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

Van Leeuwen, Pascal
2020-09-04T09:44:47Z

> -----Original Message-----
> From: Denis Efremov <efremov@linux.com>
> Sent: Friday, September 4, 2020 10:55 AM
> To: Van Leeuwen, Pascal <pvanleeuwen@rambus.com>; linux-crypto@vger.kernel.org
> Cc: Corentin Labbe <clabbe.montjoie@gmail.com>; Herbert Xu <herbert@gondor.apana.org.au>; linux-kernel@vger.kernel.org
> Subject: Re: [PATCH v2 1/4] crypto: inside-secure - use kfree_sensitive()
>
> <<< External Email >>>
> Hi,
>
> On 9/2/20 4:10 PM, Van Leeuwen, Pascal wrote:
> >> -----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.
>
> kfree_sensitive() uses memzero_explicit() internally.
>
Ok. Although formally that's still only _current_ implementation.
But given the function name, I guess it's a fair assumption that the intention is to maintain
this behavior going forward.

> > 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.
>
> There are a number of predefined allocation sizes (power of 2) for faster alloc,
> i.e. https://elixir.bootlin.com/linux/latest/source/include/linux/slab.h#L349
> and it looks like that keys we free in this patches are in bounds of these sizes.
> As far as I understand, if a key is not a power of 2 len, the buffer will be zeroed to the closest
> power of 2 size.
>
This path is for hash keys that are larger than the hash block size. Potentially, there is no
upper bound on the size of such a hash key, so it doesn't need to be in that range hence
zeroizing to the next power of 2 boundary could be expensive.
OTOH, I don't expect this path to be frequently used, and the key processing itself already
costs a lot of time, so it's probably not that relevant. Never mind.

I guess was more about whether using kfree_sensitive() is a good replacement _in general_.
For that, there should be some guaranteed upper bound on how much extra will be zeroized.

Given the above considerations (and after testing this on my hardware):

Tested-by: Pascal van Leeuwen <pvanleeuwen@rambus.com>

> For small sizes like these, performance difference should be unnoticeable because
> of cache lines and how arch-optimized memzero() works. Key freeing doesn't look like a frequent event.
>

> Thanks,
> Denis

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.
Please be so kind to update your e-mail address book with my new e-mail address.

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