linux-kernel - Re: [PATCH v2 2/2] kasan: Unpoison vms[area] addresses with a common tag

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Message-ID: <phrugqbctcakjmy2jhea56k5kwqszuua646cxfj4afrj5wk4wg@gdji4pf7kzhz>
Date: Wed, 03 Dec 2025 16:24:36 +0000
From: Maciej Wieczór-Retman <m.wieczorretman@...me>
To: Andrey Konovalov <andreyknvl@...il.com>
Cc: jiayuan.chen@...ux.dev, Andrey Ryabinin <ryabinin.a.a@...il.com>, Alexander Potapenko <glider@...gle.com>, Dmitry Vyukov <dvyukov@...gle.com>, Vincenzo Frascino <vincenzo.frascino@....com>, Andrew Morton <akpm@...ux-foundation.org>, Marco Elver <elver@...gle.com>, stable@...r.kernel.org, Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>, kasan-dev@...glegroups.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] kasan: Unpoison vms[area] addresses with a common tag

On 2025-12-03 at 16:53:01 +0100, Andrey Konovalov wrote:
>On Tue, Dec 2, 2025 at 3:29 PM Maciej Wieczor-Retman
><m.wieczorretman@...me> wrote:
>>
>> From: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.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.
>>
>> Use the modified __kasan_unpoison_vmalloc() to pass the tag of the first
>> vm_struct's address when vm_structs are unpoisoned in
>> pcpu_get_vm_areas(). Assigning a common tag resolves the pcpu chunk
>> address mismatch.
>>
>> Fixes: 1d96320f8d53 ("kasan, vmalloc: add vmalloc tagging for SW_TAGS")
>> Cc: <stable@...r.kernel.org> # 6.1+
>> Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
>> ---
>> Changelog v2:
>> - Revise the whole patch to match the fixed refactorization from the
>>   first patch.
>>
>> Changelog v1:
>> - 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  |  3 ++-
>>  mm/kasan/hw_tags.c | 12 ++++++++----
>>  mm/kasan/shadow.c  | 15 +++++++++++----
>>  3 files changed, 21 insertions(+), 9 deletions(-)
>>
>> diff --git a/mm/kasan/common.c b/mm/kasan/common.c
>> index 7884ea7d13f9..e5a867a5670b 100644
>> --- a/mm/kasan/common.c
>> +++ b/mm/kasan/common.c
>> @@ -591,11 +591,12 @@ void kasan_unpoison_vmap_areas(struct vm_struct **vms, int nr_vms,
>>         unsigned long size;
>>         void *addr;
>>         int area;
>> +       u8 tag = get_tag(vms[0]->addr);
>>
>>         for (area = 0 ; area < nr_vms ; area++) {
>>                 size = vms[area]->size;
>>                 addr = vms[area]->addr;
>> -               vms[area]->addr = __kasan_unpoison_vmap_areas(addr, size, flags);
>> +               vms[area]->addr = __kasan_unpoison_vmap_areas(addr, size, flags, tag);
>
>I'm thinking what you can do here is:
>
>vms[area]->addr = set_tag(addr, tag);
>__kasan_unpoison_vmalloc(addr, size, flags | KASAN_VMALLOC_KEEP_TAG);


I noticed that something like this wouldn't work once I started trying
to rebase my work onto Jiayuan's. The line:
+       u8 tag = get_tag(vms[0]->addr);
is wrong and should be
+       u8 tag = kasan_random_tag();
I was sure the vms[0]->addr was already tagged (I recall checking this
so I'm not sure if something changed or my previous check was wrong) but
the problem here is that vms[0]->addr, vms[1]->addr ... were unpoisoned
with random addresses, specifically different random addresses. So then
later in the pcpu chunk code vms[1] related pointers would get the tag
from vms[0]->addr.

So I think we still need a separate way to do __kasan_unpoison_vmalloc
with a specific tag.

>
>This is with the assumption that Jiayuan's patch is changed to add
>KASAN_VMALLOC_KEEP_TAG to kasan_vmalloc_flags_t.
>
>Then you should not need that extra __kasan_random_unpoison_vmalloc helper.

I already rewrote the patch rebased onto Jiayuan's patch. I was able to
ditch the __kasan_random_unpoison_vmalloc but I needed to add
__kasan_unpoison_vrealloc - so I can pass the tag of the start pointer
to __kasan_unpoison_vmalloc. I was hoping to post it today/tomorrow so
Jiayuan can check my changes don't break his solution. I'm just waiting
to check it compiles against all the fun kernel configs.

-- 
kind regards
Maciej Wieczór-Retman