Re: [PATCH v3 4/6] x86/mm/KASLR: Fix the wrong calculation of memory region initial size

[Kees Cook]

On Sat, Feb 16, 2019 at 6:03 AM Baoquan He <bhe@xxxxxxxxxx> wrote:
>
> In memory region KASLR, __PHYSICAL_MASK_SHIFT is taken to calculate
> the initial size of the direct mapping region. This is correct in
> the old code where __PHYSICAL_MASK_SHIFT was equal to MAX_PHYSMEM_BITS,
> 46 bits, and only 4-level mode was supported.
>
> Later, in commit b83ce5ee91471d ("x86/mm/64: Make __PHYSICAL_MASK_SHIFT
> always 52"), __PHYSICAL_MASK_SHIFT was changed to be always 52 bits, no
> matter it's 5-level or 4-level. This is wrong for 4-level paging. Then
> when we adapt physical memory region size based on available memory, it
> will overflow if the amount of system RAM and the padding is bigger
> than 64 TB.
>
> In fact, here MAX_PHYSMEM_BITS should be used 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>

Nice catch! I wish I had a system with >64TB RAM. ;)

Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>

> ---
>  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 bf680929fe26..97768df923e3 100644
> --- a/arch/x86/mm/kaslr.c
> +++ b/arch/x86/mm/kaslr.c
> @@ -137,7 +137,7 @@ 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
>


-- 
Kees Cook