From patchwork Sun Jun 14 14:45:34 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Alexander Potapenko X-Patchwork-Id: 11603349 Return-Path: Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id 989591392 for ; Sun, 14 Jun 2020 14:45:43 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7EFF920578 for ; Sun, 14 Jun 2020 14:45:43 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="pMwlKrrH" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726868AbgFNOpm (ORCPT ); Sun, 14 Jun 2020 10:45:42 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:45614 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725815AbgFNOpm (ORCPT ); Sun, 14 Jun 2020 10:45:42 -0400 Received: from mail-qk1-x749.google.com (mail-qk1-x749.google.com [IPv6:2607:f8b0:4864:20::749]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 48689C03E969 for ; Sun, 14 Jun 2020 07:45:40 -0700 (PDT) Received: by mail-qk1-x749.google.com with SMTP id d187so12154128qke.4 for ; Sun, 14 Jun 2020 07:45:40 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=date:message-id:mime-version:subject:from:to:cc; bh=cES73WJaLMoUweYIM+OlRJ2Jm853+1vLwtEwV6Uh388=; b=pMwlKrrHagVLWTUat4QRNSBaTmseF6Z+raXgWIIqRaGm32qet+ibqsVdxjZ1TQZOP6 3NrTDjoo72m0GXXw+6rq1i0wbY/80L2a/P2kf/i1+bbxZ01cJUPNcMcnD4Jfu/9mR5Hp 5DQCr+bpfXTbpo1LOmHyCtcIyoInzk+NtksLYTG/HmRUXIyLGd9NP3wpuRvXdTncf8fT cJUQZsbItoIyZI/pn6Dt3j4v0X8B1MKBr57SpiusXEiIQKstqowTgHYgC5i0NkpphVgU GNowbTVhXFd7DuYMnR+WXEcYSTXUL1dapX3wiNm8nCF7l49dweq/s+0WHf/HEkIY+qeT VDgw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:message-id:mime-version:subject:from:to:cc; bh=cES73WJaLMoUweYIM+OlRJ2Jm853+1vLwtEwV6Uh388=; b=Ojt1AvuTeTrx5XWQ1G1m1wZZ3PqahQW91KHB2/vnXCZKJ9/b2w+3Qwfx04PtFL9gNm D0rhNoHsewvhtnWQwwRiTfkOiGlF8769c5/pyxPLkShWCsDbnPPKqr6OSRFzWQ8AhU8b mL4aC/ofGfPy8Z035xaXtIUxepRGAPxEFi5UgR+rnr5IFmDQJlEwimXqd23of4RgIXY4 Ub2M5NO99HUopBvAbG9e/AMwoc8XyPoD3g4V5W98+3yDPwtvIQO3f+JJb8fLukfsLris R6cBowNiPFq7DoBx4qrsPkrWvMdLbTImz2nozsF4WZGHj8D2IF+t74vEoG2AcG+RXZVM WzAw== X-Gm-Message-State: AOAM533+pG5MDYzqfdkPuNiY4UGmV0CwS01gUYI3w30lTJjBac+Lfzyw zsxWyM7mJKqEWhfHnD+Lgza1f96Hhho= X-Google-Smtp-Source: ABdhPJz1k1RNu9dAXsmdsawTe7TyraBkMndQDppnCMV2qrWO1j7+QMn1X+iW+F38INV+PsKSU5IK3AvNUXA= X-Received: by 2002:a05:6214:713:: with SMTP id b19mr20890435qvz.199.1592145938185; Sun, 14 Jun 2020 07:45:38 -0700 (PDT) Date: Sun, 14 Jun 2020 16:45:34 +0200 Message-Id: <20200614144534.237035-1-glider@google.com> Mime-Version: 1.0 X-Mailer: git-send-email 2.27.0.290.gba653c62da-goog Subject: [PATCH] [RFC] security: allow using Clang's zero initialization for stack variables From: glider@google.com To: yamada.masahiro@socionext.com, keescook@chromium.org, jmorris@namei.org Cc: maze@google.com, ndesaulniers@google.com, linux-security-module@vger.kernel.org, linux-kernel@vger.kernel.org, Alexander Potapenko Sender: owner-linux-security-module@vger.kernel.org Precedence: bulk List-ID: In addition to -ftrivial-auto-var-init=pattern (used by CONFIG_INIT_STACK_ALL now) Clang also supports zero initialization for locals enabled by -ftrivial-auto-var-init=zero. The future of this flag is still being debated, see https://bugs.llvm.org/show_bug.cgi?id=45497 Right now it is guarded by another flag, -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang, which means it may not be supported by