linux-hardening - Re: Limited/Broken functionality of ASLR for Libs >= 2MB

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Date: Tue, 16 Jan 2024 09:09:45 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: Matthew Wilcox <willy@...radead.org>, Kees Cook <keescook@...omium.org>, 
	Linux ARM <linux-arm-kernel@...ts.infradead.org>
Cc: mail@...otw.com, linux-hardening@...r.kernel.org, 
	Jakub Wilk <jwilk@...lk.net>, Salvatore Bonaccorso <carnil@...ian.org>, 
	Linux Memory Management List <linux-mm@...ck.org>, William Kucharski <william.kucharski@...cle.com>
Subject: Re: Limited/Broken functionality of ASLR for Libs >= 2MB

(cc Kees, LAKML)

https://lkml.kernel.org/r/69fa6015256613ed10aee996e181ebd4%40horotw.com

On Mon, 15 Jan 2024 at 21:46, Matthew Wilcox <willy@...radead.org> wrote:
>
...
> Yeah, I don't know either.  Outside my scope of expertise.
>
> I received a suggestion off-list that we only do the PMD alignment on
> 64-bit, which seems quite reasonable to me.  After all, I don't care
> about performance on 32-bit just as much as I don't care about security
> on 32-bit.
>

For context, the culprit is

commit 1854bc6e2420472676c5c90d3d6b15f6cd640e40
Author: William Kucharski <william.kucharski@...cle.com>
Date:   Sun Sep 22 08:43:15 2019 -0400

    mm/readahead: Align file mappings for non-DAX

    When we have the opportunity to use PMDs to map a file, we want to follow
    the same rules as DAX.

    Signed-off-by: William Kucharski <william.kucharski@...cle.com>
    Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>

which affects *all* 32-bit architectures not just i686. 32-bit ARM
user space is still being deployed widely, even on arm64 Chromebooks
running 64-bit kernels (at least up until recently) so unfortunately,
we're not quite at the point yet where we can just let it rot.