[PATCH] arm64: kaslr: consider parange is bigger than linear_region_size

[Keun-O Park]

On Mon, Feb 24, 2025 at 10:21 AM Keun-O Park <kpark3469 at gmail.com> wrote:
>
> From: Keuno Park <keun-o.park at katim.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 at katim.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());
> +               }
> +
>                 /*
>                  * If the size of the linear region exceeds, by a sufficient
>                  * margin, the size of the region that the physical memory can
> --
> 2.34.1
>

36bit: linear_region_size=0x800000000 32 GB
39bit: linear_region_size=0x4000000000 256 GB
42bit: linear_region_size=0x20000000000 2048 GB, 2 TB
47bit: linear_region_size=0x400000000000 65536 GB, 64 TB
48bit: linear_region_size=0x800000000000 131072 GB, 128 TB
52bit: linear_region_size=0xf800000000000 4063232 GB, 3968 TB

The problem happens only when linear_region_size is smaller than
BIT(id_aa64mmfr0_parange_to_phys_shift(parange)).
In my case, SoC returns 2 as parange value. (va_bits is 39bit, so the
linear_region_size gets 0x4000000000, 256GB.)
#define ID_AA64MMFR0_PARANGE_40         0x2
BIT(id_aa64mmfr0_parange_to_phys_shift(parange)) == 0x10000000000, 1TB

                s64 range = linear_region_size -
                            BIT(id_aa64mmfr0_parange_to_phys_shift(parange));
As a result, range gets a negative value. 0xffffff4000000000 in here.
So this makes randomization code bypass as range is smaller than
ARM64_MEMSTART_ALIGN.

                if (memstart_offset_seed > 0 && range >=
(s64)ARM64_MEMSTART_ALIGN) {

In most cases, the hotplug memory code will be working the same as before.