Re: [PATCH v5 4/4] x86/mm/KASLR: Adjust the padding size for the direct mapping.

[Baoquan He <bhe@redhat.com>]

On 11/15/19 at 09:49am, Masayoshi Mizuma wrote:
> From: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>
> 
> The system sometimes crashes while memory hot-adding on KASLR
> enabled system. The crash happens because the regions pointed by
> kaslr_regions[].base are overwritten by the hot-added memory.
> 
> It happens because of the padding size for kaslr_regions[].base isn't
> enough for the system whose physical memory layout has huge space for
> memory hotplug. kaslr_regions[].base points "actual installed
> memory size + padding" or higher address. So, if the "actual + padding"
> is lower address than the maximum memory address, which means the memory
> address reachable by memory hot-add, kaslr_regions[].base is destroyed by
> the overwritten.
> 
>   address
>     ^
>     |------- maximum memory address (Hotplug)
>     |                                    ^
>     |------- kaslr_regions[0].base       | Hotadd-able region
>     |     ^                              |
>     |     | padding                      |
>     |     V                              V
>     |------- actual memory address (Installed on boot)
>     |
> 
> Fix it by getting the maximum memory address from SRAT and store
> the value in boot_param, then set the padding size while KASLR
> initializing if the default padding size isn't enough.
> 
> Signed-off-by: Baoquan He <bhe@redhat.com>
> Signed-off-by: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>
> ---
>  arch/x86/mm/kaslr.c | 57 +++++++++++++++++++++++++++++++++------------
>  1 file changed, 42 insertions(+), 15 deletions(-)
> 
> diff --git a/arch/x86/mm/kaslr.c b/arch/x86/mm/kaslr.c
> index dc6182eecefa..1f0aa9e68226 100644
> --- a/arch/x86/mm/kaslr.c
> +++ b/arch/x86/mm/kaslr.c
> @@ -70,15 +70,52 @@ 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.

The doc doesn't reflect the the case of boot_params.max_addr existed?

Otherwise, it looks good to me and easy to understand. You can remove my
'Signed-off' and feel free to add my ack.

Acked-by: Baoquan He <bhe@redhat.com>

Thanks
Baoquan

> + */
> +static inline unsigned long calc_direct_mapping_size(void)
> +{
> +	unsigned long size_tb, memory_tb;
> +
> +	memory_tb = DIV_ROUND_UP(max_pfn << PAGE_SHIFT, 1UL << TB_SHIFT) +
> +		CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING;
> +
> +#ifdef CONFIG_MEMORY_HOTPLUG
> +	if (boot_params.max_addr) {
> +		unsigned long maximum_tb;
> +
> +		maximum_tb = DIV_ROUND_UP(boot_params.max_addr,
> +				1UL << TB_SHIFT);
> +
> +		if (maximum_tb > memory_tb)
> +			memory_tb = maximum_tb;
> +	}
> +#endif
> +	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 +132,10 @@ 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
>