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=-4.1 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, 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 6BB37C433E0 for ; Mon, 22 Jun 2020 22:05:02 +0000 (UTC) Received: from mother.openwall.net (mother.openwall.net [195.42.179.200]) by mail.kernel.org (Postfix) with SMTP id BDE2220716 for ; Mon, 22 Jun 2020 22:05:01 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=chromium.org header.i=@chromium.org header.b="Om+hyYPX" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org BDE2220716 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=chromium.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=kernel-hardening-return-19046-kernel-hardening=archiver.kernel.org@lists.openwall.com Received: (qmail 25876 invoked by uid 550); 22 Jun 2020 22:04:55 -0000 Mailing-List: contact kernel-hardening-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Received: (qmail 25841 invoked from network); 22 Jun 2020 22:04:54 -0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=chromium.org; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=tiLvWsLlWI5Mz8yJb26QV5MdQxD2Cn7z8TLlvShCGfU=; b=Om+hyYPXJ8zWabYbc6j0NJhRsYt8jMK2RZ8VkZvizn/0iWwTmfV5CpCspe1f49JHQW 6gV2PbvOVcBmbS6Dkxhwac9tkJmvLeq9OhxwTB6IOmUuEIfpwzibKOjcJaaH8shijbHK ngd3RTBVPCWAn6skAEgoIsGV4+tetMcj/rxhM= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=tiLvWsLlWI5Mz8yJb26QV5MdQxD2Cn7z8TLlvShCGfU=; b=ovSQmgCeEmHohn+icVB+5odkkdpf64i3c5Rvy2WNZ3ImMIHCKb6XjnwL4Ltn9yFi0V XH8v7nhDZMJu32QA7rliqmZrcjOgO7ogcFfKLNKhI1wVaTFzXMSSqVio/24CZ8XDakqP g5tKsDUIXXzCo0MYzygiBbUTplVy5IcYbStvgpJCh8EmsWeV0FwMWzIeqRhUyBeK10+W PtJTv8xZq5YIxAD8F5Gnntey2Iw3cAGifR1uxnb/VlbWR4Cho12Ai6kHts6TYuImffhl qeF3kavRuP6AcG4dWChJcNmDhjdLPsKekUlQhlKttai2X923NEBID4it66LGmD9v2w3M dUAw== X-Gm-Message-State: AOAM530D92adL7gao/BIBXlN60XlJ8+W4pBW0GXzOMnBda9JbPFLCcio d9FmQd2vda8S4roSgOmAtBtE/w== X-Google-Smtp-Source: ABdhPJzYYstfMbj2bKl5qg6BjBnX7jxPh+vFVKET3pf6cfx1vakKGSEBc4Spw9TWrQtmJz0BX3mj8Q== X-Received: by 2002:a17:902:8342:: with SMTP id z2mr21209390pln.300.1592863482442; Mon, 22 Jun 2020 15:04:42 -0700 (PDT) Date: Mon, 22 Jun 2020 15:04:40 -0700 From: Kees Cook To: Jann Horn Cc: Thomas Gleixner , Elena Reshetova , the arch/x86 maintainers , Andy Lutomirski , Peter Zijlstra , Catalin Marinas , Will Deacon , Mark Rutland , Alexander Potapenko , Alexander Popov , Ard Biesheuvel , Kernel Hardening , Linux ARM , Linux-MM , kernel list Subject: Re: [PATCH v4 3/5] stack: Optionally randomize kernel stack offset each syscall Message-ID: <202006221451.2E80C90FF7@keescook> References: <20200622193146.2985288-1-keescook@chromium.org> <20200622193146.2985288-4-keescook@chromium.org> <202006221426.CEEE0B8@keescook> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Mon, Jun 22, 2020 at 11:42:29PM +0200, Jann Horn wrote: > No, at least on x86-64 and x86 Linux overrides the normal ABI. From > arch/x86/Makefile: Ah! Thanks for the pointer. > > # For gcc stack alignment is specified with -mpreferred-stack-boundary, > # clang has the option -mstack-alignment for that purpose. > ifneq ($(call cc-option, -mpreferred-stack-boundary=4),) > cc_stack_align4 := -mpreferred-stack-boundary=2 > cc_stack_align8 := -mpreferred-stack-boundary=3 > else ifneq ($(call cc-option, -mstack-alignment=16),) > cc_stack_align4 := -mstack-alignment=4 > cc_stack_align8 := -mstack-alignment=8 > endif > [...] > ifeq ($(CONFIG_X86_32),y) > [...] > # Align the stack to the register width instead of using the default > # alignment of 16 bytes. This reduces stack usage and the number of > # alignment instructions. > KBUILD_CFLAGS += $(call cc-option,$(cc_stack_align4)) > [...] > else > [...] > # By default gcc and clang use a stack alignment of 16 bytes for x86. > # However the standard kernel entry on x86-64 leaves the stack on an > # 8-byte boundary. If the compiler isn't informed about the actual > # alignment it will generate extra alignment instructions for the > # default alignment which keep the stack *mis*aligned. > # Furthermore an alignment to the register width reduces stack usage > # and the number of alignment instructions. > KBUILD_CFLAGS += $(call cc-option,$(cc_stack_align8)) > [...] > endif And it seems that only x86 does this. No other architecture specifies -mpreferred-stack-boundary... > Normal x86-64 ABI has 16-byte stack alignment; Linux kernel x86-64 ABI > has 8-byte stack alignment. > Similarly, the normal Linux 32-bit x86 ABI is 16-byte aligned; > meanwhile Linux kernel x86 ABI has 4-byte stack alignment. > > This is because userspace code wants the stack to be sufficiently > aligned for fancy SSE instructions and such; the kernel, on the other > hand, never uses those in normal code, and cares about stack usage and > such very much. This makes it nicer for Clang: diff --git a/include/linux/randomize_kstack.h b/include/linux/randomize_kstack.h index 1df0dc52cadc..f7e1f68fb50c 100644 --- a/include/linux/randomize_kstack.h +++ b/include/linux/randomize_kstack.h @@ -10,6 +10,14 @@ DECLARE_STATIC_KEY_MAYBE(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, randomize_kstack_offset); DECLARE_PER_CPU(u32, kstack_offset); +#ifdef CONFIG_X86_64 +#define ARCH_STACK_ALIGN_MASK ~((1 << 8) - 1) +#elif defined(CONFIG_X86_32) +#define ARCH_STACK_ALIGN_MASK ~((1 << 4) - 1) +#else +#define ARCH_STACK_ALIGN_MASK ~(0) +#endif + /* * Do not use this anywhere else in the kernel. This is used here because * it provides an arch-agnostic way to grow the stack with correct @@ -23,7 +31,8 @@ void *__builtin_alloca(size_t size); if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT, \ &randomize_kstack_offset)) { \ u32 offset = this_cpu_read(kstack_offset); \ - u8 *ptr = __builtin_alloca(offset & 0x3FF); \ + u8 *ptr = __builtin_alloca(offset & 0x3FF & \ + ARCH_STACK_ALIGN_MASK); \ asm volatile("" : "=m"(*ptr)); \ } \ } while (0) But I don't like open-coding the x86-ony stack alignment... it should be in Kconfig or something, I think? -- Kees Cook