linux-kernel - Re: [PATCH] mm: Stop PMD alignment for PIE shared objects

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Message-ID: <Z2W1ZCc1oN67dOHW@athena.aquini.home>
Date: Fri, 20 Dec 2024 13:20:20 -0500
From: Rafael Aquini <raquini@...hat.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: Audra Mitchell <audra@...hat.com>, linux-mm@...ck.org, aris@...hat.com,
	akpm@...ux-foundation.org, william.kucharski@...cle.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: Stop PMD alignment for PIE shared objects

On Thu, Dec 19, 2024 at 09:30:58PM +0000, Matthew Wilcox wrote:
> On Thu, Dec 19, 2024 at 04:15:52PM -0500, Audra Mitchell wrote:
> > After commit 1854bc6e2420 ("mm/readahead: Align file mappings for non-DAX")
> > any request through thp_get_unmapped_area would align to a PMD_SIZE,
> > causing shared objects to have less randomization than previously (9 less
> > bits for 2MB PMDs). As these lower 9 bits are the most impactful for
> > ASLR, this change could be argued to have an impact on security.
> 
> Yes, very tiresome people have been making that argument for a long
> time.  Do you have anything further to add to the discussion that
> happened here:
> 
> https://lore.kernel.org/linux-mm/20240118133504.2910955-1-shy828301@gmail.com/
> 
> particularly in light of 3afb76a66b55 existing.
>

Linus, however, disliked the config question and ended up reverting 
3afb76a66b55 a while after.