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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,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 EC265CA9EA1 for ; Fri, 18 Oct 2019 13:40:52 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id CCE7B222C2 for ; Fri, 18 Oct 2019 13:40:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2410411AbfJRNkv (ORCPT ); Fri, 18 Oct 2019 09:40:51 -0400 Received: from [217.140.110.172] ([217.140.110.172]:39666 "EHLO foss.arm.com" rhost-flags-FAIL-FAIL-OK-OK) by vger.kernel.org with ESMTP id S1728150AbfJRNkv (ORCPT ); Fri, 18 Oct 2019 09:40:51 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id D6CC1EC0; Fri, 18 Oct 2019 06:40:30 -0700 (PDT) Received: from arm.com (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 7C9A73F6C4; Fri, 18 Oct 2019 06:40:27 -0700 (PDT) Date: Fri, 18 Oct 2019 14:40:25 +0100 From: Dave Martin To: Mark Rutland Cc: Dave Kleikamp , Paul Elliott , Peter Zijlstra , Catalin Marinas , Will Deacon , Andrew Jones , Amit Kachhap , Vincenzo Frascino , linux-arch@vger.kernel.org, Eugene Syromiatnikov , Szabolcs Nagy , Benjamin Herrenschmidt , "H.J. Lu" , Yu-cheng Yu , Kees Cook , Arnd Bergmann , Jann Horn , Richard Henderson , Kristina =?utf-8?Q?Mart=C5=A1enko?= , Mark Brown , Thomas Gleixner , linux-arm-kernel@lists.infradead.org, Florian Weimer , linux-kernel@vger.kernel.org, Andrew Morton , Sudakshina Das Subject: Re: [PATCH v2 05/12] arm64: Basic Branch Target Identification support Message-ID: <20191018134024.GE27757@arm.com> References: <1570733080-21015-1-git-send-email-Dave.Martin@arm.com> <1570733080-21015-6-git-send-email-Dave.Martin@arm.com> <20191011151028.GE33537@lakrids.cambridge.arm.com> <20191011172013.GQ27757@arm.com> <20191018111603.GD27759@lakrids.cambridge.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191018111603.GD27759@lakrids.cambridge.arm.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Oct 18, 2019 at 12:16:03PM +0100, Mark Rutland wrote: > [adding mm folk] > > On Fri, Oct 11, 2019 at 06:20:15PM +0100, Dave Martin wrote: > > On Fri, Oct 11, 2019 at 04:10:29PM +0100, Mark Rutland wrote: > > > On Thu, Oct 10, 2019 at 07:44:33PM +0100, Dave Martin wrote: > > > > +#define arch_validate_prot(prot, addr) arm64_validate_prot(prot, addr) > > > > +static inline int arm64_validate_prot(unsigned long prot, unsigned long addr) > > > > +{ > > > > + unsigned long supported = PROT_READ | PROT_WRITE | PROT_EXEC | PROT_SEM; > > > > + > > > > + if (system_supports_bti()) > > > > + supported |= PROT_BTI; > > > > + > > > > + return (prot & ~supported) == 0; > > > > +} > > > > > > If we have this check, can we ever get into arm64_calc_vm_prot_bits() > > > with PROT_BIT but !system_supports_bti()? > > > > > > ... or can that become: > > > > > > return (prot & PROT_BTI) ? VM_ARM64_BTI : 0; > > > > We can reach this via mmap() and friends IIUC. > > > > Since this function only gets called once-ish per vma I have a weak > > preference for keeping the check here to avoid code fragility. > > > > > > It does feel like arch_validate_prot() is supposed to be a generic gate > > for prot flags coming into the kernel via any route though, but only the > > mprotect() path actually uses it. > > > > This function originally landed in v2.6.27 as part of the powerpc strong > > access ordering support (PROT_SAO): > > > > b845f313d78e ("mm: Allow architectures to define additional protection bits") > > ef3d3246a0d0 ("powerpc/mm: Add Strong Access Ordering support") > > > > where the mmap() path uses arch_calc_vm_prot_bits() without > > arch_validate_prot(), just as in the current code. powerpc's original > > arch_calc_vm_prot_bits() does no obvious policing. > > > > This might be a bug. I can draft a patch to add it for the mmap() path > > for people to comment on ... I can't figure out yet whether or not the > > difference is intentional or there's some subtlety that I'm missed. > > From reading those two commit messages, it looks like this was an > oversight. I'd expect that we should apply this check for any > user-provided prot (i.e. it should apply to both mprotect and mmap). > > Ben, Andrew, does that make sense to you? > > ... or was there some reason to only do this for mprotect? > > Thanks, > Mark. For now, I'll drop a comment under the tearoff noting this outstanding question. The resulting behaviour is slightly odd, but doesn't seem unsafe, and we can of course tidy it up later. I think the risk of userspace becoming dependent on randomly passing PROT_BTI to mprotect() even when unsupported is low. [...] Cheers ---Dave