RE: [PATCH v2 1/4] crypto: inside-secure - use kfree_sensitive()

Van Leeuwen, Pascal
2020-09-02T13:15:08Z

> -----Original Message-----
> From: linux-crypto-owner@vger.kernel.org <linux-crypto-owner@vger.kernel.org> On Behalf Of Denis Efremov
> Sent: Thursday, August 27, 2020 8:44 AM
> To: linux-crypto@vger.kernel.org
> Cc: Denis Efremov <efremov@linux.com>; Corentin Labbe <clabbe.montjoie@gmail.com>; Herbert Xu
> <herbert@gondor.apana.org.au>; linux-kernel@vger.kernel.org
> Subject: [PATCH v2 1/4] crypto: inside-secure - use kfree_sensitive()
>
> <<< External Email >>>
> Use kfree_sensitive() instead of open-coding it.
>
> Signed-off-by: Denis Efremov <efremov@linux.com>
> ---
> drivers/crypto/inside-secure/safexcel_hash.c | 3 +--
> 1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/drivers/crypto/inside-secure/safexcel_hash.c b/drivers/crypto/inside-secure/safexcel_hash.c
> index 16a467969d8e..5ffdc1cd5847 100644
> --- a/drivers/crypto/inside-secure/safexcel_hash.c
> +++ b/drivers/crypto/inside-secure/safexcel_hash.c
> @@ -1082,8 +1082,7 @@ static int safexcel_hmac_init_pad(struct ahash_request *areq,
> }
>
> /* Avoid leaking */
> -memzero_explicit(keydup, keylen);
> -kfree(keydup);
> +kfree_sensitive(keydup);
>
I'm not sure here ... I verified it does not break the driver (not a big surprise), but ...

memzero_explicit guarantees that it will not get optimized away and the keydata _always_
gets overwritten. Does kfree_sensitive also come with such a guarantee? I could not find a
hard statement on that in its documentation. Although the "sensitive" part surely suggests
it.

Additionally, this remark is made in the documentation for kfree_sensitive: "this function
zeroes the whole allocated buffer which can be a good deal bigger than the requested buffer
size passed to kmalloc(). So be careful when using this function in performance sensitive
code"

While the memzero_explicit does not zeroize anything beyond keylen.
Which is all you really need here, so why would you want to zeroize potentially a lot more?
In any case the two are not fully equivalent.

Any opinions?

> if (ret)
> return ret;
> --
> 2.26.2

Regards,
Pascal van Leeuwen
Silicon IP Architect Multi-Protocol Engines, Rambus Security
Rambus ROTW Holding BV
+31-73 6581953

Note: The Inside Secure/Verimatrix Silicon IP team was recently acquired by Rambus.
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