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

[Thomas Garnier]

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