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

Michal Hocko
2020-06-16T06:42:27Z

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@vger.kernel.org
> Signed-off-by: Waiman Long <longman@redhat.com>

Acked-by: Michal Hocko <mhocko@suse.com>

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