[PATCH v2 1/2] x86/mm: Add an option to change the padding used for the physical memory mapping

[Masayoshi Mizuma]

From: Masayoshi Mizuma <m.mizuma@xxxxxxxxxxxxxx>

There are some exceptional cases that the padding used for the physical
memory mapping section is not enough.

For example of the cases:
- As Baoquan reported in the following, SGI UV system.
  https://lkml.org/lkml/2017/9/7/87
- Each node of physical memory layout has huge space for hotplug.
  For exapmle of the layout:
    SRAT: Node 6 PXM 4 [mem 0x100000000000-0x13ffffffffff] hotplug
    SRAT: Node 7 PXM 5 [mem 0x140000000000-0x17ffffffffff] hotplug
    SRAT: Node 2 PXM 6 [mem 0x180000000000-0x1bffffffffff] hotplug
    SRAT: Node 3 PXM 7 [mem 0x1c0000000000-0x1fffffffffff] hotplug

We can increase the padding by CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING,
however, the available entropy is decreased if the padding is increased.
So the config change is not very good for the other most of systems.
And, the needed padding size depends on the system environment, for
example physical memory layout, so the kernel option is better than
changing the config.

Signed-off-by: Masayoshi Mizuma <m.mizuma@xxxxxxxxxxxxxx>
---
 arch/x86/mm/kaslr.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm/kaslr.c b/arch/x86/mm/kaslr.c
index 61db77b0eda9..aeb62615bd46 100644
--- a/arch/x86/mm/kaslr.c
+++ b/arch/x86/mm/kaslr.c
@@ -40,6 +40,7 @@
  */
 static const unsigned long vaddr_end = CPU_ENTRY_AREA_BASE;
 
+static int __initdata rand_mem_physical_padding = CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING;
 /*
  * Memory regions randomized by KASLR (except modules that use a separate logic
  * earlier during boot). The list is ordered based on virtual addresses. This
@@ -69,6 +70,20 @@ static inline bool kaslr_memory_enabled(void)
 	return kaslr_enabled() && !IS_ENABLED(CONFIG_KASAN);
 }
 
+static int __init rand_mem_physical_padding_setup(char *str)
+{
+	int max_padding = (1 << (MAX_PHYSMEM_BITS - TB_SHIFT)) - 1;
+
+	get_option(&str, &rand_mem_physical_padding);
+	if (rand_mem_physical_padding < 0)
+		rand_mem_physical_padding = 0;
+	else if (rand_mem_physical_padding > max_padding)
+		rand_mem_physical_padding = max_padding;
+
+	return 0;
+}
+early_param("rand_mem_physical_padding", rand_mem_physical_padding_setup);
+
 /* Initialize base and padding for each memory region randomized with KASLR */
 void __init kernel_randomize_memory(void)
 {
@@ -102,7 +117,7 @@ void __init kernel_randomize_memory(void)
 	 */
 	BUG_ON(kaslr_regions[0].base != &page_offset_base);
 	memory_tb = DIV_ROUND_UP(max_pfn << PAGE_SHIFT, 1UL << TB_SHIFT) +
-		CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING;
+		rand_mem_physical_padding;
 
 	/* Adapt phyiscal memory region size based on available memory */
 	if (memory_tb < kaslr_regions[0].size_tb)
-- 
2.18.0