Re: [PATCH] kasan: fix quarantine conflicting with init_on_free

[Marco Elver <elver@google.com>]

On Mon, 20 Dec 2021 at 17:37, <andrey.konovalov@linux.dev> wrote:
>
> From: Andrey Konovalov <andreyknvl@google.com>
>
> KASAN's quarantine might save its metadata inside freed objects. As
> this happens after the memory is zeroed by the slab allocator when
> init_on_free is enabled, the memory coming out of quarantine is not
> properly zeroed.
>
> This causes lib/test_meminit.c tests to fail with Generic KASAN.
>
> Zero the metadata when the object is removed from quarantine.
>
> Fixes: 6471384af2a6 ("mm: security: introduce init_on_alloc=1 and init_on_free=1 boot options")
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
> ---
>  mm/kasan/quarantine.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
>
> diff --git a/mm/kasan/quarantine.c b/mm/kasan/quarantine.c
> index 587da8995f2d..2e50869fd8e2 100644
> --- a/mm/kasan/quarantine.c
> +++ b/mm/kasan/quarantine.c
> @@ -132,11 +132,22 @@ static void *qlink_to_object(struct qlist_node *qlink, struct kmem_cache *cache)
>  static void qlink_free(struct qlist_node *qlink, struct kmem_cache *cache)
>  {
>         void *object = qlink_to_object(qlink, cache);
> +       struct kasan_free_meta *meta = kasan_get_free_meta(cache, object);
>         unsigned long flags;
>
>         if (IS_ENABLED(CONFIG_SLAB))
>                 local_irq_save(flags);
>
> +       /*
> +        * If init_on_free is enabled and KASAN's free metadata is stored in
> +        * the object, zero the metadata. Otherwise, the object's memory will
> +        * not be properly zeroed, as KASAN saves the metadata after the slab
> +        * allocator zeroes the object.
> +        */
> +       if (slab_want_init_on_free(cache) &&
> +           cache->kasan_info.free_meta_offset == 0)
> +               memset(meta, 0, sizeof(*meta));

memzero_explicit()

although in this case it probably doesn't matter much, because AFAIK
memzero_explicit() only exists to prevent the compiler from eliding
the zeroing. Up to you.

> +
>         /*
>          * As the object now gets freed from the quarantine, assume that its
>          * free track is no longer valid.
> --
> 2.25.1
>