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

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Date:   Tue, 3 Aug 2021 17:12:47 +0200
From:   Lukas Bulwahn <lukas.bulwahn@...il.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        X86 ML <x86@...nel.org>, Randy Dunlap <rdunlap@...radead.org>,
        "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 <kernel-janitors@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH 0/9] Kconfig symbol clean-up on ./arch/x86/

On Tue, Aug 3, 2021 at 4:32 PM Dave Hansen <dave.hansen@...el.com> wrote:
>
> 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;
> ...
>

Yes, that is what I referred to.

> 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.

Thanks, Dave.

If the maintainers consider updates to comments making them consistent
with the code as worth being picked, I will turn your analysis into a
proper commit message and provide a patch to update that comment.

Lukas