Re: [PATCH v5 18/19] mm: Unpoison vms[area] addresses with a common tag

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

On 2025-09-06 at 19:19:11 +0200, Andrey Konovalov wrote:
>On Mon, Aug 25, 2025 at 10:31 PM Maciej Wieczor-Retman
><maciej.wieczor-retman@intel.com> wrote:
>>
>> The problem presented here is related to NUMA systems and tag-based
>> KASAN mode. It can be explained in the following points:
>>
>>         1. There can be more than one virtual memory chunk.
>>         2. Chunk's base address has a tag.
>>         3. The base address points at the first chunk and thus inherits
>>            the tag of the first chunk.
>>         4. The subsequent chunks will be accessed with the tag from the
>>            first chunk.
>>         5. Thus, the subsequent chunks need to have their tag set to
>>            match that of the first chunk.
>>
>> Unpoison all vms[]->addr memory and pointers with the same tag to
>> resolve the mismatch.
>>
>> Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
>> ---
>> Changelog v4:
>> - Move tagging the vms[]->addr to this new patch and leave refactoring
>>   there.
>> - Comment the fix to provide some context.
>>
>>  mm/kasan/shadow.c | 10 +++++++++-
>>  1 file changed, 9 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/kasan/shadow.c b/mm/kasan/shadow.c
>> index b41f74d68916..ee2488371784 100644
>> --- a/mm/kasan/shadow.c
>> +++ b/mm/kasan/shadow.c
>> @@ -646,13 +646,21 @@ void __kasan_poison_vmalloc(const void *start, unsigned long size)
>>         kasan_poison(start, size, KASAN_VMALLOC_INVALID, false);
>>  }
>>
>> +/*
>> + * A tag mismatch happens when calculating per-cpu chunk addresses, because
>> + * they all inherit the tag from vms[0]->addr, even when nr_vms is bigger
>> + * than 1. This is a problem because all the vms[]->addr come from separate
>> + * allocations and have different tags so while the calculated address is
>> + * correct the tag isn't.
>> + */
>>  void __kasan_unpoison_vmap_areas(struct vm_struct **vms, int nr_vms)
>>  {
>>         int area;
>>
>>         for (area = 0 ; area < nr_vms ; area++) {
>>                 kasan_poison(vms[area]->addr, vms[area]->size,
>> -                            arch_kasan_get_tag(vms[area]->addr), false);
>> +                            arch_kasan_get_tag(vms[0]->addr), false);
>> +               arch_kasan_set_tag(vms[area]->addr, arch_kasan_get_tag(vms[0]->addr));
>>         }
>>  }
>>
>> --
>> 2.50.1
>>
>
>Do we need this fix for the HW_TAGS mode too?

Oh, I suppose it could also affect the hardware mode since this is related to
tagged pointers and NUMA nodes. I'll try to also make it work for HW_TAGS.

-- 
Kind regards
Maciej Wieczór-Retman