Re: [PATCH 10/15] x86: KASAN raw shadow memory PTE init

[Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>]

On 2025-02-06 at 00:45:49 +0100, Andrey Konovalov wrote:
>On Tue, Feb 4, 2025 at 6:36 PM Maciej Wieczor-Retman
><maciej.wieczor-retman@intel.com> wrote:
>>
>> In KASAN's generic mode the default value in shadow memory is zero.
>> During initialization of shadow memory pages they are allocated and
>> zeroed.
>>
>> In KASAN's tag-based mode the default tag for the arm64 architecture is
>> 0xFE which corresponds to any memory that should not be accessed. On x86
>> (where tags are 4-bit wide instead of 8-bit wide) that tag is 0xE so
>> during the initializations all the bytes in shadow memory pages should
>> be filled with 0xE or 0xEE if two tags should be packed in one shadow
>> byte.
>>
>> Use memblock_alloc_try_nid_raw() instead of memblock_alloc_try_nid() to
>> avoid zeroing out the memory so it can be set with the KASAN invalid
>> tag.
>>
>> Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
>> ---
>>  arch/x86/mm/kasan_init_64.c | 19 ++++++++++++++++---
>>  include/linux/kasan.h       | 25 +++++++++++++++++++++++++
>>  mm/kasan/kasan.h            | 19 -------------------
>>  3 files changed, 41 insertions(+), 22 deletions(-)
>>
>> diff --git a/arch/x86/mm/kasan_init_64.c b/arch/x86/mm/kasan_init_64.c
>> index 9dddf19a5571..55d468d83682 100644
>> --- a/arch/x86/mm/kasan_init_64.c
>> +++ b/arch/x86/mm/kasan_init_64.c
>> @@ -35,6 +35,18 @@ static __init void *early_alloc(size_t size, int nid, bool should_panic)
>>         return ptr;
>>  }
>>
>> +static __init void *early_raw_alloc(size_t size, int nid, bool should_panic)
>> +{
>> +       void *ptr = memblock_alloc_try_nid_raw(size, size,
>> +                       __pa(MAX_DMA_ADDRESS), MEMBLOCK_ALLOC_ACCESSIBLE, nid);
>> +
>> +       if (!ptr && should_panic)
>> +               panic("%pS: Failed to allocate page, nid=%d from=%lx\n",
>> +                     (void *)_RET_IP_, nid, __pa(MAX_DMA_ADDRESS));
>> +
>> +       return ptr;
>> +}
>> +
>>  static void __init kasan_populate_pmd(pmd_t *pmd, unsigned long addr,
>>                                       unsigned long end, int nid)
>>  {
>> @@ -64,8 +76,9 @@ static void __init kasan_populate_pmd(pmd_t *pmd, unsigned long addr,
>>                 if (!pte_none(*pte))
>>                         continue;
>>
>> -               p = early_alloc(PAGE_SIZE, nid, true);
>> -               entry = pfn_pte(PFN_DOWN(__pa(p)), PAGE_KERNEL);
>> +               p = early_raw_alloc(PAGE_SIZE, nid, true);
>> +               memset(p, PAGE_SIZE, kasan_dense_tag(KASAN_SHADOW_INIT));
>> +               entry = pfn_pte(PFN_DOWN(__pa_nodebug(p)), PAGE_KERNEL);
>>                 set_pte_at(&init_mm, addr, pte, entry);
>>         } while (pte++, addr += PAGE_SIZE, addr != end);
>>  }
>> @@ -437,7 +450,7 @@ void __init kasan_init(void)
>>          * it may contain some garbage. Now we can clear and write protect it,
>>          * since after the TLB flush no one should write to it.
>>          */
>> -       memset(kasan_early_shadow_page, 0, PAGE_SIZE);
>> +       kasan_poison(kasan_early_shadow_page, PAGE_SIZE, KASAN_SHADOW_INIT, false);
>>         for (i = 0; i < PTRS_PER_PTE; i++) {
>>                 pte_t pte;
>>                 pgprot_t prot;
>> diff --git a/include/linux/kasan.h b/include/linux/kasan.h
>> index 83146367170a..af8272c74409 100644
>> --- a/include/linux/kasan.h
>> +++ b/include/linux/kasan.h
>> @@ -151,6 +151,31 @@ static __always_inline void kasan_unpoison_range(const void *addr, size_t size)
>>                 __kasan_unpoison_range(addr, size);
>>  }
>>
>> +#ifdef CONFIG_KASAN_HW_TAGS
>> +
>> +static inline void kasan_poison(const void *addr, size_t size, u8 value, bool init)
>> +{
>> +       if (WARN_ON((unsigned long)addr & KASAN_GRANULE_MASK))
>> +               return;
>> +       if (WARN_ON(size & KASAN_GRANULE_MASK))
>> +               return;
>> +
>> +       hw_set_mem_tag_range(kasan_reset_tag(addr), size, value, init);
>> +}
>> +
>> +#else /* CONFIG_KASAN_HW_TAGS */
>> +
>> +/**
>> + * kasan_poison - mark the memory range as inaccessible
>> + * @addr - range start address, must be aligned to KASAN_GRANULE_SIZE
>> + * @size - range size, must be aligned to KASAN_GRANULE_SIZE
>> + * @value - value that's written to metadata for the range
>> + * @init - whether to initialize the memory range (only for hardware tag-based)
>> + */
>> +void kasan_poison(const void *addr, size_t size, u8 value, bool init);
>> +
>> +#endif /* CONFIG_KASAN_HW_TAGS */
>
>Please keep kasan_poison() and kasan_unpoison() in mm/kasan/kasan.h:
>these are intended as internal-only functions (perhaps, we should add
>this into the comment). Instead, add a purpose-specific wrapper
>similar to the ones in include/linux/kasan.h.
>

Okay, got it, I'll pass it through a wrapper.

>
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-- 
Kind regards
Maciej Wieczór-Retman