Re: [PATCH 13/32] kmsan: Support SLAB_POISON

Alexander Potapenko
2023-11-16T14:56:30Z

On Wed, Nov 15, 2023 at 9:34 PM Ilya Leoshkevich <iii@linux.ibm.com> wrote:
>
> Avoid false KMSAN negatives with SLUB_DEBUG by allowing
> kmsan_slab_free() to poison the freed memory, and by preventing
> init_object() from unpoisoning new allocations.
>
> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
> ---
> mm/kmsan/hooks.c | 2 +-
> mm/slub.c | 3 ++-
> 2 files changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/mm/kmsan/hooks.c b/mm/kmsan/hooks.c
> index 7b5814412e9f..7a30274b893c 100644
> --- a/mm/kmsan/hooks.c
> +++ b/mm/kmsan/hooks.c
> @@ -76,7 +76,7 @@ void kmsan_slab_free(struct kmem_cache *s, void *object)
> return;
>
> /* RCU slabs could be legally used after free within the RCU period */
> - if (unlikely(s->flags & (SLAB_TYPESAFE_BY_RCU | SLAB_POISON)))
> + if (unlikely(s->flags & SLAB_TYPESAFE_BY_RCU))
> return;
> /*
> * If there's a constructor, freed memory must remain in the same state
> diff --git a/mm/slub.c b/mm/slub.c
> index 63d281dfacdb..8d9aa4d7cb7e 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1024,7 +1024,8 @@ static __printf(3, 4) void slab_err(struct kmem_cache *s, struct slab *slab,
> add_taint(TAINT_BAD_PAGE, LOCKDEP_NOW_UNRELIABLE);
> }
>
> -static void init_object(struct kmem_cache *s, void *object, u8 val)
> +__no_sanitize_memory static void

__no_sanitize_memory should be used with great care, because it drops
all instrumentation from the function, and any shadow writes will be
lost.
Won't it be better to add kmsan_poison() to init_object() if you want
it to stay uninitialized?