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    Subject[PATCH 4/4] x86, boot: Memory hotplug support for KASLR memory randomization
    Date
    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@google.com>
    ---
    Based on next-20160422
    ---
    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 7c786d4..cc01b69 100644
    --- a/arch/x86/Kconfig
    +++ b/arch/x86/Kconfig
    @@ -2018,6 +2018,21 @@ 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
    +
    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;

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