From: Kristen Carlson Accardi <kristen@linux.intel.com> To: Kees Cook <keescook@chromium.org> Cc: 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, 06 Feb 2020 11:41:15 -0800 [thread overview] Message-ID: <75f0bd0365857ba4442ee69016b63764a8d2ad68.camel@linux.intel.com> (raw) In-Reply-To: <202002060408.84005CEFFD@keescook> 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. > > And we can't adjust function section alignment, or there is some > benefit to a larger alignment, I would like to have a way for the > linker > to specify the inter-section padding (or fill) bytes. Right now, the > FILL(0xnn) (or =0xnn) can be used to specify the padding bytes > _within_ > a section, but not between sections. Right now, BFD appears to 0-pad. > I'd > like that to be 0xCC so "guessing" addresses incorrectly will trigger > a trap. Padding the space between functions with int3 is easy to add during boot time, and I've got it on my todo list.
next prev parent reply other threads:[~2020-02-06 19:41 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 [this message] 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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