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
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