Re: [PATCH v4 1/3] mm/slab: Use memzero_explicit() in kzfree()

[Michal Hocko]

On Mon 15-06-20 21:57:16, Waiman Long wrote:
> The kzfree() function is normally used to clear some sensitive
> information, like encryption keys, in the buffer before freeing it back
> to the pool. Memset() is currently used for the buffer clearing. However,
> it is entirely possible that the compiler may choose to optimize away the
> memory clearing especially if LTO is being used. To make sure that this
> optimization will not happen, memzero_explicit(), which is introduced
> in v3.18, is now used in kzfree() to do the clearing.
> 
> Fixes: 3ef0e5ba4673 ("slab: introduce kzfree()")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Waiman Long <longman@xxxxxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

Although I am not really sure this is a stable material. Is there any
known instance where the memset was optimized out from kzfree?

> ---
>  mm/slab_common.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 9e72ba224175..37d48a56431d 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -1726,7 +1726,7 @@ void kzfree(const void *p)
>  	if (unlikely(ZERO_OR_NULL_PTR(mem)))
>  		return;
>  	ks = ksize(mem);
> -	memset(mem, 0, ks);
> +	memzero_explicit(mem, ks);
>  	kfree(mem);
>  }
>  EXPORT_SYMBOL(kzfree);
> -- 
> 2.18.1
> 

-- 
Michal Hocko
SUSE Labs