From patchwork Thu May 21 16:56:31 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Kristen Carlson Accardi X-Patchwork-Id: 11563491 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 C7315912 for ; Thu, 21 May 2020 16:57:24 +0000 (UTC) Received: from mother.openwall.net (mother.openwall.net [195.42.179.200]) by mail.kernel.org (Postfix) with SMTP id 8D45E207F7 for ; Thu, 21 May 2020 16:57:23 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 8D45E207F7 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=linux.intel.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=kernel-hardening-return-18835-patchwork-kernel-hardening=patchwork.kernel.org@lists.openwall.com Received: (qmail 30588 invoked by uid 550); 21 May 2020 16:57:19 -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: Delivered-To: mailing list kernel-hardening@lists.openwall.com Received: (qmail 30559 invoked from network); 21 May 2020 16:57:18 -0000 IronPort-SDR: rkIalUNhVAf6nmeEgpj54GjT3cbcbDkYjLPtR3W3CxoItn9nTFLuVM/ro5D43YM5oEKLPMfphB kPUJYE8cbWGw== X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False IronPort-SDR: GDu0AE+ARIYtin+MvSNXQy5yQWheRh35nBBasTrUJ+oWZ5PJP9U4MOs64Ob+Ilw8PdTF7i/zN/ rPA3zY0UTwRQ== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.73,418,1583222400"; d="scan'208";a="309094621" From: Kristen Carlson Accardi To: keescook@chromium.org, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de Cc: arjan@linux.intel.com, x86@kernel.org, linux-kernel@vger.kernel.org, kernel-hardening@lists.openwall.com, rick.p.edgecombe@intel.com, Kristen Carlson Accardi Subject: [PATCH v2 0/9] Function Granular KASLR Date: Thu, 21 May 2020 09:56:31 -0700 Message-Id: <20200521165641.15940-1-kristen@linux.intel.com> X-Mailer: git-send-email 2.20.1 MIME-Version: 1.0 Function Granular Kernel Address Space Layout Randomization (fgkaslr) --------------------------------------------------------------------- This patch set is an implementation of finer grained kernel address space randomization. It rearranges your kernel code at load time on a per-function level granularity, with only around a second added to boot time. Changes in v2: -------------- * Fix to address i386 build failure * Allow module reordering patch to be configured separately so that arm (or other non-x86_64 arches) can take advantage of module function reordering. This support has not be tested by me, but smoke tested by Ard Biesheuvel on arm. * Fix build issue when building on arm as reported by Ard Biesheuvel * minor chages for certain checkpatch warnings and review feedback. Patches to objtool are included because they are dependencies for this patchset, however they have been submitted by their maintainer separately. Background ---------- KASLR was merged into the kernel with the objective of increasing the difficulty of code reuse attacks. Code reuse attacks reused existing code snippets to get around existing memory protections. They exploit software bugs which expose addresses of useful code snippets to control the flow of execution for their own nefarious purposes. KASLR moves the entire kernel code text as a unit at boot time in order to make addresses less predictable. The order of the code within the segment is unchanged - only the base address is shifted. There are a few shortcomings to this algorithm. 1. Low Entropy - there are only so many locations the kernel can fit in. This means an attacker could guess without too much trouble. 2. Knowledge of a single address can reveal the offset of the base address, exposing all other locations for a published/known kernel image. 3. Info leaks abound. Finer grained ASLR has been proposed as a way to make ASLR more resistant to info leaks. It is not a new concept at all, and there are many variations possible. Function reordering is an implementation of finer grained ASLR which randomizes the layout of an address space on a function level granularity. We use the term "fgkaslr" in this document to refer to the technique of function reordering when used with KASLR, as well as finer grained KASLR in general. Proposed Improvement -------------------- This patch set proposes adding function reordering on top of the existing KASLR base address randomization. The over-arching objective is incremental improvement over what we already have. It is designed to work in combination with the existing solution. The implementation is really pretty simple, and there are 2 main area where changes occur: * Build time GCC has had an option to place functions into individual .text sections for many years now. This option can be used to implement function reordering at load time. The final compiled