Re: [PATCH v7 3/4] mm/kasan: Introduce helpers for lazy MMU mode sanitizer

On Mon, Aug 17, 2026 at 1:33 PM Alexander Gordeev
<[email protected]> wrote:
>
> Provide helpers that allow architectures implement
> illegitimate PTE direct accesses while the lazy MMU
> mode is enabled, such as:
>
>         pte_t pte = *ptep;
>         *ptep = pte;
>
> By contrast, these would have to be:
>
>         pte_t pte = ptep_get(ptep);
>         set_pte(ptep, pte);
>
> The direct PTE accesses pose a real issue on s390.
>
> Suggested-by: Ilya Leoshkevich <[email protected]>
> Signed-off-by: Alexander Gordeev <[email protected]>
> ---
>  include/linux/kasan.h     | 19 +++++++++++++++++--
>  mm/kasan/common.c         | 14 ++++++++++++++
>  mm/kasan/kasan.h          |  2 ++
>  mm/kasan/report_generic.c |  3 +++
>  4 files changed, 36 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/kasan.h b/include/linux/kasan.h
> index bf233bde68c7..1aa4d9dd2485 100644
> --- a/include/linux/kasan.h
> +++ b/include/linux/kasan.h
> @@ -6,6 +6,7 @@
>  #include <linux/kasan-enabled.h>
>  #include <linux/kasan-tags.h>
>  #include <linux/kernel.h>
> +#include <linux/pgtable.h>
>  #include <linux/static_key.h>
>  #include <linux/types.h>
>
> @@ -35,8 +36,6 @@ typedef unsigned int __bitwise kasan_vmalloc_flags_t;
>
>  #if defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KASAN_SW_TAGS)
>
> -#include <linux/pgtable.h>
> -
>  /* Software KASAN implementations use shadow memory. */
>
>  #ifdef CONFIG_KASAN_SW_TAGS
> @@ -134,6 +133,20 @@ static __always_inline void kasan_poison_slab(struct slab *slab)
>                 __kasan_poison_slab(slab);
>  }
>
> +void __kasan_poison_pte(pte_t *pte, int nr);
> +static __always_inline void kasan_poison_pte(pte_t *pte, int nr)
> +{
> +       if (kasan_enabled())
> +               __kasan_poison_pte(pte, nr);
> +}
> +
> +void __kasan_unpoison_pte(pte_t *pte, int nr);
> +static __always_inline void kasan_unpoison_pte(pte_t *pte, int nr)
> +{
> +       if (kasan_enabled())
> +               __kasan_unpoison_pte(pte, nr);
> +}
> +
>  void __kasan_unpoison_new_object(struct kmem_cache *cache, void *object);
>  /**
>   * kasan_unpoison_new_object - Temporarily unpoison a new slab object.
> @@ -414,6 +427,8 @@ static inline bool kasan_unpoison_pages(struct page *page, unsigned int order,
>         return false;
>  }
>  static inline void kasan_poison_slab(struct slab *slab) {}
> +static inline void kasan_poison_pte(pte_t *pte, int nr) {}
> +static inline void kasan_unpoison_pte(pte_t *pte, int nr) {}
>  static inline void kasan_unpoison_new_object(struct kmem_cache *cache,
>                                         void *object) {}
>  static inline void kasan_poison_new_object(struct kmem_cache *cache,
> diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> index b7d05c2a6d93..94d106094989 100644
> --- a/mm/kasan/common.c
> +++ b/mm/kasan/common.c
> @@ -163,6 +163,20 @@ void __kasan_poison_slab(struct slab *slab)
>                      KASAN_SLAB_REDZONE, false);
>  }
>
> +void __kasan_poison_pte(pte_t *pte, int nr)
> +{
> +       if (IS_ALIGNED(sizeof(*pte), KASAN_GRANULE_SIZE))
> +               kasan_poison(pte, sizeof(*pte) * nr, KASAN_LAZY_MMU_PTE, false);
> +}
> +EXPORT_SYMBOL_GPL(__kasan_poison_pte);
> +
> +void __kasan_unpoison_pte(pte_t *pte, int nr)
> +{
> +       if (IS_ALIGNED(sizeof(*pte), KASAN_GRANULE_SIZE))
> +               kasan_unpoison(pte, sizeof(*pte) * nr, false);
> +}
> +EXPORT_SYMBOL_GPL(__kasan_unpoison_pte);
> +
>  void __kasan_unpoison_new_object(struct kmem_cache *cache, void *object)
>  {
>         kasan_unpoison(object, cache->object_size, false);
> diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
> index fc9169a54766..1a2d18cdb21d 100644
> --- a/mm/kasan/kasan.h
> +++ b/mm/kasan/kasan.h
> @@ -144,12 +144,14 @@ static inline bool kasan_requires_meta(void)
>  #define KASAN_PAGE_REDZONE     0xFE  /* redzone for kmalloc_large allocation */
>  #define KASAN_SLAB_REDZONE     0xFC  /* redzone for slab object */
>  #define KASAN_SLAB_FREE                0xFB  /* freed slab object */
> +#define KASAN_LAZY_MMU_PTE     0xFD  /* direct pte access in lazy mmu mode */
>  #define KASAN_VMALLOC_INVALID  0xF8  /* inaccessible space in vmap area */
>  #else
>  #define KASAN_PAGE_FREE                KASAN_TAG_INVALID
>  #define KASAN_PAGE_REDZONE     KASAN_TAG_INVALID
>  #define KASAN_SLAB_REDZONE     KASAN_TAG_INVALID
>  #define KASAN_SLAB_FREE                KASAN_TAG_INVALID
> +#define KASAN_LAZY_MMU_PTE     KASAN_TAG_INVALID
>  #define KASAN_VMALLOC_INVALID  KASAN_TAG_INVALID /* only used for SW_TAGS */
>  #endif
>
> diff --git a/mm/kasan/report_generic.c b/mm/kasan/report_generic.c
> index f5b8e37b3805..489d4a8d6902 100644
> --- a/mm/kasan/report_generic.c
> +++ b/mm/kasan/report_generic.c
> @@ -113,6 +113,9 @@ static const char *get_shadow_bug_type(struct kasan_report_info *info)
>         case KASAN_SLAB_FREE_META:
>                 bug_type = "slab-use-after-free";
>                 break;
> +       case KASAN_LAZY_MMU_PTE:
> +               bug_type = "lazy-mmu-pte-access";
> +               break;
>         case KASAN_ALLOCA_LEFT:
>         case KASAN_ALLOCA_RIGHT:
>                 bug_type = "alloca-out-of-bounds";
> --
> 2.53.0
>
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Hi Alexander,

Should all this be limited to only s390?

Thanks!