linux-kernel - Re: [PATCH] arm64: kaslr: consider parange is bigger than linear_region

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Message-ID: <CAMj1kXG5uTUH3--A3OtPQcMqj4hiNrONL6tUKkmwqmjkOYMuyQ@mail.gmail.com>
Date: Mon, 24 Feb 2025 09:10:52 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: keun-o.park@...im.com
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	catalin.marinas@....com, will@...nel.org
Subject: Re: [PATCH] arm64: kaslr: consider parange is bigger than linear_region_size

On Mon, 24 Feb 2025 at 07:21, Keun-O Park <kpark3469@...il.com> wrote:
>
> From: Keuno Park <keun-o.park@...im.com>
>
> On systems using 4KB pages and having 39 VA_BITS, linear_region_size
> gets 256GiB space. It was observed that some SoCs such as Qualcomm
> QCM8550 returns 40bits of PA range from MMFR0_EL1. This leads range
> value to have minus as the variable range is s64, so that all the
> calculations for randomizing linear address space are skpped.
> As a result of this, the kernel's linear region is not randomized.
> For this case, this patch sets the range by calculating memblock
> DRAM range to randomize the linear region of kernel.
>
> Change-Id: Ib29e45f44928937881d514fb87b4cac828b5a3f5
> Fixes: 97d6786e0669 ("arm64: mm: account for hotplug memory when randomizing the linear region")
> Signed-off-by: Keuno Park <keun-o.park@...im.com>
> ---
>  arch/arm64/mm/init.c | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> index 9c0b8d9558fc..2ee657e2d60f 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -290,6 +290,11 @@ void __init arm64_memblock_init(void)
>                 s64 range = linear_region_size -
>                             BIT(id_aa64mmfr0_parange_to_phys_shift(parange));
>
> +               if (range < 0) {
> +                       range = linear_region_size -
> +                               (memblock_end_of_DRAM() - memblock_start_of_DRAM());
> +               }
> +

Please explain how this ensures that hotplug memory still works as expected.