[PATCH 2/2] x86/mm/KASLR: Calculate the actual size of vmemmap region

[Baoquan He]

Vmemmap region has different maximum size depending on paging mode.
However, its size is hardcoded as 1TB during memory KASLR handling,
this is not right for 5-level paging mode since it will cause vital
stumping if vmemmap region is randomized to be very close to the
following cpu_entry_area region and the actual size of vmemmap is much
bigger than 1 TB.

So here calculate how many TB needed by the actual size of vmemmap
region and align up to 1TB boundary. In 4-level the size will be
1 TB always since the max is 1 TB. In 5-level it's variable so that
space can be saved for randomization.

Signed-off-by: Baoquan He <bhe@xxxxxxxxxx>
---
 arch/x86/mm/kaslr.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm/kaslr.c b/arch/x86/mm/kaslr.c
index 387d4ed25d7c..4679a0075048 100644
--- a/arch/x86/mm/kaslr.c
+++ b/arch/x86/mm/kaslr.c
@@ -52,7 +52,7 @@ static __initdata struct kaslr_memory_region {
 } kaslr_regions[] = {
 	{ &page_offset_base, 0 },
 	{ &vmalloc_base, 0 },
-	{ &vmemmap_base, 1 },
+	{ &vmemmap_base, 0 },
 };
 
 /* Get size in bytes used by the memory region */
@@ -78,6 +78,7 @@ void __init kernel_randomize_memory(void)
 	unsigned long rand, memory_tb;
 	struct rnd_state rand_state;
 	unsigned long remain_entropy;
+	unsigned long vmemmap_size;
 
 	vaddr_start = pgtable_l5_enabled() ? __PAGE_OFFSET_BASE_L5 : __PAGE_OFFSET_BASE_L4;
 	vaddr = vaddr_start;
@@ -109,6 +110,14 @@ void __init kernel_randomize_memory(void)
 	if (memory_tb < kaslr_regions[0].size_tb)
 		kaslr_regions[0].size_tb = memory_tb;
 
+	/**
+	 * Calculate how many TB vmemmap region needs, and aligned to
+	 * 1TB boundary.
+	 */
+	vmemmap_size = (kaslr_regions[0].size_tb << (TB_SHIFT - PAGE_SHIFT)) *
+		sizeof(struct page);
+	kaslr_regions[2].size_tb = DIV_ROUND_UP(vmemmap_size, 1UL << TB_SHIFT);
+
 	/* Calculate entropy available between regions */
 	remain_entropy = vaddr_end - vaddr_start;
 	for (i = 0; i < ARRAY_SIZE(kaslr_regions); i++)
-- 
2.17.2