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From: Jann Horn <jannh@google.com>
To: Kees Cook <keescook@chromium.org>
Cc: Kristen Carlson Accardi <kristen@linux.intel.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	 Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	"H . Peter Anvin" <hpa@zytor.com>,
	 Arjan van de Ven <arjan@linux.intel.com>,
	Rick Edgecombe <rick.p.edgecombe@intel.com>,
	 "the arch/x86 maintainers" <x86@kernel.org>,
	kernel list <linux-kernel@vger.kernel.org>,
	 Kernel Hardening <kernel-hardening@lists.openwall.com>
Subject: Re: [RFC PATCH 06/11] x86: make sure _etext includes function sections
Date: Thu, 6 Feb 2020 14:15:30 +0100
Message-ID: <CAG48ez19kRC_5+ykvQCnZxLq6Qg3xUy7fEMf3pYrG46vBZt6jQ@mail.gmail.com> (raw)
In-Reply-To: <202002060408.84005CEFFD@keescook>

On Thu, Feb 6, 2020 at 1:26 PM Kees Cook <keescook@chromium.org> wrote:
> I know x86_64 stack alignment is 16 bytes.

That's true for the standard sysv ABI that is used in userspace; but
the kernel uses a custom ABI with 8-byte stack alignment. See
arch/x86/Makefile:

# 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
[...]
        # 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))

> I cannot find evidence for
> what function start alignment should be.

There is no architecturally required alignment for functions, but
Intel's Optimization Manual
(<https://www.intel.com/content/dam/www/public/us/en/documents/manuals/64-ia-32-architectures-optimization-manual.pdf>)
recommends in section 3.4.1.5, "Code Alignment":

| Assembly/Compiler Coding Rule 12. (M impact, H generality)
| All branch targets should be 16-byte aligned.

AFAIK this is recommended because, as documented in section 2.3.2.1,
"Legacy Decode Pipeline" (describing the frontend of Sandy Bridge, and
used as the base for newer microarchitectures):

| An instruction fetch is a 16-byte aligned lookup through the ITLB
and into the instruction cache.
| The instruction cache can deliver every cycle 16 bytes to the
instruction pre-decoder.

AFAIK this means that if a branch ends close to the end of a 16-byte
block, the frontend is less efficient because it may have to run two
instruction fetches before the first instruction can even be decoded.

  reply index

Thread overview: 73+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-05 22:39 [RFC PATCH 00/11] Finer grained kernel address space randomization Kristen Carlson Accardi
2020-02-05 22:39 ` [RFC PATCH 01/11] modpost: Support >64K sections Kristen Carlson Accardi
2020-02-06 12:38   ` Kees Cook
2020-02-05 22:39 ` [RFC PATCH 02/11] x86: tools/relocs: Support >64K section headers Kristen Carlson Accardi
2020-02-06 12:39   ` Kees Cook
2020-02-05 22:39 ` [RFC PATCH 03/11] x86/boot: Allow a "silent" kaslr random byte fetch Kristen Carlson Accardi
2020-02-06  1:08   ` Andy Lutomirski
2020-02-06 11:48     ` Kees Cook
2020-02-06 16:58     ` Kristen Carlson Accardi
2020-02-05 22:39 ` [RFC PATCH 04/11] x86/boot/KASLR: Introduce PRNG for faster shuffling Kristen Carlson Accardi
2020-02-06  1:11   ` Andy Lutomirski
2020-02-06 15:10   ` Jason A. Donenfeld
2020-02-07  7:23     ` Jean-Philippe Aumasson
2020-02-07  9:05       ` Kees Cook
2020-02-07 16:52         ` Kristen Carlson Accardi
2020-02-05 22:39 ` [RFC PATCH 05/11] x86: Makefile: Add build and config option for CONFIG_FG_KASLR Kristen Carlson Accardi
2020-02-06 10:30   ` Peter Zijlstra
2020-02-06 11:52     ` Kees Cook
2020-02-25 17:55   ` Arvind Sankar
2020-02-26 19:13     ` Kristen Carlson Accardi
2020-03-24 21:24     ` Kristen Carlson Accardi
2020-03-25 15:34       ` Kees Cook
2020-02-05 22:39 ` [RFC PATCH 06/11] x86: make sure _etext includes function sections Kristen Carlson Accardi
2020-02-06 12:26   ` Kees Cook
2020-02-06 13:15     ` Jann Horn [this message]
2020-02-06 16:27       ` David Laight
2020-02-06 14:39     ` Arvind Sankar
2020-02-06 15:29       ` Arvind Sankar
2020-02-06 16:11         ` Andy Lutomirski
2020-02-06 14:57     ` Arvind Sankar
2020-02-06 15:45       ` Arvind Sankar
2020-02-06 19:41     ` Kristen Carlson Accardi
2020-02-06 20:02       ` Andy Lutomirski
2020-02-07  9:24         ` Peter Zijlstra
2020-02-10  1:43           ` Kees Cook
2020-02-10 10:51             ` Peter Zijlstra
2020-02-10 15:54               ` Arjan van de Ven
2020-02-10 16:36                 ` Arvind Sankar
2020-02-21 19:50                   ` Josh Poimboeuf
2020-02-21 23:05                     ` Arvind Sankar
2020-02-05 22:39 ` [RFC PATCH 07/11] x86/tools: Adding relative relocs for randomized functions Kristen Carlson Accardi
2020-02-06 12:37   ` Kees Cook
2020-02-05 22:39 ` [RFC PATCH 08/11] x86: Add support for finer grained KASLR Kristen Carlson Accardi
2020-02-06  1:17   ` Andy Lutomirski
2020-02-06 11:56     ` Kees Cook
2020-02-06 17:36       ` Kristen Carlson Accardi
2020-02-06 10:38   ` Peter Zijlstra
2020-02-06 12:06     ` Kees Cook
2020-02-06 14:52       ` Peter Zijlstra
2020-02-06 17:25         ` Kristen Carlson Accardi
2020-02-06 17:35           ` Peter Zijlstra
2020-02-06 17:43             ` Kristen Carlson Accardi
2020-02-25 17:49   ` Arvind Sankar
2020-02-26 19:26     ` Kristen Carlson Accardi
2020-02-05 22:39 ` [RFC PATCH 09/11] kallsyms: hide layout and expose seed Kristen Carlson Accardi
2020-02-06 12:32   ` Kees Cook
2020-02-06 17:51     ` Kristen Carlson Accardi
2020-02-06 19:27       ` Jann Horn
2020-03-02 19:01         ` Kristen Carlson Accardi
2020-03-02 19:08           ` Kees Cook
2020-03-02 19:19             ` Kristen Carlson Accardi
2020-02-27  2:42       ` Baoquan He
2020-02-27 16:02         ` Kees Cook
2020-02-28  3:36           ` Baoquan He
2020-02-05 22:39 ` [RFC PATCH 10/11] module: Reorder functions Kristen Carlson Accardi
2020-02-06 12:41   ` Kees Cook
2020-02-11 12:39     ` Jessica Yu
2020-02-05 22:39 ` [RFC PATCH 11/11] x86/boot: Move "boot heap" out of .bss Kristen Carlson Accardi
2020-02-06  0:11   ` Arvind Sankar
2020-02-06  0:33     ` Kristen Carlson Accardi
2020-02-06 11:13     ` Kees Cook
2020-02-06 14:25       ` Arvind Sankar
2020-02-06 21:32         ` Kees Cook

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