On Tue, May 10, 2016 at 11:24 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> 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.
>
Sure, I thought I had a bug when enabling it before hotplug but it
seems to work so I can drop it.
>> +
>> 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?
>
Yes it can, I will do it on next iteration.
> -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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