Re: [PATCH v3 15/34] mm: slub: Unpoison the memchr_inv() return value

[Vlastimil Babka]

On 12/14/23 00:24, 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`.
> 
> The intention behind guarding memchr_inv() behind
> metadata_access_enable() is to touch poisoned metadata without
> triggering KMSAN, so unpoison its return value.
> 
> Signed-off-by: Ilya Leoshkevich <iii@xxxxxxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

> ---
>  mm/slub.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 2d29d368894c..802702748925 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1076,6 +1076,7 @@ static int check_bytes_and_report(struct kmem_cache *s, struct slab *slab,
>  	metadata_access_enable();
>  	fault = memchr_inv(kasan_reset_tag(start), value, bytes);
>  	metadata_access_disable();
> +	kmsan_unpoison_memory(&fault, sizeof(fault));
>  	if (!fault)
>  		return 1;
>  
> @@ -1182,6 +1183,7 @@ static void slab_pad_check(struct kmem_cache *s, struct slab *slab)
>  	metadata_access_enable();
>  	fault = memchr_inv(kasan_reset_tag(pad), POISON_INUSE, remainder);
>  	metadata_access_disable();
> +	kmsan_unpoison_memory(&fault, sizeof(fault));
>  	if (!fault)
>  		return;
>  	while (end > fault && end[-1] == POISON_INUSE)