Re: [RFC PATCH 06/11] x86: make sure _etext includes function sections

Andy Lutomirski
2020-02-06T16:11:52Z

> On Feb 6, 2020, at 7:29 AM, Arvind Sankar <nivedita@alum.mit.edu> wrote:
>
> ﻿On Thu, Feb 06, 2020 at 09:39:43AM -0500, Arvind Sankar wrote:
>>> On Thu, Feb 06, 2020 at 04:26:23AM -0800, Kees Cook wrote:
>>> 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.
>>>
>>
>> The default function alignment is 16-bytes for x64 at least with gcc.
>> You can use -falign-functions to specify a different alignment.
>>
>> There was some old discussion on reducing it [1] but it doesn't seem to
>> have been merged.
>>
>> [1] https://lore.kernel.org/lkml/tip-4874fe1eeb40b403a8c9d0ddeb4d166cab3f37ba@git.kernel.org/
>
> Though I don't think the entropy loss is real. With 50k functions, you
> can use at most log(50k!) = ~35 KiB worth of entropy in permuting them,
> no matter what the alignment is. The only way you can get more is if you
> have more than 50k slots to put them in.

There is a security consideration here that has nothing to do with entropy per se. If an attacker locates two functions, they learn the distance between them. This constrains what can fit in the gap. Padding reduces the strength of this type of attack, as would some degree of random padding.