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Message-ID: <72ea7056-5289-3eb0-eca9-a8444524667e@arm.com>
Date: Wed, 7 Oct 2020 08:20:14 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: David Hildenbrand <david@...hat.com>,
linux-arm-kernel@...ts.infradead.org
Cc: will@...nel.org, catalin.marinas@....com,
Mark Rutland <mark.rutland@....com>,
Ard Biesheuvel <ardb@...nel.org>,
Steven Price <steven.price@....com>,
Robin Murphy <robin.murphy@....com>,
Andrew Morton <akpm@...ux-foundation.org>,
linux-kernel@...r.kernel.org, Michal Hocko <mhocko@...nel.org>
Subject: Re: [PATCH] arm64/mm: Validate hotplug range before creating linear
mapping
On 10/06/2020 09:04 PM, David Hildenbrand wrote:
> 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@....com>
>> Cc: Will Deacon <will@...nel.org>
>> Cc: Mark Rutland <mark.rutland@....com>
>> Cc: Ard Biesheuvel <ardb@...nel.org>
>> Cc: Steven Price <steven.price@....com>
>> Cc: Robin Murphy <robin.murphy@....com>
>> Cc: David Hildenbrand <david@...hat.com>
>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>> Cc: linux-arm-kernel@...ts.infradead.org
>> Cc: linux-kernel@...r.kernel.org
>> Fixes: 4ab215061554 ("arm64: Add memory hotplug support")
>> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
>> ---
>> 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.
Initially, I thought about check_hotplug_memory_addressable() but the
existing check that asserts end of hotplug wrt MAX_PHYSMEM_BITS, is
generic in nature. AFAIK the linear mapping problem is arm64 specific,
hence I was not sure whether to add an arch specific callback which
will give platform an opportunity to weigh in for these ranges.
But hold on, check_hotplug_memory_addressable() only gets called from
__add_pages() after linear mapping creation in arch_add_memory(). How
would it help ? We need some thing for add_memory(), its variants and
also possibly for memremap_pages() when it calls arch_add_memory().
>
> 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
A structure based range.
>
> 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();
What would you suggest as the default fallback range if a platform
does not define this callback.
>
> if (range.start > max_phys) {
> range.start = 0;
> range.end = 0;
> }
> range.end = max_t(u64, range.end, max_phys);
min_t instead ?
>
> return range;
> }
>
>
> That, we can use in check_hotplug_memory_addressable(), and also allow
> add_memory*() users to make use of it.
So this check would happen twice during a hotplug ?
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