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

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Date:   Thu,  4 Apr 2019 10:03:14 +0800
From:   Baoquan He <bhe@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     x86@...nel.org, tglx@...utronix.de, mingo@...nel.org, bp@...en8.de,
        hpa@...or.com, kirill@...temov.name, keescook@...omium.org,
        yamada.masahiro@...ionext.com, dave.hansen@...ux.intel.com,
        luto@...nel.org, peterz@...radead.org, thgarnie@...gle.com,
        mike.travis@....com, frank.ramsay@....com,
        Baoquan He <bhe@...hat.com>
Subject: [PATCH 2/2] x86/mm/KASLR: Calculate the actual size of vmemmap region

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@...hat.com>
---
 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