Re: [PATCH v4 12/35] kmsan: Support SLAB_POISON

SeongJae Park
2024-06-13T23:30:48Z

Hi Ilya,

On Thu, 13 Jun 2024 17:34:14 +0200 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 by using __memset().
>
> There are two alternatives to this approach. First, init_object()
> can be marked with __no_sanitize_memory. This annotation should be used
> with great care, because it drops all instrumentation from the
> function, and any shadow writes will be lost. Even though this is not a
> concern with the current init_object() implementation, this may change
> in the future.
>
> Second, kmsan_poison_memory() calls may be added after memset() calls.
> The downside is that init_object() is called from
> free_debug_processing(), in which case poisoning will erase the
> distinction between simply uninitialized memory and UAF.
>
> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
> ---
> mm/kmsan/hooks.c | 2 +-
> mm/slub.c | 13 +++++++++----
> 2 files changed, 10 insertions(+), 5 deletions(-)
>
[...]
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1139,7 +1139,12 @@ static void init_object(struct kmem_cache *s, void *object, u8 val)
> unsigned int poison_size = s->object_size;
>
> if (s->flags & SLAB_RED_ZONE) {
> - memset(p - s->red_left_pad, val, s->red_left_pad);
> + /*
> + * Use __memset() here and below in order to avoid overwriting
> + * the KMSAN shadow. Keeping the shadow makes it possible to
> + * distinguish uninit-value from use-after-free.
> + */
> + __memset(p - s->red_left_pad, val, s->red_left_pad);

I found my build test[1] fails with below error on latest mm-unstable branch.
'git bisect' points me this patch.

CC mm/slub.o
/mm/slub.c: In function 'init_object':
/mm/slub.c:1147:17: error: implicit declaration of function '__memset'; did you mean 'memset'? [-Werror=implicit-function-declaration]
1147 | __memset(p - s->red_left_pad, val, s->red_left_pad);
| ^~~~~~~~
| memset
cc1: some warnings being treated as errors

I haven't looked in deep, but reporting first. Do you have any idea?

[1] https://github.com/awslabs/damon-tests/blob/next/corr/tests/build_m68k.sh

Thanks,
SJ …