Re: [PATCH v3 5/6] x86/mm/KASLR: Calculate the actual size of vmemmap region

[Kees Cook]

On Sat, Feb 16, 2019 at 6:04 AM Baoquan He <bhe@xxxxxxxxxx> wrote:
>
> Vmemmap region has different maximum size depending on paging mode.
> Now its size is hardcoded as 1TB in memory KASLR, this is not
> right for 5-level paging mode. It will cause overflow if vmemmap
> region is randomized to be adjacent to cpu_entry_area region and
> its actual size is bigger than 1TB.
>
> So here calculate how many TB by the actual size of vmemmap region
> and align up to 1TB boundary.
>
> Signed-off-by: Baoquan He <bhe@xxxxxxxxxx>
> ---
>  arch/x86/mm/kaslr.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/mm/kaslr.c b/arch/x86/mm/kaslr.c
> index 97768df923e3..ca12ed4e5239 100644
> --- a/arch/x86/mm/kaslr.c
> +++ b/arch/x86/mm/kaslr.c
> @@ -101,7 +101,7 @@ static __initdata struct kaslr_memory_region {
>  } kaslr_regions[] = {
>         { &page_offset_base, 0 },
>         { &vmalloc_base, 0 },
> -       { &vmemmap_base, 1 },
> +       { &vmemmap_base, 0 },
>  };
>
>  /*
> @@ -121,6 +121,7 @@ void __init kernel_randomize_memory(void)
>         unsigned long rand, memory_tb;
>         struct rnd_state rand_state;
>         unsigned long remain_entropy;
> +       unsigned long vmemmap_size;
>
>         vaddr_start = pgtable_l5_enabled() ? __PAGE_OFFSET_BASE_L5 : __PAGE_OFFSET_BASE_L4;
>         vaddr = vaddr_start;
> @@ -152,6 +153,14 @@ void __init kernel_randomize_memory(void)
>         if (memory_tb < kaslr_regions[0].size_tb)
>                 kaslr_regions[0].size_tb = memory_tb;
>
> +       /*
> +        * Calculate how many TB vmemmap region needs, and align to
> +        * 1TB boundary.

Can you describe why this is the right calculation? (This will help
explain why 4-level is different from 5-level here.)

> +        */
> +       vmemmap_size = (kaslr_regions[0].size_tb << (TB_SHIFT - PAGE_SHIFT)) *
> +               sizeof(struct page);
> +       kaslr_regions[2].size_tb = DIV_ROUND_UP(vmemmap_size, 1UL << TB_SHIFT);
> +
>         /* Calculate entropy available between regions */
>         remain_entropy = vaddr_end - vaddr_start;
>         for (i = 0; i < ARRAY_SIZE(kaslr_regions); i++)
> --
> 2.17.2
>

Otherwise, yeah, looks great!

-- 
Kees Cook