linux-kernel - Re: __randomize

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Message-ID: <1B6E0664-D1B5-4FCC-A874-8A9587924687@kernel.org>
Date: Wed, 27 Mar 2024 23:26:51 -0600
From: Kees Cook <kees@...nel.org>
To: "Luck, Tony" <tony.luck@...el.com>, Kees Cook <keescook@...omium.org>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 linux-hardening@...r.kernel.org
Subject: Re: __randomize_layout;



On March 27, 2024 9:21:59 AM MDT, "Luck, Tony" <tony.luck@...el.com> wrote:
>This e-mail is to check you on whether that __randomize_layout can shuffle the
>fields inside that nested union/structure. I tried some experiments, and in a
>few kernel builds I saw the whole block move to different offsets, but the order
>of x86_vendor, x86, x86_model, and x86_reserved was preserved.

Yes, this is an intentional behavior: __randomize_layout will only apply to the struct it is attached to, and is not enabled for any substructs (anonymous or otherwise).

>But experiments aren't proof. Nor defense against future versions of
>scripts/gcc-plugins/randomize_layout_plugin.c becoming smarter or
>more aggressive about changing layout.

The behavior is also supported natively by Clang -- neither implementation is likely to ever change its treatment of substructs as it would kind of cause chaos.

So you're all good! :)

-- 
Kees Cook