 4.92) (envelope-from ) id 1jKJA9-0002Zz-E3; Fri, 03 Apr 2020 11:02:57 +0100 Date: Fri, 3 Apr 2020 11:02:57 +0100 From: Russell King - ARM Linux admin To: Kees Cook Cc: Al Viro , 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, Christian Borntraeger Subject: Re: [PATCH RESEND 1/4] uaccess: Add user_read_access_begin/end and user_write_access_begin/end Message-ID: <20200403100257.GB25745@shell.armlinux.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> User-Agent: Mutt/1.10.1 (2018-07-13) 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: > On Thu, Apr 02, 2020 at 06:50:32PM +0100, Al Viro wrote: > > On Thu, Apr 02, 2020 at 07:03:28PM +0200, Christophe Leroy wrote: > > > > > user_access_begin() grants both read and write. > > > > > > This patch adds user_read_access_begin() and user_write_access_begin() but > > > it doesn't remove user_access_begin() > > > > Ouch... So the most generic name is for the rarest case? > > > > > > What should we do about that? Do we prohibit such blocks outside > > > > of arch? > > > > > > > > What should we do about arm and s390? There we want a cookie passed > > > > from beginning of block to its end; should that be a return value? > > > > > > That was the way I implemented it in January, see > > > https://patchwork.ozlabs.org/patch/1227926/ > > > > > > There was some discussion around that and most noticeable was: > > > > > > H. Peter (hpa) said about it: "I have *deep* concern with carrying state in > > > a "key" variable: it's a direct attack vector for a crowbar attack, > > > especially since it is by definition live inside a user access region." > > > > > This patch minimises the change by just adding user_read_access_begin() and > > > user_write_access_begin() keeping the same parameters as the existing > > > user_access_begin(). > > > > Umm... What about the arm situation? The same concerns would apply there, > > wouldn't they? Currently we have > > static __always_inline unsigned int uaccess_save_and_enable(void) > > { > > #ifdef CONFIG_CPU_SW_DOMAIN_PAN > > unsigned int old_domain = get_domain(); > > > > /* Set the current domain access to permit user accesses */ > > set_domain((old_domain & ~domain_mask(DOMAIN_USER)) | > > domain_val(DOMAIN_USER, DOMAIN_CLIENT)); > > > > return old_domain; > > #else > > return 0; > > #endif > > } > > and > > static __always_inline void uaccess_restore(unsigned int flags) > > { > > #ifdef CONFIG_CPU_SW_DOMAIN_PAN > > /* Restore the user access mask */ > > set_domain(flags); > > #endif > > } > > > > How much do we need nesting on those, anyway? rmk? It's that way because it's easy, logical, and actually *more* efficient to do it that way, rather than read-modify-write the domain register each time we want to change it. > 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. There is one known nesting, which is __clear_user() when used with the (IMHO horrid and I don't care about) UACCESS_WITH_MEMCPY feature. That's not intentional however. When I introduced this on ARM, the placement I adopted was to locate it _as close as sanely possible_ to the userspace access so we minimised the kernel accesses, so we minimise the number of accesses that could go stray because of the domain issue - we ideally only want the access done by the accessor itself to be affected, which we achieve for most accesses. Thinking laterally, maybe we should get rid of the whole KERNEL_DS stuff entirely, and come up with an alternative way of handling the kernel-wants-to-access-kernelspace-via-user-accessors problem. Such as, copying some data back to userspace memory? -- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line in suburbia: sync at 10.2Mbps down 587kbps up