future Clang releases. Another possible resolution is that -ftrivial-auto-var-init=zero will persist (as certain users have already started depending on it), but the name of the guard flag will change. In the meantime, zero initialization has proven itself as a good production mitigation measure against uninitialized locals. Unlike pattern initialization, which has a higher chance of triggering existing bugs, zero initialization provides safe defaults for strings, pointers, indexes, and sizes. On the other hand, pattern initialization remains safer for return values. Performance-wise, the difference between pattern and zero initialization is usually negligible, although the generated code for zero initialization is more compact. The proposed config, CONFIG_USE_CLANG_ZERO_INITIALIZATION, makes CONFIG_INIT_STACK_ALL use zero initialization if the corresponding flags are supported by Clang. Cc: Kees Cook Cc: Nick Desaulniers Signed-off-by: Alexander Potapenko --- Makefile | 15 ++++++++++++++- security/Kconfig.hardening | 16 ++++++++++++++++ 2 files changed, 30 insertions(+), 1 deletion(-) diff --git a/Makefile b/Makefile index fd31992bf918..2860bad7e39a 100644 --- a/Makefile +++ b/Makefile @@ -802,9 +802,22 @@ KBUILD_CFLAGS += -fomit-frame-pointer endif endif -# Initialize all stack variables with a pattern, if desired. +# Initialize all stack variables, if desired. ifdef CONFIG_INIT_STACK_ALL + +# Use pattern initialization by default. +ifndef CONFIG_USE_CLANG_ZERO_INITIALIZATION KBUILD_CFLAGS += -ftrivial-auto-var-init=pattern +else + +ifdef CONFIG_CC_HAS_AUTO_VAR_ZERO_INIT +# Future support for zero initialization is still being debated, see +# https://bugs.llvm.org/show_bug.cgi?id=45497. These flags are subject to being +# renamed or dropped. +KBUILD_CFLAGS += -ftrivial-auto-var-init=zero -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang +endif + +endif endif DEBUG_CFLAGS := $(call cc-option, -fno-var-tracking-assignments) diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening index af4c979b38ee..299d27c6d78c 100644 --- a/security/Kconfig.hardening +++ b/security/Kconfig.hardening @@ -22,6 +22,9 @@ menu "Memory initialization" config CC_HAS_AUTO_VAR_INIT def_bool $(cc-option,-ftrivial-auto-var-init=pattern) +config CC_HAS_AUTO_VAR_ZERO_INIT + def_bool $(cc-option,-ftrivial-auto-var-init=zero -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang) + choice prompt "Initialize kernel stack variables at function entry" default GCC_PLUGIN_STRUCTLEAK_BYREF_ALL if COMPILE_TEST && GCC_PLUGINS @@ -100,6 +103,19 @@ choice endchoice +config USE_CLANG_ZERO_INITIALIZATION + bool "Use Clang's zero initialization for local variables" + depends on CC_HAS_AUTO_VAR_ZERO_INIT + depends on INIT_STACK_ALL + help + If set, uses zeros instead of 0xAA to initialize local variables in + INIT_STACK_ALL. Zeroing the stack provides safer defaults for strings, + pointers, indexes, and sizes. The downsides are less-safe defaults for + return values, and exposing fewer bugs where the underlying code + relies on zero initialization. + The corresponding flag isn't officially supported by Clang and may + sooner or later go away or get renamed. + config GCC_PLUGIN_STRUCTLEAK_VERBOSE bool "Report forcefully initialized variables" depends on GCC_PLUGIN_STRUCTLEAK