Re: [PATCH v2 1/1] arm64: mm: correct the inside linear map boundaries during hotplug check

[Ard Biesheuvel <ardb@kernel.org>]

On Mon, 15 Feb 2021 at 20:22, Pavel Tatashin <pasha.tatashin@soleen.com> wrote:
>
> Memory hotplug may fail on systems with CONFIG_RANDOMIZE_BASE because the
> linear map range is not checked correctly.
>
> The start physical address that linear map covers can be actually at the
> end of the range because of randomization. Check that and if so reduce it
> to 0.
>
> This can be verified on QEMU with setting kaslr-seed to ~0ul:
>
> memstart_offset_seed = 0xffff
> START: __pa(_PAGE_OFFSET(vabits_actual)) = ffff9000c0000000
> END:   __pa(PAGE_END - 1) =  1000bfffffff
>
> Signed-off-by: Pavel Tatashin <pasha.tatashin@soleen.com>
> Fixes: 58284a901b42 ("arm64/mm: Validate hotplug range before creating linear mapping")
> Tested-by: Tyler Hicks <tyhicks@linux.microsoft.com>

> ---
>  arch/arm64/mm/mmu.c | 20 ++++++++++++++++++--
>  1 file changed, 18 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index ae0c3d023824..cc16443ea67f 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -1444,14 +1444,30 @@ static void __remove_pgd_mapping(pgd_t *pgdir, unsigned long start, u64 size)
>
>  static bool inside_linear_region(u64 start, u64 size)
>  {
> +       u64 start_linear_pa = __pa(_PAGE_OFFSET(vabits_actual));
> +       u64 end_linear_pa = __pa(PAGE_END - 1);
> +
> +       if (IS_ENABLED(CONFIG_RANDOMIZE_BASE)) {
> +               /*
> +                * Check for a wrap, it is possible because of randomized linear
> +                * mapping the start physical address is actually bigger than
> +                * the end physical address. In this case set start to zero
> +                * because [0, end_linear_pa] range must still be able to cover
> +                * all addressable physical addresses.
> +                */
> +               if (start_linear_pa > end_linear_pa)
> +                       start_linear_pa = 0;
> +       }
> +
> +       WARN_ON(start_linear_pa > end_linear_pa);
> +
>         /*
>          * Linear mapping region is the range [PAGE_OFFSET..(PAGE_END - 1)]
>          * accommodating both its ends but excluding PAGE_END. Max physical
>          * range which can be mapped inside this linear mapping range, must
>          * also be derived from its end points.
>          */
> -       return start >= __pa(_PAGE_OFFSET(vabits_actual)) &&
> -              (start + size - 1) <= __pa(PAGE_END - 1);

Can't we simply use signed arithmetic here? This expression works fine
if the quantities are all interpreted as s64 instead of u64


> +       return start >= start_linear_pa && (start + size - 1) <= end_linear_pa;
>  }
>
>  int arch_add_memory(int nid, u64 start, u64 size,
> --
> 2.25.1
>