[PATCH] x86/mm/KASLR: Fix the size of the direct mapping section

[Baoquan He]

kernel_randomize_memory() uses __PHYSICAL_MASK_SHIFT to calculate
the maximum amount of system RAM supported. The size of the direct
mapping section is obtained from the smaller one of the below two
values:

 (actual system RAM size + padding size) vs (max system RAM size supported)

This calculation is wrong since commit:
b83ce5ee91471d ("x86/mm/64: Make __PHYSICAL_MASK_SHIFT always 52").

In commit b83ce5ee91471d, __PHYSICAL_MASK_SHIFT was changed to be 52,
regardless of whether it's using 4-level or 5-level page tables.
It will always use 4 PB as the maximum amount of system RAM, even
in 4-level paging mode where it should be 64 TB.  Thus the size of
the direct mapping section will always be the sum of the actual
system RAM size plus the padding size.

Even when the amount of system RAM is 64 TB, the following layout will
still be used. Obviously KALSR will be weakened significantly.

   |____|_______actual RAM_______|_padding_|______the rest_______|
   0            64TB                                            ~120TB

What we want is the following:

   |____|_______actual RAM_______|_________the rest______________|
   0            64TB                                            ~120TB

Here, the size of padding region can be configured with
CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING, 10 TB by default. The above
issue only exists when CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING is set
to a non-zero value. Otherwise, using __PHYSICAL_MASK_SHIFT doesn't
affect KASLR either.

So the code should use MAX_PHYSMEM_BITS instead. Fix it by replacing
__PHYSICAL_MASK_SHIFT with MAX_PHYSMEM_BITS.

Fixes: b83ce5ee9147 ("x86/mm/64: Make __PHYSICAL_MASK_SHIFT always 52")
Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Reviewed-by: Thomas Garnier <thgarnie@xxxxxxxxxx>
Signed-off-by: Baoquan He <bhe@xxxxxxxxxx>
---
 arch/x86/mm/kaslr.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/mm/kaslr.c b/arch/x86/mm/kaslr.c
index 78974ee5d97f..4679a0075048 100644
--- a/arch/x86/mm/kaslr.c
+++ b/arch/x86/mm/kaslr.c
@@ -95,7 +95,7 @@ void __init kernel_randomize_memory(void)
 	if (!kaslr_memory_enabled())
 		return;
 
-	kaslr_regions[0].size_tb = 1 << (__PHYSICAL_MASK_SHIFT - TB_SHIFT);
+	kaslr_regions[0].size_tb = 1 << (MAX_PHYSMEM_BITS - TB_SHIFT);
 	kaslr_regions[1].size_tb = VMALLOC_SIZE_TB;
 
 	/*
-- 
2.17.2