From: Andy Lutomirski <luto@amacapital.net>
To: Kristen Carlson Accardi <kristen@linux.intel.com>
Cc: Kees Cook <keescook@chromium.org>,
tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
hpa@zytor.com, arjan@linux.intel.com, rick.p.edgecombe@intel.com,
x86@kernel.org, linux-kernel@vger.kernel.org,
kernel-hardening@lists.openwall.com
Subject: Re: [RFC PATCH 06/11] x86: make sure _etext includes function sections
Date: Thu, 6 Feb 2020 12:02:36 -0800 [thread overview]
Message-ID: <B413445A-F1F0-4FB7-AA9F-C5FF4CEFF5F5@amacapital.net> (raw)
In-Reply-To: <75f0bd0365857ba4442ee69016b63764a8d2ad68.camel@linux.intel.com>
> On Feb 6, 2020, at 11:41 AM, Kristen Carlson Accardi <kristen@linux.intel.com> wrote:
>
> On Thu, 2020-02-06 at 04:26 -0800, Kees Cook wrote:
>>> On Wed, Feb 05, 2020 at 02:39:45PM -0800, Kristen Carlson Accardi
>>> wrote:
>>> We will be using -ffunction-sections to place each function in
>>> it's own text section so it can be randomized at load time. The
>>> linker considers these .text.* sections "orphaned sections", and
>>> will place them after the first similar section (.text). However,
>>> we need to move _etext so that it is after both .text and .text.*
>>> We also need to calculate text size to include .text AND .text.*
>>
>> The dependency on the linker's orphan section handling is, I feel,
>> rather fragile (during work on CFI and generally building kernels
>> with
>> Clang's LLD linker, we keep tripping over difference between how BFD
>> and
>> LLD handle orphans). However, this is currently no way to perform a
>> section "pass through" where input sections retain their name as an
>> output section. (If anyone knows a way to do this, I'm all ears).
>>
>> Right now, you can only collect sections like this:
>>
>> .text : AT(ADDR(.text) - LOAD_OFFSET) {
>> *(.text.*)
>> }
>>
>> or let them be orphans, which then the linker attempts to find a
>> "similar" (code, data, etc) section to put them near:
>> https://sourceware.org/binutils/docs-2.33.1/ld/Orphan-Sections.html
>>
>> So, basically, yes, this works, but I'd like to see BFD and LLD grow
>> some kind of /PASSTHRU/ special section (like /DISCARD/), that would
>> let
>> a linker script specify _where_ these sections should roughly live.
>>
>> Related thoughts:
>>
>> I know x86_64 stack alignment is 16 bytes. I cannot find evidence for
>> what function start alignment should be. It seems the linker is 16
>> byte
>> aligning these functions, when I think no alignment is needed for
>> function starts, so we're wasting some memory (average 8 bytes per
>> function, at say 50,000 functions, so approaching 512KB) between
>> functions. If we can specify a 1 byte alignment for these orphan
>> sections, that would be nice, as mentioned in the cover letter: we
>> lose
>> a 4 bits of entropy to this alignment, since all randomized function
>> addresses will have their low bits set to zero.
>
> So, when I was developing this patch set, I initially ignored the value
> of sh_addralign and just packed the functions in one right after
> another when I did the new layout. They were byte aligned :). I later
> realized that I should probably pay attention to alignment and thus
> started respecting the value that was in sh_addralign. There is
> actually nothing stopping me from ignoring it again, other than I am
> concerned that I will make runtime performance suffer even more than I
> already have.
If you start randomizing *data* sections, then alignment matters.
Also, in the shiny new era of Intel-CPUs-can’t-handle-Jcc-spanning-a-cacheline, function alignment may actually matter. Sigh. The symptom will be horrible maybe-exploitable crashes on old microcode and “minimal performance impact” on new microcode. In this context, “minimal” may actually mean “huge, throw away your CPU and replace it with one from a different vendor.”
Of course, there doesn’t appear to be anything resembling credible public documentation for any of this.
next prev parent reply other threads:[~2020-02-06 20:02 UTC|newest]
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
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 [this message]
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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