linux-kernel - Re: [PATCH v4 4/4] x86/mm/KASLR: Adjust the padding size for the direct mapping.

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Date:   Mon, 4 Nov 2019 08:48:25 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Masayoshi Mizuma <msys.mizuma@...il.com>
Cc:     Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Masayoshi Mizuma <m.mizuma@...fujitsu.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 4/4] x86/mm/KASLR: Adjust the padding size for the
 direct  mapping.

On 11/01/19 at 09:09pm, Masayoshi Mizuma wrote:
> ---
>  arch/x86/mm/kaslr.c | 65 ++++++++++++++++++++++++++++++++++-----------
>  1 file changed, 50 insertions(+), 15 deletions(-)
> 
> diff --git a/arch/x86/mm/kaslr.c b/arch/x86/mm/kaslr.c
> index dc6182eec..a80eed563 100644
> --- a/arch/x86/mm/kaslr.c
> +++ b/arch/x86/mm/kaslr.c
> @@ -70,15 +70,60 @@ static inline bool kaslr_memory_enabled(void)
>  	return kaslr_enabled() && !IS_ENABLED(CONFIG_KASAN);
>  }
>  
> +/*
> + * Even though a huge virtual address space is reserved for the direct
> + * mapping of physical memory, e.g in 4-level paging mode, it's 64TB,
> + * rare system can own enough physical memory to use it up, most are
> + * even less than 1TB. So with KASLR enabled, we adapt the size of
> + * direct mapping area to the size of actual physical memory plus the
> + * configured padding CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING.
> + * The left part will be taken out to join memory randomization.
> + */
> +static inline unsigned long calc_direct_mapping_size(void)
> +{
> +	unsigned long padding = CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING;
> +	unsigned long size_tb, memory_tb;
> +#ifdef CONFIG_MEMORY_HOTPLUG
> +	unsigned long actual, maximum, base;
> +
> +	if (boot_params.max_addr) {
> +		/*
> +		 * The padding size should set to get for kaslr_regions[].base
> +		 * bigger address than the maximum memory address the system can
> +		 * have. kaslr_regions[].base points "actual size + padding" or
> +		 * higher address. If "actual size + padding" points the lower
> +		 * address than the maximum memory size, fix the padding size.
> +		 */
> +		actual = roundup(PFN_PHYS(max_pfn), 1UL << TB_SHIFT);
> +		maximum = roundup(boot_params.max_addr, 1UL << TB_SHIFT);
> +		base = actual + (padding << TB_SHIFT);
> +
> +		if (maximum > base)
> +			padding = (maximum - actual) >> TB_SHIFT;
> +	}
> +#endif
> +	memory_tb =  DIV_ROUND_UP(max_pfn << PAGE_SHIFT, 1UL << TB_SHIFT) +
> +			padding;

Yes, wrapping up the whole adjusting code block for the direct mapping
area into a function looks much better. This was also suggested by Ingo
when I posted UV system issue fix before, just later the UV system issue
is not seen in the current code.

However, I have a small concern about the memory_tb calculateion here.
We can treat the (actual RAM + CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING)
as the default memory_tb, then check if we need adjst it according to
boot_params.max_addr. Discarding the local padding variable can make
code much simpler? And it is a little confusing when mix with the
later padding concept when doing randomization, I mean the get_padding()
thing.


	memory_tb = DIV_ROUND_UP(max_pfn << PAGE_SHIFT, 1UL << TB_SHIFT) +
                CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING;

	if (boot_params.max_addr) {
		maximum = roundup(boot_params.max_addr, 1UL << TB_SHIFT);

		if (maximum > memory_tb)
			memory_tb = maximum;
	}
#endif

Personal opinion. Anyway, this patch looks good to me. Thanks.

Thanks
Baoquan


> +
> +	size_tb = 1 << (MAX_PHYSMEM_BITS - TB_SHIFT);
> +
> +	/*
> +	 * Adapt physical memory region size based on available memory
> +	 */
> +	if (memory_tb < size_tb)
> +		size_tb = memory_tb;
> +
> +	return size_tb;
> +}
> +
>  /* Initialize base and padding for each memory region randomized with KASLR */
>  void __init kernel_randomize_memory(void)
>  {
> -	size_t i;
> -	unsigned long vaddr_start, vaddr;
> -	unsigned long rand, memory_tb;
> -	struct rnd_state rand_state;
> +	unsigned long vaddr_start, vaddr, rand;
>  	unsigned long remain_entropy;
>  	unsigned long vmemmap_size;
> +	struct rnd_state rand_state;
> +	size_t i;
>  
>  	vaddr_start = pgtable_l5_enabled() ? __PAGE_OFFSET_BASE_L5 : __PAGE_OFFSET_BASE_L4;
>  	vaddr = vaddr_start;
> @@ -95,20 +140,10 @@ void __init kernel_randomize_memory(void)
>  	if (!kaslr_memory_enabled())
>  		return;
>  
> -	kaslr_regions[0].size_tb = 1 << (MAX_PHYSMEM_BITS - TB_SHIFT);
> +	kaslr_regions[0].size_tb = calc_direct_mapping_size();
>  	kaslr_regions[1].size_tb = VMALLOC_SIZE_TB;
>  
> -	/*
> -	 * Update Physical memory mapping to available and
> -	 * add padding if needed (especially for memory hotplug support).
> -	 */
>  	BUG_ON(kaslr_regions[0].base != &page_offset_base);
> -	memory_tb = DIV_ROUND_UP(max_pfn << PAGE_SHIFT, 1UL << TB_SHIFT) +
> -		CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING;
> -
> -	/* Adapt phyiscal memory region size based on available memory */
> -	if (memory_tb < kaslr_regions[0].size_tb)
> -		kaslr_regions[0].size_tb = memory_tb;
>  
>  	/*
>  	 * Calculate the vmemmap region size in TBs, aligned to a TB
> -- 
> 2.20.1
>