linux-kernel - Re: [PATCH v3 RESEND 2/2] x86/mm/KASLR: Fix the size of vmemmap section

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Date:   Sun, 28 Apr 2019 21:54:08 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Baoquan He <bhe@...hat.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org, tglx@...utronix.de,
        mingo@...nel.org, bp@...en8.de, hpa@...or.com,
        kirill.shutemov@...ux.intel.com, keescook@...omium.org,
        peterz@...radead.org, thgarnie@...gle.com,
        herbert@...dor.apana.org.au, mike.travis@....com,
        frank.ramsay@....com, yamada.masahiro@...ionext.com
Subject: Re: [PATCH v3 RESEND 2/2] x86/mm/KASLR: Fix the size of vmemmap
 section

On Mon, Apr 22, 2019 at 05:10:45PM +0800, 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 method,
> the left space can be saved to join randomization to increase the entropy.
> 
> Signed-off-by: Baoquan He <bhe@...hat.com>
> ---
> v2->v3:
>   Fix typo Boris pointed out. 
> 
>  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;
>  
> +	/**

Nit: that is weird style for inline comment.

> +	 * 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);

Hm. Don't we need to take into account alignment requirements for struct
page here? I'm worried about some exotic debug kernel config where
sizeof(struct page) doesn't satify __alignof__(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++)
-- 
 Kirill A. Shutemov