Re: [PATCH v3 4/4] x86, boot: Memory hotplug support for KASLR memory randomization

[Kees Cook]

On Tue, May 3, 2016 at 12:31 PM, Thomas Garnier <thgarnie@xxxxxxxxxx> wrote:
> Add a new option (CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING) to define
> the padding used for the physical memory mapping section when KASLR
> memory is enabled. It ensures there is enough virtual address space when
> CONFIG_MEMORY_HOTPLUG is used. The default value is 10 terabytes. If
> CONFIG_MEMORY_HOTPLUG is not used, no space is reserved increasing the
> entropy available.
>
> Signed-off-by: Thomas Garnier <thgarnie@xxxxxxxxxx>
> ---
> Based on next-20160502
> ---
>  arch/x86/Kconfig    | 15 +++++++++++++++
>  arch/x86/mm/kaslr.c | 14 ++++++++++++--
>  2 files changed, 27 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 60f33c7..5124d9c 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -2003,6 +2003,21 @@ config RANDOMIZE_MEMORY
>
>            If unsure, say N.
>
> +config RANDOMIZE_MEMORY_PHYSICAL_PADDING
> +       hex "Physical memory mapping padding" if EXPERT
> +       depends on RANDOMIZE_MEMORY
> +       default "0xa" if MEMORY_HOTPLUG
> +       default "0x0"
> +       range 0x1 0x40 if MEMORY_HOTPLUG
> +       range 0x0 0x40
> +       ---help---
> +          Define the padding in terabyte added to the existing physical memory
> +          size during kernel memory randomization. It is useful for memory
> +          hotplug support but reduces the entropy available for address
> +          randomization.
> +
> +          If unsure, leave at the default value.
> +
>  config HOTPLUG_CPU
>         bool "Support for hot-pluggable CPUs"
>         depends on SMP
> diff --git a/arch/x86/mm/kaslr.c b/arch/x86/mm/kaslr.c
> index 3b330a9..ef3dc19 100644
> --- a/arch/x86/mm/kaslr.c
> +++ b/arch/x86/mm/kaslr.c
> @@ -68,15 +68,25 @@ void __init kernel_randomize_memory(void)
>  {
>         size_t i;
>         unsigned long addr = memory_rand_start;
> -       unsigned long padding, rand, mem_tb;
> +       unsigned long padding, rand, mem_tb, page_offset_padding;
>         struct rnd_state rnd_st;
>         unsigned long remain_padding = memory_rand_end - memory_rand_start;
>
>         if (!kaslr_enabled())
>                 return;
>
> +       /*
> +        * Update Physical memory mapping to available and
> +        * add padding if needed (especially for memory hotplug support).
> +        */
> +       page_offset_padding = CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING;
> +
> +#ifdef CONFIG_MEMORY_HOTPLUG
> +       page_offset_padding = max(1UL, page_offset_padding);
> +#endif

Can't the ifdef and max lines be dropped? The Kconfig already enforces
the range to have a minimum of 1 when CONFIG_MEMORY_HOTPLUG is used,
IIUC.

> +
>         BUG_ON(kaslr_regions[0].base != &page_offset_base);
> -       mem_tb = ((max_pfn << PAGE_SHIFT) >> TB_SHIFT);
> +       mem_tb = ((max_pfn << PAGE_SHIFT) >> TB_SHIFT) + page_offset_padding;

In fact, can't this variable be entirely dropped and the mem_tb
calculation could just refer to RANDOMIZE_MEMORY_PHYSICAL_PADDING
directly?

-Kees

>
>         if (mem_tb < kaslr_regions[0].size_tb)
>                 kaslr_regions[0].size_tb = mem_tb;
> --
> 2.8.0.rc3.226.g39d4020
>



-- 
Kees Cook
Chrome OS & Brillo Security