[PATCH v6 3/3] x86/mm: Memory hotplug support for KASLR memory randomization

From: Kees Cook
Date: Wed May 25 2016 - 19:05:57 EST


From: Thomas Garnier <thgarnie@xxxxxxxxxx>

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>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
arch/x86/Kconfig | 15 +++++++++++++++
arch/x86/mm/kaslr.c | 7 ++++++-
2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index adab3fef3bb4..214b3fadbc11 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2010,6 +2010,21 @@ 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 terabytes 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 7a1aa44cff1b..0c9264ed9357 100644
--- a/arch/x86/mm/kaslr.c
+++ b/arch/x86/mm/kaslr.c
@@ -109,8 +109,13 @@ void __init kernel_randomize_memory(void)
if (!kaslr_enabled())
return;

+ /*
+ * Update Physical memory mapping to available and
+ * add padding if needed (especially for memory hotplug support).
+ */
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) +
+ CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING;

if (mem_tb < kaslr_regions[0].size_tb)
kaslr_regions[0].size_tb = mem_tb;
--
2.6.3