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Date: Sat, 10 Jun 2017 10:43:34 +0300
From: Gilad Ben-Yossef <gilad@...yossef.com>
To: "Jason A. Donenfeld" <Jason@...c4.com>
Cc: Linux kernel mailing list <linux-kernel@...r.kernel.org>, kernel-hardening@...ts.openwall.com, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, stable@...r.kernel.org
Subject: Re: [PATCH 3/6] ccree: use constant time memory comparison for macs
 and tags

Thank you Jason,

I think what you are doing is very important.

On Sat, Jun 10, 2017 at 5:59 AM, Jason A. Donenfeld <Jason@...c4.com> wrote:
> Otherwise, we enable several different forgeries via timing attack.
>
> While the C inside this file is nearly incomprehensible, I did notice a
> high volume of "FIPS" and "NIST", which makes this kind of bug slightly
> more embarrassing.
>

The code you are referring to implements, as the function name states,
FIPS power up tests[*].
Specifically, this is the code that compares computed results to known
good results.

As far as I understand the purpose of timing and memory side channel
attacks is to deduce
key material by measurement of time and/or memory usage. However, this
being a FIPS power
up test, the key material is actually part of the source code, so not
much use here.

So, unless I've missed something, I'm going to NAK this one. Your
patch however did inspire me
to look in the ccree driver for other places where not using these
mechanisms is more dangerous,
so thank you for that.

[*] whose implementation inside the driver itself is questionable and
will probably go away as part
of staging clean-ups.

