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

[SeongJae Park <sj@kernel.org>]

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

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