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

[Anshuman Khandual]

On 09/29/2020 08:52 PM, Will Deacon wrote:
> On Tue, Sep 29, 2020 at 01:34:24PM +0530, Anshuman Khandual wrote:
>>
>>
>> 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 ?
> 
> I just think having a function called "inside_linear_region()" as well as a
> macro called "__is_lm_address()" is weird when they have completely separate
> implementations. They're obviously trying to do the same thing, just the
> first one gets given physical address as parameters.
> 
> Implementing one in terms of the other is much better for maintenance.

/*
 * The linear kernel range starts at the bottom of the virtual address
 * space. Testing the top bit for the start of the region is a
 * sufficient check and avoids having to worry about the tag.
 */
#define __is_lm_address(addr)   (!(((u64)addr) & BIT(vabits_actual - 1)))

__is_lm_address() currently just check the highest bit in a virtual address
where the linear mapping ends i.e the lower half and all other kernel mapping
starts i.e the upper half. But I would believe, it misses the blind range
[_PAGE_OFFSET(VA_BITS).._PAGE_OFFSET(vabits_actual)] in some configurations,
even though it does not really affect anything because it gets ignored by the
MMU. Hence in current form __is_lm_address() cannot be used to derive maximum
linear range and it's corresponding physical range for hotplug range check.

But if __is_lm_address() checks against the effective linear range instead
i.e [_PAGE_OFFSET(vabits_actual)..(PAGE_END - 1)], it can be used for hot
plug physical range check there after. Perhaps something like this, though
not tested properly.

diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
index afa722504bfd..6da046b479d4 100644
--- a/arch/arm64/include/asm/memory.h
+++ b/arch/arm64/include/asm/memory.h
@@ -238,7 +238,10 @@ static inline const void *__tag_set(const void *addr, u8 tag)
  * space. Testing the top bit for the start of the region is a
  * sufficient check and avoids having to worry about the tag.
  */
-#define __is_lm_address(addr)  (!(((u64)addr) & BIT(vabits_actual - 1)))
+static inline bool __is_lm_address(unsigned long addr)
+{
+       return ((addr >= _PAGE_OFFSET(vabits_actual)) && (addr <= (PAGE_END - 1)));
+}
 
 #define __lm_to_phys(addr)     (((addr) + physvirt_offset))
 #define __kimg_to_phys(addr)   ((addr) - kimage_voffset)
diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index d59ffabb9c84..5750370a7e8c 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -1451,8 +1451,7 @@ static bool inside_linear_region(u64 start, u64 size)
         * 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)));
+       return __is_lm_address(__va(start)) && __is_lm_address(__va(start + size));
 }
 
 int arch_add_memory(int nid, u64 start, u64 size,