Re: [PATCH v4] x86/mm/KASLR: Fix the size of vmemmap section

[Baoquan He]

On 05/08/19 at 04:04pm, Baoquan He wrote:
> 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 way,
> the left space can be saved to join randomization to increase the entropy.
> 
> Signed-off-by: Baoquan He <bhe@xxxxxxxxxx>
> Acked-by: Kirill A. Shutemov <kirill@xxxxxxxxxxxxxxx>
> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

I think this's worth noticing stable tree:

Cc: stable@xxxxxxxxxxxxxxx

> ---
> v3->v4:
>   Fix the incorrect style of code comment;
>   Add ack tags from Kirill and Kees.
> v3 discussion is here:
>   http://lkml.kernel.org/r/20190422091045.GB3584@xxxxxxxxxxxxxxxxxxxxx
> 
>  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 dc3f058bdf9b..c0eedb85a92f 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
>