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References: <20230711134623.12695-3-vbabka@suse.cz>
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From:   Andrey Konovalov <andreyknvl@gmail.com>
Date:   Wed, 12 Jul 2023 18:23:09 +0200
Message-ID: <CA+fCnZci9E8Snjuc-rJqSeX+Gn84_AVO5OjQwyFT=vL+pw22HQ@mail.gmail.com>
Subject: Re: [PATCH 1/2] mm/slub: remove redundant kasan_reset_tag() from
 freelist_ptr calculations
To:     Vlastimil Babka <vbabka@suse.cz>
Cc:     Christoph Lameter <cl@linux.com>,
        David Rientjes <rientjes@google.com>,
        Pekka Enberg <penberg@kernel.org>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Hyeonggon Yoo <42.hyeyoo@gmail.com>,
        Roman Gushchin <roman.gushchin@linux.dev>, linux-mm@kvack.org,
        patches@lists.linux.dev, linux-kernel@vger.kernel.org,
        Matteo Rizzo <matteorizzo@google.com>,
        Jann Horn <jannh@google.com>,
        Andrey Konovalov <andreyknvl@google.com>,
        Marco Elver <elver@google.com>,
        Alexander Potapenko <glider@google.com>,
        kasan-dev@googlegroups.com, Kees Cook <keescook@chromium.org>,
        linux-hardening@vger.kernel.org
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On Tue, Jul 11, 2023 at 3:46=E2=80=AFPM Vlastimil Babka <vbabka@suse.cz> wr=
ote:
>
> 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@suse.cz>
> ---
> These 2 patches build on top of:
> https://lore.kernel.org/all/20230704135834.3884421-1-matteorizzo@google.c=
om/
>
>  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 tag=
ged.
> -        * Normally, this doesn't cause any issues, as both set_freepoint=
er()
> -        * and get_freepointer() are called with a pointer with the same =
tag.
> -        * However, there are some issues with CONFIG_SLUB_DEBUG code. Fo=
r
> -        * 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 =3D (unsigned long)ptr ^ s->random ^
> -                       swab((unsigned long)kasan_reset_tag((void *)ptr_a=
ddr))};
> +       encoded =3D (unsigned long)ptr ^ s->random ^ swab(ptr_addr);
>  #else
> -       return (freeptr_t){.v =3D (unsigned long)ptr};
> +       encoded =3D (unsigned long)ptr;
>  #endif
> +       return (freeptr_t){.v =3D 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 =3D (void *)(ptr.v ^ s->random ^
> -               swab((unsigned long)kasan_reset_tag((void *)ptr_addr)));
> +       decoded =3D (void *)(ptr.v ^ s->random ^ swab(ptr_addr));
>  #else
>         decoded =3D (void *)ptr.v;
>  #endif
> --
> 2.41.0

Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>

Thanks!