[PATCH v4 4/4] x86, boot: Memory hotplug support for KASLR memory randomization

[Thomas Garnier]

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-20160511
---
 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 f5ed150..0e5087d 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2005,6 +2005,21 @@ 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 2415178..5e86082 100644
--- a/arch/x86/mm/kaslr.c
+++ b/arch/x86/mm/kaslr.c
@@ -88,8 +88,13 @@ 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;
-- 
2.8.0.rc3.226.g39d4020