[RFC v1 4/4] x86, boot: Memory hotplug support for KASLR memory randomization

Thomas Garnier
2016-04-15T22:04:07Z

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-20160413
---
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 9de807d..f7dc477 100644
--- a/arch/x86/mm/kaslr.c
+++ b/arch/x86/mm/kaslr.c
@@ -63,7 +63,7 @@ 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;

@@ -74,8 +74,18 @@ void __init kernel_randomize_memory(void)
if (!xen_domain())
page_offset_base -= __XEN_SPACE;

+ /*
+ * 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