linux-hardening - Re: [PATCH v4 22/26] arm64: mm: move ro_after

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Date:   Thu, 16 Jun 2022 18:31:55 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-hardening@...r.kernel.org, Marc Zyngier <maz@...nel.org>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Brown <broonie@...nel.org>,
        Anshuman Khandual <anshuman.khandual@....com>
Subject: Re: [PATCH v4 22/26] arm64: mm: move ro_after_init section into the
 data segment

On Thu, 16 Jun 2022 at 18:18, Kees Cook <keescook@...omium.org> wrote:
>
> On Thu, Jun 16, 2022 at 01:31:23PM +0200, Ard Biesheuvel wrote:
> > We used to have
> >
> > text
> > --
> > rodata
> > (ro_after_init)
> > --
> > inittext
> > --
> > initdata
> > --
> > data
> > bss
> >
> > where -- are the segment boundaries, which are always aligned to 64k on arm64
> >
> > After this patch, we get
> >
> > text
> > --
> > rodata
> > --
> > inittext
> > --
> > initdata
> > --
> > (ro_after_init)
> > data
> > bss
> >
> > so in terms of padding due to alignment, there is not a lot of difference.
>
> But how is ro_after_init read-only and data isn't, if there isn't a
> segment alignment to make that work out?
>

Actually, there is a segment alignment between ro_after_init and data
- my diagram is inaccurate. But we don't actually need that to remap
this slice of memory r/o