linux-kernel - Re: [PATCH 02/23] x86, kaiser: do not set _PAGE_USER for init

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Date:   Wed, 1 Nov 2017 22:11:37 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        moritz.lipp@...k.tugraz.at, daniel.gruss@...k.tugraz.at,
        michael.schwarz@...k.tugraz.at, luto@...nel.org,
        torvalds@...ux-foundation.org, keescook@...gle.com,
        hughd@...gle.com, x86@...nel.org
Subject: Re: [PATCH 02/23] x86, kaiser: do not set _PAGE_USER for init_mm
 page tables

On Tue, 31 Oct 2017, Dave Hansen wrote:

> 
> init_mm is for kernel-exclusive use.  If someone is allocating page
> tables in it, do not set _PAGE_USER on them.  This ensures that
> we do *not* set NX on these page tables in the KAISER code.

This changelog is confusing at best.

Why is this a kaiser issue? Nothing should ever create _PAGE_USER entries
in init_mm, right?

So this is a general improvement and creating a _PAGE_USER entry in init_mm
should be considered a bug in the first place.

> +/*
> + * _KERNPG_TABLE has _PAGE_USER clear which tells the KAISER code
> + * that this mapping is for kernel use only.  That makes sure that
> + * we leave the mapping usable by the kernel and do not try to
> + * sabotage it by doing stuff like setting _PAGE_NX on it.

So this comment should not mention KAISER at all. As I explained above
there are no user mappings in init_mm and this should be expressed here.

The fact that KAISER can make use of this information is a different story.

Other than that:

      Reviewed-by: Thomas Gleixner <tglx@...utronix.de>

Thanks,

	tglx