linux-kernel - Re: [PATCH] x86/mm: Do not shuffle CPU entry areas without KASLR

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Date:   Sat, 4 Mar 2023 00:04:30 +0100
From:   Michal Koutný <mkoutny@...e.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] x86/mm: Do not shuffle CPU entry areas without KASLR

On Fri, Mar 03, 2023 at 01:24:53PM -0800, Dave Hansen <dave.hansen@...el.com> wrote:
> Should this be kaslr_memory_enabled() or kaslr_enabled()?

Originally, I had chosen kaslr_enabled(), seeing the PGD requirement of
KASAN (whole randomization area CPU_ENTRY_AREA_MAP_SIZE would fit in PGD
afterall).

> The delta seems to be CONFIG_KASAN, and the cpu entry area randomization
> works just fine with KASAN after some recent fixes.

But then I found KASAN code trying to be smart and having the fixups,
hence I chickened out to kaslr_memory_enabled().

> I _think_ that makes cpu entry area randomization more like module
> randomization which would point toward kaslr_enabled().

<del>I understood the only difference between kaslr_enabled and
kaslr_memory_enabled is the PGD alignment of the respective regions.
(Although, I don't see where KASAN breaks with unaligned ranges except
for better efficiency of page tables.)</del>

I've just found your [1], wondering the similar.

That being said, I will send v2 with just kaslr_enabled() guard and
updated commit message to beware of KASAN fixups (when backporting).

Thanks,
Michal

[1] https://lore.kernel.org/r/299fbb80-e3ab-3b7c-3491-e85cac107930@intel.com/