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SubjectRe: [PATCH v2 1/2] x86/mm/KASLR: Fix the size of the direct mapping section
On 04/12/19 at 02:55pm, Baoquan He wrote:
> 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 74TB ~120TB
~~ I could use tab, will resend to
correct this.
>
> What we want is the following:
>
> |_______actual RAM_______|_________the rest_______________|
> 0 64TB ~120TB
>
> 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@linux.intel.com>
> Reviewed-by: Thomas Garnier <thgarnie@google.com>
> Signed-off-by: Baoquan He <bhe@redhat.com>
> ---
> 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 9a8756517504..387d4ed25d7c 100644
> --- a/arch/x86/mm/kaslr.c
> +++ b/arch/x86/mm/kaslr.c
> @@ -94,7 +94,7 @@ 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
>

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