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

[Anshuman Khandual]

On 09/29/2020 02:05 AM, Will Deacon wrote:
> On Thu, Sep 17, 2020 at 02:16:42PM +0530, 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)));
>> +}
> 
> Why isn't this implemented using the existing __is_lm_address()?

Not sure, if I understood your suggestion here. The physical address range
[start..start + size] needs to be checked against maximum physical range
that can be represented inside effective boundaries for the linear mapping
i.e [__pa(_PAGE_OFFSET(vabits_actual)..__pa(PAGE_END - 1)].

Are you suggesting [start..start + size] should be first be converted into
a virtual address range and then checked against __is_lm_addresses() ? But
is not deriving the physical range from from know limits of linear mapping
much cleaner ?