linux-kernel - Re: [PATCH 0/9] Kconfig symbol clean-up on ./arch/x86/

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Date:   Tue, 3 Aug 2021 07:32:39 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Lukas Bulwahn <lukas.bulwahn@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, Randy Dunlap <rdunlap@...radead.org>
Cc:     "H . Peter Anvin" <hpa@...or.com>,
        Darren Hart <dvhart@...radead.org>,
        Andy Shevchenko <andy@...radead.org>,
        Lubomir Rintel <lkundrak@...sk>, Pavel Machek <pavel@....cz>,
        Lee Jones <lee.jones@...aro.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        platform-driver-x86@...r.kernel.org,
        kernel-janitors@...r.kernel.org, linux-kernel@...r.kernel.org,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH 0/9] Kconfig symbol clean-up on ./arch/x86/

On 8/3/21 4:35 AM, Lukas Bulwahn wrote:
>   - a reference to STRICT_IOMEM in arch/x86/mm/init.c
>     unclear to me: which exact config this refers to

Are you referring to the reference in this comment?

> +       /*
> +        * This must follow RAM test, since System RAM is considered a
> +        * restricted resource under CONFIG_STRICT_IOMEM.
> +        */
> +       if (iomem_is_exclusive(pagenr << PAGE_SHIFT)) {
> +               /* Low 1MB bypasses iomem restrictions. */
> +               if (pagenr < 256)
> +                       return 1;
...

That came from here:

> commit a4866aa812518ed1a37d8ea0c881dc946409de94
> Author: Kees Cook <keescook@...omium.org>
> Date:   Wed Apr 5 09:39:08 2017 -0700
>
>     mm: Tighten x86 /dev/mem with zeroing reads

Which also added this hunk:

>  #ifdef CONFIG_STRICT_DEVMEM
> +static inline int page_is_allowed(unsigned long pfn)
> +{
> +       return devmem_is_allowed(pfn);
> +}

and talks about CONFIG_STRICT_DEVMEM in the changelog:

>     mm: Tighten x86 /dev/mem with zeroing reads
>     
>     Under CONFIG_STRICT_DEVMEM, reading System RAM through /dev/mem is
>     disallowed. However, on x86, the first 1MB was always allowed for BIOS
...

It's a pretty safe guess that STRICT_IOMEM refers to CONFIG_STRICT_DEVMEM.