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=-12.1 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED autolearn=unavailable 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 4EAA3C43461 for ; Thu, 3 Sep 2020 21:45:04 +0000 (UTC) Received: from mother.openwall.net (mother.openwall.net [195.42.179.200]) by mail.kernel.org (Postfix) with SMTP id 8A207206D4 for ; Thu, 3 Sep 2020 21:45:03 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=chromium.org header.i=@chromium.org header.b="O5o59mUd" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 8A207206D4 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-19748-kernel-hardening=archiver.kernel.org@lists.openwall.com Received: (qmail 32609 invoked by uid 550); 3 Sep 2020 21:44:56 -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 32577 invoked from network); 3 Sep 2020 21:44:55 -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=Wkni4eXx9E7Yx63VN0+vnviLhaoHNM+yB8xZjp7w5hM=; b=O5o59mUdRprA27/bBNjMETG5FChmb/k7jW6G/CEqPxqkNExuhCDFNEGkwkwsAAOk9B S/ND1TkN2Kb21Yt+dermQ1eJzMamhppfZcA/5BLAHvK0xooF6lH1S7/GgEK+o+mFn5sC Q+9Q6xNOHekU+wFfZY8e2e5GVn6y3TV0ytjI0= 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=Wkni4eXx9E7Yx63VN0+vnviLhaoHNM+yB8xZjp7w5hM=; b=TQZkQDP4wSOJOLQShZ6LpCqNgF/MZB5FBH5V7qV2JIl6OWvhYRytprPd5XJsjm5D6V 1Hw2N2EwIkxlxrWjywLtW+dxpeu9n1lWWcHS2GDje500I5yJyq+gTc65JYSpetaFKwue 1d8ORDGUu92vkMVBUurZFa7/11Y5qaU2XlSLFOPtN+kX6El5bAUSZtElFjGPpIG7PYUU /rz+1Dtl29niXQzxY5EZQOZKjYYZ67/gVcV61s3ELxHPrYsnpGyqhTA43boUDjZ/Ii2R x8S6mc+1d2PziFR0X8iNkaY3YTqmR3ORndfJSFdhr9q69KxkKAveVvEhfEGb46TsOZuo pAIg== X-Gm-Message-State: AOAM530v9HDoce7PxmAZLx+IGmdYTTtxBgAvhUBGxK7gRU9EhfLoX9bv nhYrsdbUG0lRFYMa7QYXiu0Smg== X-Google-Smtp-Source: ABdhPJzAv2fIwHx5fjsh6FyQ4DUXacQqnpd1HrcO3mYGfEva7Q9zQka0PVlCAE3Ap8Ww7DWt19mfFg== X-Received: by 2002:a62:1809:: with SMTP id 9mr5726960pfy.217.1599169483379; Thu, 03 Sep 2020 14:44:43 -0700 (PDT) Date: Thu, 3 Sep 2020 14:44:41 -0700 From: Kees Cook To: Sami Tolvanen Cc: Masahiro Yamada , Will Deacon , Peter Zijlstra , Steven Rostedt , Greg Kroah-Hartman , "Paul E. McKenney" , Nick Desaulniers , clang-built-linux@googlegroups.com, kernel-hardening@lists.openwall.com, linux-arch@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org, linux-pci@vger.kernel.org, x86@kernel.org, Arvind Sankar Subject: Re: [PATCH v2 01/28] x86/boot/compressed: Disable relocation relaxation Message-ID: <202009031444.F2ECA89E@keescook> References: <20200624203200.78870-1-samitolvanen@google.com> <20200903203053.3411268-1-samitolvanen@google.com> <20200903203053.3411268-2-samitolvanen@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200903203053.3411268-2-samitolvanen@google.com> On Thu, Sep 03, 2020 at 01:30:26PM -0700, Sami Tolvanen wrote: > From: Arvind Sankar > > The x86-64 psABI [0] specifies special relocation types > (R_X86_64_[REX_]GOTPCRELX) for indirection through the Global Offset > Table, semantically equivalent to R_X86_64_GOTPCREL, which the linker > can take advantage of for optimization (relaxation) at link time. This > is supported by LLD and binutils versions 2.26 onwards. > > The compressed kernel is position-independent code, however, when using > LLD or binutils versions before 2.27, it must be linked without the -pie > option. In this case, the linker may optimize certain instructions into > a non-position-independent form, by converting foo@GOTPCREL(%rip) to $foo. > > This potential issue has been present with LLD and binutils-2.26 for a > long time, but it has never manifested itself before now: > - LLD and binutils-2.26 only relax > movq foo@GOTPCREL(%rip), %reg > to > leaq foo(%rip), %reg > which is still position-independent, rather than > mov $foo, %reg > which is permitted by the psABI when -pie is not enabled. > - gcc happens to only generate GOTPCREL relocations on mov instructions. > - clang does generate GOTPCREL relocations on non-mov instructions, but > when building the compressed kernel, it uses its integrated assembler > (due to the redefinition of KBUILD_CFLAGS dropping -no-integrated-as), > which has so far defaulted to not generating the GOTPCRELX > relocations. > > Nick Desaulniers reports [1,2]: > A recent change [3] to a default value of configuration variable > (ENABLE_X86_RELAX_RELOCATIONS OFF -> ON) in LLVM now causes Clang's > integrated assembler to emit R_X86_64_GOTPCRELX/R_X86_64_REX_GOTPCRELX > relocations. LLD will relax instructions with these relocations based > on whether the image is being linked as position independent or not. > When not, then LLD will relax these instructions to use absolute > addressing mode (R_RELAX_GOT_PC_NOPIC). This causes kernels built with > Clang and linked with LLD to fail to boot. > > Patch series [4] is a solution to allow the compressed kernel to be > linked with -pie unconditionally, but even if merged is unlikely to be > backported. As a simple solution that can be applied to stable as well, > prevent the assembler from generating the relaxed relocation types using > the -mrelax-relocations=no option. For ease of backporting, do this > unconditionally. > > [0] https://gitlab.com/x86-psABIs/x86-64-ABI/-/blob/master/x86-64-ABI/linker-optimization.tex#L65 > [1] https://lore.kernel.org/lkml/20200807194100.3570838-1-ndesaulniers@google.com/ > [2] https://github.com/ClangBuiltLinux/linux/issues/1121 > [3] https://reviews.llvm.org/rGc41a18cf61790fc898dcda1055c3efbf442c14c0 > [4] https://lore.kernel.org/lkml/20200731202738.2577854-1-nivedita@alum.mit.edu/ > > Reported-by: Nick Desaulniers > Signed-off-by: Arvind Sankar Reviewed-by: Kees Cook -- Kees Cook