Re: [PATCH] arm64/mm: Validate hotplug range before creating linear mapping

[David Hildenbrand]

On 17.09.20 10:46, Anshuman Khandual wrote:
> During memory hotplug process, the linear mapping should not be created for
> a given memory range if that would fall outside the maximum allowed linear
> range. Else it might cause memory corruption in the kernel virtual space.
> 
> Maximum linear mapping region is [PAGE_OFFSET..(PAGE_END -1)] accommodating
> both its ends but excluding PAGE_END. Max physical range that can be mapped
> inside this linear mapping range, must also be derived from its end points.
> 
> When CONFIG_ARM64_VA_BITS_52 is enabled, PAGE_OFFSET is computed with the
> assumption of 52 bits virtual address space. However, if the CPU does not
> support 52 bits, then it falls back using 48 bits instead and the PAGE_END
> is updated to reflect this using the vabits_actual. As for PAGE_OFFSET,
> bits [51..48] are ignored by the MMU and remain unchanged, even though the
> effective start address of linear map is now slightly different. Hence, to
> reliably check the physical address range mapped by the linear map, the
> start address should be calculated using vabits_actual. This ensures that
> arch_add_memory() validates memory hot add range for its potential linear
> mapping requirement, before creating it with __create_pgd_mapping().
> 
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: Ard Biesheuvel <ardb@xxxxxxxxxx>
> Cc: Steven Price <steven.price@xxxxxxx>
> Cc: Robin Murphy <robin.murphy@xxxxxxx>
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Fixes: 4ab215061554 ("arm64: Add memory hotplug support")
> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> ---
>  arch/arm64/mm/mmu.c | 27 +++++++++++++++++++++++++++
>  1 file changed, 27 insertions(+)
> 
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 75df62fea1b6..d59ffabb9c84 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -1433,11 +1433,38 @@ static void __remove_pgd_mapping(pgd_t *pgdir, unsigned long start, u64 size)
>  	free_empty_tables(start, end, PAGE_OFFSET, PAGE_END);
>  }
>  
> +static bool inside_linear_region(u64 start, u64 size)
> +{
> +	/*
> +	 * Linear mapping region is the range [PAGE_OFFSET..(PAGE_END - 1)]
> +	 * accommodating both its ends but excluding PAGE_END. Max physical
> +	 * range which can be mapped inside this linear mapping range, must
> +	 * also be derived from its end points.
> +	 *
> +	 * With CONFIG_ARM64_VA_BITS_52 enabled, PAGE_OFFSET is defined with
> +	 * the assumption of 52 bits virtual address space. However, if the
> +	 * CPU does not support 52 bits, it falls back using 48 bits and the
> +	 * PAGE_END is updated to reflect this using the vabits_actual. As
> +	 * for PAGE_OFFSET, bits [51..48] are ignored by the MMU and remain
> +	 * unchanged, even though the effective start address of linear map
> +	 * is now slightly different. Hence, to reliably check the physical
> +	 * address range mapped by the linear map, the start address should
> +	 * be calculated using vabits_actual.
> +	 */
> +	return ((start >= __pa(_PAGE_OFFSET(vabits_actual)))
> +			&& ((start + size) <= __pa(PAGE_END - 1)));
> +}
> +
>  int arch_add_memory(int nid, u64 start, u64 size,
>  		    struct mhp_params *params)
>  {
>  	int ret, flags = 0;
>  
> +	if (!inside_linear_region(start, size)) {
> +		pr_err("[%llx %llx] is outside linear mapping region\n", start, start + size);
> +		return -EINVAL;
> +	}
> +
>  	if (rodata_full || debug_pagealloc_enabled())
>  		flags = NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
>  
> 

Can we please provide a generic way to figure limits like that out,
especially, before calling add_memory() and friends?

We do have __add_pages()->check_hotplug_memory_addressable() where we
already check against MAX_PHYSMEM_BITS.

What I'd prefer is a way to get

1. Lower address limit we can use for add_memory() and friends
2. Upper address limit we can use for add_memory() and friends

something like


struct range memhp_get_addressable_range(void)
{
	const u64 max_phys = (1ull << (MAX_PHYSMEM_BITS + 1)) - 1;
	struct range range = arch_get_mappable_range();

	if (range.start > max_phys) {
		range.start = 0;
		range.end = 0;
	}
	range.end = max_t(u64, range.end, max_phys);

	return range;
}


That, we can use in check_hotplug_memory_addressable(), and also allow
add_memory*() users to make use of it.

-- 
Thanks,

David / dhildenb