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,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 533C6C352A3 for ; Tue, 11 Feb 2020 13:57:31 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 2A7C92086A for ; Tue, 11 Feb 2020 13:57:31 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729987AbgBKN5a (ORCPT ); Tue, 11 Feb 2020 08:57:30 -0500 Received: from foss.arm.com ([217.140.110.172]:46418 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728564AbgBKN52 (ORCPT ); Tue, 11 Feb 2020 08:57:28 -0500 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 C768030E; Tue, 11 Feb 2020 05:57:27 -0800 (PST) Received: from [10.1.196.105] (eglon.cambridge.arm.com [10.1.196.105]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 909D33F68F; Tue, 11 Feb 2020 05:57:25 -0800 (PST) Subject: Re: [PATCH v7 00/11] add support for Clang's Shadow Call Stack To: Sami Tolvanen Cc: Will Deacon , Catalin Marinas , Steven Rostedt , Masami Hiramatsu , Ard Biesheuvel , Mark Rutland , Dave Martin , Kees Cook , Laura Abbott , Marc Zyngier , Nick Desaulniers , Jann Horn , Miguel Ojeda , Masahiro Yamada , clang-built-linux@googlegroups.com, kernel-hardening@lists.openwall.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org References: <20191018161033.261971-1-samitolvanen@google.com> <20200128184934.77625-1-samitolvanen@google.com> From: James Morse Message-ID: <63517cff-4bd6-bb6c-9a54-23de4f5fbb4a@arm.com> Date: Tue, 11 Feb 2020 13:57:24 +0000 User-Agent: Mozilla/5.0 (X11; Linux aarch64; rv:60.0) Gecko/20100101 Thunderbird/60.9.0 MIME-Version: 1.0 In-Reply-To: <20200128184934.77625-1-samitolvanen@google.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-GB Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Sami, On 28/01/2020 18:49, Sami Tolvanen wrote: > This patch series adds support for Clang's Shadow Call Stack > (SCS) mitigation, which uses a separately allocated shadow stack > to protect against return address overwrites. More information > can be found here: > > https://clang.llvm.org/docs/ShadowCallStack.html > > SCS provides better protection against traditional buffer > overflows than CONFIG_STACKPROTECTOR_*, but it should be noted > that SCS security guarantees in the kernel differ from the ones > documented for user space. The kernel must store addresses of > shadow stacks used by inactive tasks and interrupt handlers in > memory, which means an attacker capable reading and writing > arbitrary memory may be able to locate them and hijack control > flow by modifying shadow stacks that are not currently in use. > > SCS is currently supported only on arm64, where the compiler > requires the x18 register to be reserved for holding the current > task's shadow stack pointer. I found I had to add: | KBUILD_CFLAGS := $(filter-out -ffixed-x18 $(CC_FLAGS_SCS), $(KBUILD_CFLAGS)) to drivers/firmware/efi/libstub/Makefile, to get this going. I don't think there is much point supporting SCS for the EFIstub, its already isolated from the rest of the kernel's C code by the __efistub symbol prefix machinery, and trying to use it would expose us to buggy firmware at a point we can't handle it! I can send a patch if its easier for you, Thanks, James