linux-kernel - [PATCH v2 2/2] x86/mm/KASLR: Fix the size of vmemmap section

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Message-Id: <20190412065542.18431-3-bhe@redhat.com>
Date:   Fri, 12 Apr 2019 14:55:42 +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,
        peterz@...radead.org, thgarnie@...gle.com,
        herbert@...dor.apana.org.au, mike.travis@....com,
        frank.ramsay@....com, yamada.masahiro@...ionext.com,
        Baoquan He <bhe@...hat.com>
Subject: [PATCH v2 2/2] x86/mm/KASLR: Fix the size of vmemmap section

kernel_randomize_memory() hardcodes the size of vmemmap section as 1 TB,
to support the maximum amount of system RAM in 4-level paging mode, 64 TB.

However, 1 TB is not enough in 5-level paging mode. Assuming the size of
struct page is 64 Bytes, To support 4 PB system RAM in 5-level, 64 TB of
vmemmap area is needed. The wrong hardcoding may cause vmemmap stamping
into the following cpu_entry_area section, if KASLR puts vmemmap very close
to cpu_entry_area , and the needed area of vmemmap is much bigger than
1 TB.

So here calculate the actual size of vmemmap region, then align up to 1 TB
boundary. In 4-level it's always 1 TB. In 5-level it's adjusted on demand.
The current code reserves 0.5 PB for vmemmap in 5-level. In this way, the
left space can be saved to join randomization to increase the entropy.

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