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 F0358C47404 for ; Fri, 11 Oct 2019 15:44:53 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id CCAAF206CD for ; Fri, 11 Oct 2019 15:44:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728179AbfJKPow (ORCPT ); Fri, 11 Oct 2019 11:44:52 -0400 Received: from foss.arm.com ([217.140.110.172]:36292 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726521AbfJKPov (ORCPT ); Fri, 11 Oct 2019 11:44:51 -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 1F8AE142F; Fri, 11 Oct 2019 08:44:50 -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 3844B3F68E; Fri, 11 Oct 2019 08:44:47 -0700 (PDT) Date: Fri, 11 Oct 2019 16:44:45 +0100 From: Dave Martin To: Mark Rutland Cc: Paul Elliott , Peter Zijlstra , Catalin Marinas , Will Deacon , Andrew Jones , Amit Kachhap , Vincenzo Frascino , linux-arch@vger.kernel.org, Eugene Syromiatnikov , Szabolcs Nagy , "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, Sudakshina Das Subject: Re: [PATCH v2 05/12] arm64: Basic Branch Target Identification support Message-ID: <20191011154444.GN27757@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> <4e09ca54-f353-9448-64ed-4ba1e38c6ebc@linaro.org> <20191011153225.GL27757@arm.com> <20191011154043.GG33537@lakrids.cambridge.arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191011154043.GG33537@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 11, 2019 at 04:40:43PM +0100, Mark Rutland wrote: > On Fri, Oct 11, 2019 at 04:32:26PM +0100, Dave Martin wrote: > > On Fri, Oct 11, 2019 at 11:25:33AM -0400, Richard Henderson wrote: > > > On 10/11/19 11:10 AM, Mark Rutland wrote: > > > > On Thu, Oct 10, 2019 at 07:44:33PM +0100, Dave Martin wrote: > > > >> @@ -730,6 +730,11 @@ static void setup_return > > > >> regs->regs[29] = (unsigned long)&user->next_frame->fp; > > > >> regs->pc = (unsigned long)ka->sa.sa_handler; > > > >> > > > >> + if (system_supports_bti()) { > > > >> + regs->pstate &= ~PSR_BTYPE_MASK; > > > >> + regs->pstate |= PSR_BTYPE_CALL; > > > >> + } > > > >> + > > > > > > > > I think we might need a comment as to what we're trying to ensure here. > > > > > > > > I was under the (perhaps mistaken) impression that we'd generate a > > > > pristine pstate for a signal handler, and it's not clear to me that we > > > > must ensure the first instruction is a target instruction. > > > > > > I think it makes sense to treat entry into a signal handler as a call. Code > > > that has been compiled for BTI, and whose page has been marked with PROT_BTI, > > > will already have the pauth/bti markup at the beginning of the signal handler > > > function; we might as well verify that. > > > > > > Otherwise sigaction becomes a hole by which an attacker can force execution to > > > start at any arbitrary address. > > > > Ack, that's the intended rationale -- I also outlined this in the commit > > message. > > Ah, sorry. I evidently did not read that thoroughly enough. > > > Does this sound reasonable? > > > > > > Either way, I feel we should do this: any function in a PROT_BTI page > > should have a suitable landing pad. There's no reason I can see why > > a protection given to any other callback function should be omitted > > for a signal handler. > > > > Note, if the signal handler isn't in a PROT_BTI page then overriding > > BTYPE here will not trigger a Branch Target exception. > > > > I'm happy to drop a brief comment into the code also, once we're > > agreed on what the code should be doing. > > So long as there's a comment as to why, I have no strong feelings here. > :) OK, I think it's worth a brief comment in the code either way, so I'll add something. Cheers ---Dave