vmlinux retains all the section headers, which can be used to help find the address ranges of each function. Using this information and an expanded table of relocation addresses, individual text sections can be suffled immediately after decompression. Some data tables inside the kernel that have assumptions about order require re-sorting after being updated when applying relocations. In order to modify these tables, a few key symbols are excluded from the objcopy symbol stripping process for use after shuffling the text segments. Some highlights from the build time changes to look for: The top level kernel Makefile was modified to add the gcc flag if it is supported. Currently, I am applying this flag to everything it is possible to randomize. Anything that is written in C and is a function is randomized. Future work could turn off this flags for selected files or even entire subsystems, although obviously at the cost of security. The relocs tool is updated to add relative relocations. This information previously wasn't included because it wasn't necessary when moving the entire .text segment as a unit. A new file was created to contain a list of symbols that objcopy should keep. We use those symbols at load time as described below. * Load time The boot kernel was modified to parse the vmlinux elf file after decompression to check for our interesting symbols that we kept, and to look for any .text.* sections to randomize. We then shuffle the sections and update any tables that need to be updated or resorted. The existing code which updated relocation addresses was modified to account for not just a fixed delta from the load address, but the offset that the function section was moved to. This requires inspection of each address to see if it was impacted by a randomization. We use a bsearch to make this less horrible on performance. In order to hide our new layout, symbols reported through /proc/kallsyms will be sorted by name alphabetically rather than by address. Security Considerations ----------------------- The objective of this patch set is to improve a technology that is already merged into the kernel (KASLR). This code will not prevent all attacks, but should instead be considered as one of several tools that can be used. In particular, this code is meant to make KASLR more effective in the presence of info leaks. How much entropy we are adding to the existing entropy of standard KASLR will depend on a few variables. Firstly and most obviously, the number of functions that are randomized matters. This implementation keeps the existing .text section for code that cannot be randomized - for example, because it was assembly code. The less sections to randomize, the less entropy. In addition, due to alignment (16 bytes for x86_64), the number of bits in a address that the attacker needs to guess is reduced, as the lower bits are identical. Performance Impact ------------------ There are two areas where function reordering can impact performance: boot time latency, and run time performance. * Boot time latency This implementation of finer grained KASLR impacts the boot time of the kernel in several places. It requires additional parsing of the kernel ELF file to obtain the section headers of the sections to be randomized. It calls the random number generator for each section to be randomized to determine that section's new memory location. It copies the decompressed kernel into a new area of memory to avoid corruption when laying out the newly randomized sections. It increases the number of relocations the kernel has to perform at boot time vs. standard KASLR, and it also requires a lookup on each address that needs to be relocated to see if it was in a randomized section and needs to be adjusted by a new offset. Finally, it re-sorts a few data tables that are required to be sorted by address. Booting a test VM on a modern, well appointed system showed an increase in latency of approximately 1 second. * Run time The performance impact at run-time of function reordering varies by workload. Using kcbench, a kernel compilation benchmark, the performance of a kernel build with finer grained KASLR was about 1% slower than a kernel with standard KASLR. Analysis with perf showed a slightly higher percentage of L1-icache-load-misses. Other workloads were examined as well, with varied results. Some workloads performed significantly worse under FGKASLR, while others stayed the same or were mysteriously better. In general, it will depend on the code flow whether or not finer grained KASLR will impact your workload, and how the underlying code was designed. Because the layout changes per