[RFC v1 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-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