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

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Message-ID: <CA+fCnZedGwtMThKjFLcXqJuc6+RD_EskQGvqKhV9Ew4dKdM_Og@mail.gmail.com>
Date: Sat, 6 Sep 2025 19:19:11 +0200
From: Andrey Konovalov <andreyknvl@...il.com>
To: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
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Subject: Re: [PATCH v5 18/19] mm: Unpoison vms[area] addresses with a common tag

On Mon, Aug 25, 2025 at 10:31 PM Maciej Wieczor-Retman
<maciej.wieczor-retman@...el.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@...el.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?