boot, each time a system is rebooted the performance of a workload may change. Future work could identify hot areas that may not be randomized and either leave them in the .text section or group them together into a single section that may be randomized. If grouping things together helps, one other thing to consider is that if we could identify text blobs that should be grouped together to benefit a particular code flow, it could be interesting to explore whether this security feature could be also be used as a performance feature if you are interested in optimizing your kernel layout for a particular workload at boot time. Optimizing function layout for a particular workload has been researched and proven effective - for more information read the Facebook paper "Optimizing Function Placement for Large-Scale Data-Center Applications" (see references section below). Image Size ---------- Adding additional section headers as a result of compiling with -ffunction-sections will increase the size of the vmlinux ELF file. With a standard distro config, the resulting vmlinux was increased by about 3%. The compressed image is also increased due to the header files, as well as the extra relocations that must be added. You can expect fgkaslr to increase the size of the compressed image by about 15%. Memory Usage ------------ fgkaslr increases the amount of heap that is required at boot time, although this extra memory is released when the kernel has finished decompression. As a result, it may not be appropriate to use this feature on systems without much memory. Building -------- To enable fine grained KASLR, you need to have the following config options set (including all the ones you would use to build normal KASLR) CONFIG_FG_KASLR=y In addition, fgkaslr is only supported for the X86_64 architecture. Modules ------- Modules are randomized similarly to the rest of the kernel by shuffling the sections at load time prior to moving them into memory. The module must also have been build with the -ffunction-sections compiler option. Although fgkaslr for the kernel is only supported for the X86_64 architecture, it is possible to use fgkaslr with modules on other architectures. To enable this feature, select CONFIG_MODULE_FG_KASLR=y This option is selected automatically for X86_64 when CONFIG_FG_KASLR is set. Disabling --------- Disabling normal KASLR using the nokaslr command line option also disables fgkaslr. It is also possible to disable fgkaslr separately by booting with fgkaslr=off on the commandline. References ---------- There are a lot of academic papers which explore finer grained ASLR. This paper in particular contributed the most to my implementation design as well as my overall understanding of the problem space: Selfrando: Securing the Tor Browser against De-anonymization Exploits, M. Conti, S. Crane, T. Frassetto, et al. For more information on how function layout impacts performance, see: Optimizing Function Placement for Large-Scale Data-Center Applications, G. Ottoni, B. Maher Kees Cook (1): x86/boot: Allow a "silent" kaslr random byte fetch Kristen Carlson Accardi (8): objtool: Do not assume order of parent/child functions x86: tools/relocs: Support >64K section headers x86: Makefile: Add build and config option for CONFIG_FG_KASLR x86: Make sure _etext includes function sections x86/tools: Add relative relocs for randomized functions x86: Add support for function granular KASLR kallsyms: Hide layout module: Reorder functions Documentation/security/fgkaslr.rst | 155 +++++ Documentation/security/index.rst | 1 + Makefile | 4 + arch/x86/Kconfig | 14 + arch/x86/Makefile | 3 + arch/x86/boot/compressed/Makefile | 10 +- arch/x86/boot/compressed/fgkaslr.c | 823 +++++++++++++++++++++++ arch/x86/boot/compressed/kaslr.c | 4 - arch/x86/boot/compressed/misc.c | 109 ++- arch/x86/boot/compressed/misc.h | 34 + arch/x86/boot/compressed/utils.c | 12 + arch/x86/boot/compressed/vmlinux.symbols | 17 + arch/x86/include/asm/boot.h | 15 +- arch/x86/kernel/vmlinux.lds.S | 18 +- arch/x86/lib/kaslr.c | 18 +- arch/x86/tools/relocs.c | 143 +++- arch/x86/tools/relocs.h | 4 +- arch/x86/tools/relocs_common.c | 15 +- include/asm-generic/vmlinux.lds.h | 2 +- include/uapi/linux/elf.h | 1 + init/Kconfig | 11 + kernel/kallsyms.c | 138 +++- kernel/module.c | 81 +++ tools/objtool/elf.c | 8 +- 24 files changed, 1578 insertions(+), 62 deletions(-) create mode 100644 Documentation/security/fgkaslr.rst create mode 100644 arch/x86/boot/compressed/fgkaslr.c create mode 100644 arch/x86/boot/compressed/utils.c create mode 100644 arch/x86/boot/compressed/vmlinux.symbols base-commit: b9bbe6ed63b2b9f2c9ee5cbd0f2c946a2723f4ce