linux-kernel - Re: [PATCH 13/32] kmsan: Support SLAB

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Message-ID: <CAG_fn=WOfRvDw3r3zcZXWr8aa6MiEuKSa1etQrGVSJP+ic7=mg@mail.gmail.com>
Date:   Thu, 16 Nov 2023 15:55:45 +0100
From:   Alexander Potapenko <glider@...gle.com>
To:     Ilya Leoshkevich <iii@...ux.ibm.com>
Cc:     Alexander Gordeev <agordeev@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...ux.com>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Marco Elver <elver@...gle.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Pekka Enberg <penberg@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>,
        kasan-dev@...glegroups.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-s390@...r.kernel.org,
        linux-trace-kernel@...r.kernel.org,
        Mark Rutland <mark.rutland@....com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Sven Schnelle <svens@...ux.ibm.com>
Subject: Re: [PATCH 13/32] kmsan: Support SLAB_POISON

On Wed, Nov 15, 2023 at 9:34 PM Ilya Leoshkevich <iii@...ux.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@...ux.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?