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Subject[PATCH v2 RESEND 2/2] x86/mm/KASLR: Fix the size of vmemmap section
Date
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 for vmemmap 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 actual 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 new methor,
the left space can be saved to join randomization to increase the entropy.

Signed-off-by: Baoquan He <bhe@redhat.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
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