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

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Message-ID: <CA+fCnZefD8F7rMu3-M4uDTbWR5R8y7qfLzjrB34sK3bz4di03g@mail.gmail.com>
Date: Wed, 5 Nov 2025 02:12:49 +0100
From: Andrey Konovalov <andreyknvl@...il.com>
To: Maciej Wieczor-Retman <m.wieczorretman@...me>
Cc: 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>, Uladzislau Rezki <urezki@...il.com>, 
	Marco Elver <elver@...gle.com>, stable@...r.kernel.org, 
	Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>, Baoquan He <bhe@...hat.com>, 
	kasan-dev@...glegroups.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v1 1/2] kasan: Unpoison pcpu chunks with base address tag

On Tue, Nov 4, 2025 at 3:49 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.
>
> 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@...r.kernel.org> # 6.1+
> Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@...el.com>
> Tested-by: Baoquan He <bhe@...hat.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.
>
>  include/linux/kasan.h | 10 ++++++++++
>  mm/kasan/common.c     | 11 +++++++++++
>  mm/vmalloc.c          |  4 +---
>  3 files changed, 22 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/kasan.h b/include/linux/kasan.h
> index d12e1a5f5a9a..b00849ea8ffd 100644
> --- a/include/linux/kasan.h
> +++ b/include/linux/kasan.h
> @@ -614,6 +614,13 @@ static __always_inline void kasan_poison_vmalloc(const void *start,
>                 __kasan_poison_vmalloc(start, size);
>  }
>
> +void __kasan_unpoison_vmap_areas(struct vm_struct **vms, int nr_vms);
> +static __always_inline void kasan_unpoison_vmap_areas(struct vm_struct **vms, int nr_vms)
> +{
> +       if (kasan_enabled())
> +               __kasan_unpoison_vmap_areas(vms, nr_vms);
> +}
> +
>  #else /* CONFIG_KASAN_VMALLOC */
>
>  static inline void kasan_populate_early_vm_area_shadow(void *start,
> @@ -638,6 +645,9 @@ static inline void *kasan_unpoison_vmalloc(const void *start,
>  static inline void kasan_poison_vmalloc(const void *start, unsigned long size)
>  { }
>
> +static inline void kasan_unpoison_vmap_areas(struct vm_struct **vms, int nr_vms)
> +{ }
> +
>  #endif /* CONFIG_KASAN_VMALLOC */
>
>  #if (defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KASAN_SW_TAGS)) && \
> 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.

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





> +       }
> +}
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 798b2ed21e46..934c8bfbcebf 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -4870,9 +4870,7 @@ struct vm_struct **pcpu_get_vm_areas(const unsigned long *offsets,
>          * With hardware tag-based KASAN, marking is skipped for
>          * non-VM_ALLOC mappings, see __kasan_unpoison_vmalloc().
>          */
> -       for (area = 0; area < nr_vms; area++)
> -               vms[area]->addr = kasan_unpoison_vmalloc(vms[area]->addr,
> -                               vms[area]->size, KASAN_VMALLOC_PROT_NORMAL);
> +       kasan_unpoison_vmap_areas(vms, nr_vms);
>
>         kfree(vas);
>         return vms;
> --
> 2.51.0
>
>