Thanks,
Gilad


> Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
> Cc: Gilad Ben-Yossef <gilad@...yossef.com>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: stable@...r.kernel.org
> ---
>  drivers/staging/ccree/ssi_fips_ll.c | 17 +++++++++--------
>  1 file changed, 9 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/staging/ccree/ssi_fips_ll.c b/drivers/staging/ccree/ssi_fips_ll.c
> index d573574bbb98..3310997d8e3e 100644
> --- a/drivers/staging/ccree/ssi_fips_ll.c
> +++ b/drivers/staging/ccree/ssi_fips_ll.c
> @@ -19,6 +19,7 @@ This file defines the driver FIPS Low Level implmentaion functions,
>  that executes the KAT.
>  ***************************************************************/
>  #include <linux/kernel.h>
> +#include <crypto/algapi.h>
>
>  #include "ssi_driver.h"
>  #include "ssi_fips_local.h"
> @@ -462,7 +463,7 @@ ssi_cipher_fips_power_up_tests(struct ssi_drvdata *drvdata, void *cpu_addr_buffe
>                 }
>
>                 /* compare actual dout to expected */
> -               if (memcmp(virt_ctx->dout, cipherData->dataOut, cipherData->dataInSize) != 0)
> +               if (crypto_memneq(virt_ctx->dout, cipherData->dataOut, cipherData->dataInSize))
>                 {
>                         FIPS_LOG("dout comparison error %d - oprMode=%d, isAes=%d\n", i, cipherData->oprMode, cipherData->isAes);
>                         FIPS_LOG("  i  expected   received \n");
> @@ -586,7 +587,7 @@ ssi_cmac_fips_power_up_tests(struct ssi_drvdata *drvdata, void *cpu_addr_buffer,
>                 }
>
>                 /* compare actual mac result to expected */
> -               if (memcmp(virt_ctx->mac_res, cmac_data->mac_res, cmac_data->mac_res_size) != 0)
> +               if (crypto_memneq(virt_ctx->mac_res, cmac_data->mac_res, cmac_data->mac_res_size))
>                 {
>                         FIPS_LOG("comparison error %d - digest_size=%d \n", i, cmac_data->mac_res_size);
>                         FIPS_LOG("  i  expected   received \n");
> @@ -760,7 +761,7 @@ ssi_hash_fips_power_up_tests(struct ssi_drvdata *drvdata, void *cpu_addr_buffer,
>                  }
>
>                 /* compare actual mac result to expected */
> -               if (memcmp(virt_ctx->mac_res, hash_data->mac_res, digest_size) != 0)
> +               if (crypto_memneq(virt_ctx->mac_res, hash_data->mac_res, digest_size))
>                 {
>                         FIPS_LOG("comparison error %d - hash_mode=%d digest_size=%d \n", i, hash_data->hash_mode, digest_size);
>                         FIPS_LOG("  i  expected   received \n");
> @@ -1093,7 +1094,7 @@ ssi_hmac_fips_power_up_tests(struct ssi_drvdata *drvdata, void *cpu_addr_buffer,
>                 }
>
>                 /* compare actual mac result to expected */
> -               if (memcmp(virt_ctx->mac_res, hmac_data->mac_res, digest_size) != 0)
> +               if (crypto_memneq(virt_ctx->mac_res, hmac_data->mac_res, digest_size))
>                 {
>                         FIPS_LOG("comparison error %d - hash_mode=%d digest_size=%d \n", i, hmac_data->hash_mode, digest_size);
>                         FIPS_LOG("  i  expected   received \n");
> @@ -1310,7 +1311,7 @@ ssi_ccm_fips_power_up_tests(struct ssi_drvdata *drvdata, void *cpu_addr_buffer,
>                 }
>
>                 /* compare actual dout to expected */
> -               if (memcmp(virt_ctx->dout, ccmData->dataOut, ccmData->dataInSize) != 0)
> +               if (crypto_memneq(virt_ctx->dout, ccmData->dataOut, ccmData->dataInSize))
>                 {
>                         FIPS_LOG("dout comparison error %d - size=%d \n", i, ccmData->dataInSize);
>                          error = CC_REE_FIPS_ERROR_AESCCM_PUT;
> @@ -1318,7 +1319,7 @@ ssi_ccm_fips_power_up_tests(struct ssi_drvdata *drvdata, void *cpu_addr_buffer,
>                  }
>
>                 /* compare actual mac result to expected */
> -               if (memcmp(virt_ctx->mac_res, ccmData->macResOut, ccmData->tagSize) != 0)
> +               if (crypto_memneq(virt_ctx->mac_res, ccmData->macResOut, ccmData->tagSize))
>                 {
>                         FIPS_LOG("mac_res comparison error %d - mac_size=%d \n", i, ccmData->tagSize);
>                         FIPS_LOG("  i  expected   received \n");
> @@ -1633,7 +1634,7 @@ ssi_gcm_fips_power_up_tests(struct ssi_drvdata *drvdata, void *cpu_addr_buffer,
>
>                 if (gcmData->direction == DRV_CRYPTO_DIRECTION_ENCRYPT) {
>                         /* compare actual dout to expected */
> -                       if (memcmp(virt_ctx->dout, gcmData->dataOut, gcmData->dataInSize) != 0)
> +                       if (crypto_memneq(virt_ctx->dout, gcmData->dataOut, gcmData->dataInSize))
>                         {
>                                 FIPS_LOG("dout comparison error %d - size=%d \n", i, gcmData->dataInSize);
>                                 FIPS_LOG("  i  expected   received \n");
> @@ -1649,7 +1650,7 @@ ssi_gcm_fips_power_up_tests(struct ssi_drvdata *drvdata, void *cpu_addr_buffer,
>                 }
>
>                 /* compare actual mac result to expected */
> -               if (memcmp(virt_ctx->mac_res, gcmData->macResOut, gcmData->tagSize) != 0)
> +               if (crypto_memneq(virt_ctx->mac_res, gcmData->macResOut, gcmData->tagSize))
>                 {
>                         FIPS_LOG("mac_res comparison error %d - mac_size=%d \n", i, gcmData->tagSize);
>                         FIPS_LOG("  i  expected   received \n");
> --
> 2.13.1
>



-- 
Gilad Ben-Yossef
Chief Coffee Drinker

"If you take a class in large-scale robotics, can you end up in a
situation where the homework eats your dog?"
 -- Jean-Baptiste Queru

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