Re: [PATCH v1 2/2] kasan: Unpoison vms[area] addresses with a common tag

[Maciej Wieczor-Retman]

On 2025-11-05 at 02:13:22 +0100, Andrey Konovalov wrote:
>On Tue, Nov 4, 2025 at 3:49 PM Maciej Wieczor-Retman
><m.wieczorretman@xxxxx> wrote:
>>
>> From: Maciej Wieczor-Retman <maciej.wieczor-retman@xxxxxxxxx>
>>
>> A KASAN tag mismatch, possibly causing a kernel panic, can be observed
>> on systems with a tag-based KASAN enabled and with multiple NUMA nodes.
>> It was reported on arm64 and reproduced on x86. 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 vm_structs after allocating them for the percpu allocator.
>> Use the same tag to resolve the pcpu chunk address mismatch.
>>
>> Fixes: 1d96320f8d53 ("kasan, vmalloc: add vmalloc tagging for SW_TAGS")
>> Cc: <stable@xxxxxxxxxxxxxxx> # 6.1+
>> Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@xxxxxxxxx>
>> Tested-by: Baoquan He <bhe@xxxxxxxxxx>
>> ---
>> Changelog v1 (after splitting of from the KASAN series):
>> - Rewrite the patch message to point at the user impact of the issue.
>> - Move helper to common.c so it can be compiled in all KASAN modes.
>>
>>  mm/kasan/common.c | 10 +++++++++-
>>  1 file changed, 9 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/kasan/common.c b/mm/kasan/common.c
>> index c63544a98c24..a6bbc68984cd 100644
>> --- a/mm/kasan/common.c
>> +++ b/mm/kasan/common.c
>> @@ -584,12 +584,20 @@ bool __kasan_check_byte(const void *address, unsigned long ip)
>>         return true;
>>  }
>>
>> +/*
>> + * 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));
>
>set_tag() does not set the tag in place, its return value needs to be assigned.

Right, not sure how I missed that

>
>So if this patch fixes the issue, there's something off (is
>vms[area]->addr never used for area != 0)?

Maybe there is something off with my tests then. I'll try to run them in a
couple of different environments.

-- 
Kind regards
Maciej Wieczór-Retman