Re: [PATCH 1/2] mm/slub: remove redundant kasan_reset_tag() from freelist_ptr calculations

[Andrey Konovalov]

On Tue, Jul 11, 2023 at 3:46 PM Vlastimil Babka <vbabka@xxxxxxx> wrote:
>
> Commit d36a63a943e3 ("kasan, slub: fix more conflicts with
> CONFIG_SLAB_FREELIST_HARDENED") has introduced kasan_reset_tags() to
> freelist_ptr() encoding/decoding when CONFIG_SLAB_FREELIST_HARDENED is
> enabled to resolve issues when passing tagged or untagged pointers
> inconsistently would lead to incorrect calculations.
>
> Later, commit aa1ef4d7b3f6 ("kasan, mm: reset tags when accessing
> metadata") made sure all pointers have tags reset regardless of
> CONFIG_SLAB_FREELIST_HARDENED, because there was no other way to access
> the freepointer metadata safely with hw tag-based KASAN.
>
> Therefore the kasan_reset_tag() usage in freelist_ptr_encode()/decode()
> is now redundant, as all callers use kasan_reset_tag() unconditionally
> when constructing ptr_addr. Remove the redundant calls and simplify the
> code and remove obsolete comments.
>
> Also in freelist_ptr_encode() introduce an 'encoded' variable to make
> the lines shorter and make it similar to the _decode() one.
>
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> ---
> These 2 patches build on top of:
> https://lore.kernel.org/all/20230704135834.3884421-1-matteorizzo@xxxxxxxxxx/
>
>  mm/slub.c | 22 ++++++----------------
>  1 file changed, 6 insertions(+), 16 deletions(-)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index f8cc47eff742..07edad305512 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -374,22 +374,14 @@ typedef struct { unsigned long v; } freeptr_t;
>  static inline freeptr_t freelist_ptr_encode(const struct kmem_cache *s,
>                                             void *ptr, unsigned long ptr_addr)
>  {
> +       unsigned long encoded;
> +
>  #ifdef CONFIG_SLAB_FREELIST_HARDENED
> -       /*
> -        * When CONFIG_KASAN_SW/HW_TAGS is enabled, ptr_addr might be tagged.
> -        * Normally, this doesn't cause any issues, as both set_freepointer()
> -        * and get_freepointer() are called with a pointer with the same tag.
> -        * However, there are some issues with CONFIG_SLUB_DEBUG code. For
> -        * example, when __free_slub() iterates over objects in a cache, it
> -        * passes untagged pointers to check_object(). check_object() in turns
> -        * calls get_freepointer() with an untagged pointer, which causes the
> -        * freepointer to be restored incorrectly.
> -        */
> -       return (freeptr_t){.v = (unsigned long)ptr ^ s->random ^
> -                       swab((unsigned long)kasan_reset_tag((void *)ptr_addr))};
> +       encoded = (unsigned long)ptr ^ s->random ^ swab(ptr_addr);
>  #else
> -       return (freeptr_t){.v = (unsigned long)ptr};
> +       encoded = (unsigned long)ptr;
>  #endif
> +       return (freeptr_t){.v = encoded};
>  }
>
>  static inline void *freelist_ptr_decode(const struct kmem_cache *s,
> @@ -398,9 +390,7 @@ static inline void *freelist_ptr_decode(const struct kmem_cache *s,
>         void *decoded;
>
>  #ifdef CONFIG_SLAB_FREELIST_HARDENED
> -       /* See the comment in freelist_ptr_encode */
> -       decoded = (void *)(ptr.v ^ s->random ^
> -               swab((unsigned long)kasan_reset_tag((void *)ptr_addr)));
> +       decoded = (void *)(ptr.v ^ s->random ^ swab(ptr_addr));
>  #else
>         decoded = (void *)ptr.v;
>  #endif
> --
> 2.41.0

Reviewed-by: Andrey Konovalov <andreyknvl@xxxxxxxxx>

Thanks!