linux-kernel - Re: [PATCH v2 04/13] arm64: decouple early fixmap init from linear mapping

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Message-ID: <CAKv+Gu9KUdJy5ozRpUjTJjLNh08K-Hsq7JLuQjFbWH2wT+CpUA@mail.gmail.com>
Date:	Wed, 6 Jan 2016 17:42:56 +0100
From:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:	James Morse <james.morse@....com>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	kernel-hardening@...ts.openwall.com,
	Will Deacon <will.deacon@....com>,
	Catalin Marinas <catalin.marinas@....com>,
	Mark Rutland <mark.rutland@....com>,
	Leif Lindholm <leif.lindholm@...aro.org>,
	Kees Cook <keescook@...omium.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Stuart Yoder <stuart.yoder@...escale.com>,
	Sharma Bhupesh <bhupesh.sharma@...escale.com>,
	Arnd Bergmann <arnd@...db.de>,
	Marc Zyngier <marc.zyngier@....com>,
	Christoffer Dall <christoffer.dall@...aro.org>
Subject: Re: [PATCH v2 04/13] arm64: decouple early fixmap init from linear mapping

On 6 January 2016 at 17:35, James Morse <james.morse@....com> wrote:
> Hi Ard!
>
> On 30/12/15 15:26, Ard Biesheuvel wrote:
>> Since the early fixmap page tables are populated using pages that are
>> part of the static footprint of the kernel, they are covered by the
>> initial kernel mapping, and we can refer to them without using __va/__pa
>> translations, which are tied to the linear mapping.
>>
>> Instead, let's introduce __phys_to_kimg, which will be tied to the kernel
>> virtual mapping, regardless of whether or not it intersects with the linear
>> mapping. This will allow us to move the kernel out of the linear mapping in
>> a subsequent patch.
>>
>
> I gave your arm64-kaslr-v2 branch a go on juno r1, currently with
> ARM64_RELOCATABLE_KERNEL=n, to find it didn't boot.
>
> git bisect pointed to this patch. From the debugger it looks like
> rubbish is ending up the page tables after early_fixmap_init(), printing
> bits of bm_pmd and friends shows these aren't zeroed.
>
> I think this is because the section(".pgdir") is dragging these outside
> the __bss_start/__bss_stop range that is zeroed in head.S:__mmap_switched().
>

Thanks for spotting that! This code runs happily on my Seattle A0, but
it is obviously incorrect.

> The following inelegant patch fixes this problem for me:
> ----------------------------%<----------------------------
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index a78fc5a882da..15fc9712ddc1 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -559,6 +559,7 @@ void __init early_fixmap_init(void)
>         if (pgd_none(*pgd)) {
>                 static pud_t bm_pud[PTRS_PER_PUD] __pgdir;
>
> +               memset(bm_pud, 0, sizeof(bm_pud));
>                 pgd_populate(&init_mm, pgd, bm_pud);
>                 memblock_reserve(__pa(bm_pud), sizeof(bm_pud));
>         }
> @@ -570,6 +571,7 @@ void __init early_fixmap_init(void)
>         if (pud_none(*pud)) {
>                 static pmd_t bm_pmd[PTRS_PER_PMD] __pgdir;
>
> +               memset(bm_pmd, 0, sizeof(bm_pmd));
>                 pud_populate(&init_mm, pud, bm_pmd);
>                 memblock_reserve(__pa(bm_pmd), sizeof(bm_pmd));
>         }
> @@ -580,6 +582,7 @@ void __init early_fixmap_init(void)
>         if (pmd_none(*pmd)) {
>                 static pte_t bm_pte[PTRS_PER_PTE] __pgdir;
>
> +               memset(bm_pte, 0, sizeof(bm_pte));
>                 pmd_populate_kernel(&init_mm, pmd, bm_pte);
>                 memblock_reserve(__pa(bm_pte), sizeof(bm_pte));
>         }
> ----------------------------%<----------------------------
>

Actually, this looks fine to me. I will fold this into my patch

NOTE: I have a -v3 version up on git.linaro.org now, with a couple of changes.

Thanks!

Ard.
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