Re: [PATCH v7 18/38] mm: slub: Disable KMSAN when checking the padding bytes

Vlastimil Babka
2024-06-24T07:34:40Z

On 6/21/24 1:35 PM, Ilya Leoshkevich wrote:
> Even though the KMSAN warnings generated by memchr_inv() are suppressed
> by metadata_access_enable(), its return value may still be poisoned.
>
> The reason is that the last iteration of memchr_inv() returns
> `*start != value ? start : NULL`, where *start is poisoned. Because of
> this, somewhat counterintuitively, the shadow value computed by
> visitSelectInst() is equal to `(uintptr_t)start`.
>
> One possibility to fix this, since the intention behind guarding
> memchr_inv() behind metadata_access_enable() is to touch poisoned
> metadata without triggering KMSAN, is to unpoison its return value.
> However, this approach is too fragile. So simply disable the KMSAN
> checks in the respective functions.
>
> Reviewed-by: Alexander Potapenko <glider@google.com>
> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> ---
> mm/slub.c | 16 ++++++++++++----
> 1 file changed, 12 insertions(+), 4 deletions(-)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index b050e528112c..fcd68fcea4ab 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1176,9 +1176,16 @@ static void restore_bytes(struct kmem_cache *s, char *message, u8 data,
> memset(from, data, to - from);
> }
>
> -static int check_bytes_and_report(struct kmem_cache *s, struct slab *slab,
> - u8 *object, char *what,
> - u8 *start, unsigned int value, unsigned int bytes)
> +#ifdef CONFIG_KMSAN
> +#define pad_check_attributes noinline __no_kmsan_checks
> +#else
> +#define pad_check_attributes
> +#endif
> +
> +static pad_check_attributes int
> +check_bytes_and_report(struct kmem_cache *s, struct slab *slab,
> + u8 *object, char *what,
> + u8 *start, unsigned int value, unsigned int bytes)
> {
> u8 *fault;
> u8 *end;
> @@ -1270,7 +1277,8 @@ static int check_pad_bytes(struct kmem_cache *s, struct slab *slab, u8 *p)
> }
>
> /* Check the pad bytes at the end of a slab page */
> -static void slab_pad_check(struct kmem_cache *s, struct slab *slab)
> +static pad_check_attributes void
> +slab_pad_check(struct kmem_cache *s, struct slab *slab)
> {
> u8 *start;
> u8 *fault;