Re: [PATCH v1 1/2] kasan: Unpoison pcpu chunks with base address tag

[Maciej Wieczor-Retman <m.wieczorretman@pm.me>]

On 2025-11-05 at 02:12:49 +0100, Andrey Konovalov wrote:
>On Tue, Nov 4, 2025 at 3:49 PM Maciej Wieczor-Retman
><m.wieczorretman@pm.me> wrote:
>>
>> From: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
>>
>> 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.
>>
>> Refactor code by moving it into a helper in preparation for the actual
>> fix.
>>
>> Fixes: 1d96320f8d53 ("kasan, vmalloc: add vmalloc tagging for SW_TAGS")
>> Cc: <stable@vger.kernel.org> # 6.1+
>> Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
>> Tested-by: Baoquan He <bhe@redhat.com>
>> ---
>> Changelog v1 (after splitting of from the KASAN series):
>> - Rewrite first paragraph of 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.
...
>> diff --git a/mm/kasan/common.c b/mm/kasan/common.c
>> index d4c14359feaf..c63544a98c24 100644
>> --- a/mm/kasan/common.c
>> +++ b/mm/kasan/common.c
>> @@ -28,6 +28,7 @@
>>  #include <linux/string.h>
>>  #include <linux/types.h>
>>  #include <linux/bug.h>
>> +#include <linux/vmalloc.h>
>>
>>  #include "kasan.h"
>>  #include "../slab.h"
>> @@ -582,3 +583,13 @@ bool __kasan_check_byte(const void *address, unsigned long ip)
>>         }
>>         return true;
>>  }
>> +
>> +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);
>
>The patch description says this patch is a refactoring, but the patch
>changes the logic of the code.
>
>We don't call __kasan_unpoison_vmalloc() anymore and don't perform all
>the related checks. This might be OK, assuming the checks always
>succeed/fail, but this needs to be explained (note that there two
>versions of __kasan_unpoison_vmalloc() with different checks).
>
>And also we don't assign a random tag anymore - we should.

Thanks for the pointers, I'll revise the two versions and make it an actual
refactor.

>Also, you can just use get/set_tag(), no need to use the arch_ version
>(and in the following patch too).

Thanks :)

-- 
Kind regards
Maciej Wieczór-Retman