linux-kernel - Re: [PATCH 18/18] crypto: dh - accept only approved safe-prime groups in FIPS mode

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Date:   Wed, 1 Dec 2021 08:34:49 +0100
From:   Hannes Reinecke <hare@...e.de>
To:     Nicolai Stange <nstange@...e.de>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>
Cc:     Stephan Müller <smueller@...onox.de>,
        Torsten Duwe <duwe@...e.de>, Zaibo Xu <xuzaibo@...wei.com>,
        Giovanni Cabiddu <giovanni.cabiddu@...el.com>,
        David Howells <dhowells@...hat.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org,
        qat-linux@...el.com, keyrings@...r.kernel.org
Subject: Re: [PATCH 18/18] crypto: dh - accept only approved safe-prime groups
 in FIPS mode

On 12/1/21 1:48 AM, Nicolai Stange wrote:
> SP800-56Arev3, sec. 5.5.2 ("Assurance of Domain-Parameter Validity")
> asserts that an implementation needs to verify domain paramtere validity,
> which boils down to either
> - the domain parameters corresponding to some known safe-prime group
>    explicitly listed to be approved in the document or
> - for parameters conforming to a "FIPS 186-type parameter-size set",
>    that the implementation needs to perform an explicit domain parameter
>    verification, which would require access to the "seed" and "counter"
>    values used in their generation.
> 
> The latter is not easily feasible and moreover, SP800-56Arev3 states that
> safe-prime groups are preferred and that FIPS 186-type parameter sets
> should only be supported for backward compatibility, if it all.
> 
> Make the dh implementations reject any domain parameters which don't
> correspond to any of the approved safe-prime groups in FIPS mode. The
> approved safe-prime groups are the ones specified in RFC 7919 and RFC 3526,
> and given that all possible values of enum dh_group_id correspond to
> either groups from these RFCs or to dh_group_id_unknown, it suffices to
> make crypto_dh_decode_key() to reject any parameter set where
> ->group_id == dh_group_id_unknown.
> 
> As this change will effectively render the dh implementation unusable in
> FIPS mode if neither of the CRYPTO_DH_GROUPS_RFC7919 or
> CRYPTO_DH_GROUPS_RFC3526 Kconfig options enabled, make CRYPTO_DH imply
> these two if CRYPTO_FIPS is set.
> 
> Signed-off-by: Nicolai Stange <nstange@...e.de>
> ---
>   crypto/Kconfig     | 2 ++
>   crypto/dh_helper.c | 4 ++++
>   2 files changed, 6 insertions(+)
> 
> diff --git a/crypto/Kconfig b/crypto/Kconfig
> index 578711b02bb3..571f2271ad2e 100644
> --- a/crypto/Kconfig
> +++ b/crypto/Kconfig
> @@ -229,6 +229,8 @@ menuconfig CRYPTO_DH
>   	select CRYPTO_KPP
>   	select MPILIB
>   	select CRYPTO_RNG_DEFAULT
> +	imply CRYPTO_DH_GROUPS_RFC7919 if CRYPTO_FIPS
> +	imply CRYPTO_DH_GROUPS_RFC3526 if CRYPTO_FIPS
>   	help
>   	  Generic implementation of the Diffie-Hellman algorithm.
>   
> diff --git a/crypto/dh_helper.c b/crypto/dh_helper.c
> index cf632beca65e..f30674df0d76 100644
> --- a/crypto/dh_helper.c
> +++ b/crypto/dh_helper.c
> @@ -7,6 +7,7 @@
>   #include <linux/export.h>
>   #include <linux/err.h>
>   #include <linux/string.h>
> +#include <linux/fips.h>
>   #include <crypto/dh.h>
>   #include <crypto/kpp.h>
>   #include <crypto/rng.h>
> @@ -622,6 +623,9 @@ int crypto_dh_decode_key(const char *buf, unsigned int len, struct dh *params)
>   	    params->g_size > params->p_size)
>   		return -EINVAL;
>   
> +	/* Only safe-prime groups are allowed in FIPS mode. */
> +	if (fips_enabled && params->group_id == dh_group_id_unknown)
> +		return -EINVAL;
>   
>   	return 0;
>   }
> 
That was cheap.
Maybe merge it with the previous patch?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
hare@...e.de                              +49 911 74053 688
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