Date:   Wed, 1 Dec 2021 08:17:22 +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 03/18] crypto: dh - optimize domain parameter
 serialization for well-known groups

On 12/1/21 1:48 AM, Nicolai Stange wrote:
> DH users are supposed to set a struct dh instance's ->p and ->g domain
> parameters (as well as the secret ->key), serialize the whole struct dh
> instance via the crypto_dh_encode_key() helper and pass the encoded blob
> on to the DH's ->set_secret(). All three currently available DH
> implementations (generic, drivers/crypto/hisilicon/hpre/ and
> drivers/crypto/qat/) would then proceed to call the crypto_dh_decode_key()
> helper for unwrapping the encoded struct dh instance again.
> 
> Up to now, the only DH user has been the keyctl(KEYCTL_DH_COMPUTE) syscall
> and thus, all domain parameters have been coming from userspace. The domain
> parameter encoding scheme for DH's ->set_secret() has been a perfectly
> reasonable approach in this setting and the potential extra copy of ->p
> and ->g during the encoding phase didn't harm much.
> 
> However, recently, the need for working with the well-known safe-prime
> groups' domain parameters from RFC 3526 and RFC 7919 resp. arose from two
> independent developments:
> - The NVME in-band authentication support currently being worked on ([1])
>    needs to install the RFC 7919 ffdhe groups' domain parameters for DH
>    tfms.
> - In FIPS mode, there's effectively no sensible way for the DH
>    implementation to conform to SP800-56Arev3 other than rejecting any
>    parameter set not corresponding to some approved safe-prime group
>    specified in either of these two RFCs.
> 
> As the ->p arrays' lengths are in the range from 256 to 1024 bytes, it
> would be nice if that extra copy during the crypto_dh_encode_key() step
> from the NVME in-band authentication code could be avoided. Likewise, it
> would be great if the DH implementation's FIPS handling code could avoid
> attempting to match the input ->p and ->g against the individual approved
> groups' parameters via memcmp() if it's known in advance that the input
> corresponds to such one, as is the case for NVME.
> 
> Introduce a enum dh_group_id for referring to any of the safe-prime groups
> known to the kernel. The introduction of actual such safe-prime groups
> alongside with their resp. P and G parameters will be deferred to later
> patches. As of now, the new enum contains only a single member,
> dh_group_id_unknown, which is meant to be associated with parameter sets
> not corresponding to any of the groups known to the kernel, as is needed
> to continue to support the current keyctl(KEYCTL_DH_COMPUTE) syscall
> semantics.
> 
> Add a new 'group_id' member of type enum group_id to struct dh. Make
> crypto_dh_encode_key() include it in the serialization and to encode
> ->p and ->g only if it equals dh_group_id_unknown. For all other possible
> values of the encoded ->group_id, the receiving decoding primitive,
> crypto_dh_decode_key(), is made to not decode ->p and ->g from the encoded
> data, but to look them up in a central registry instead.
> 
> The intended usage pattern is that users like NVME wouldn't set any of
> the struct dh's ->p or ->g directly, but only the ->group_id for the group
> they're interested in. They'd then proceed as usual and call
> crypto_dh_encode_key() on the struct dh instance, pass the encoded result
> on to DH's ->set_secret() and the latter would then invoke
> crypto_dh_decode_key(), which would then in turn lookup the parameters
> associated with the passed ->group_id.
> 
> Note that this will avoid the extra copy of the ->p and ->g for the groups
> (to be made) known to the kernel and also, that a future patch can easily
> introduce a validation of ->group_id if in FIPS mode.
> 
> As mentioned above, the introduction of actual safe-prime groups will be
> deferred to later patches, so for now, only introduce an empty placeholder
> array safe_prime_groups[] to be queried by crypto_dh_decode_key() for
> domain parameters associated with a given ->group_id as outlined above.
> Make its elements to be of the new internal struct safe_prime_group type.
> Among the members ->group_id, ->p and ->p_size with obvious meaning, there
> will also be a ->max_strength member for storing the maximum security
> strength supported by the associated group -- its value will be needed for
> the upcoming private key generation support.
> 
> Finally, update the encoded secrets provided by the testmgr's DH test
> vectors in order to account for the additional ->group_id field expected
> by crypto_dh_decode_key() now.
> 
> [1] https://lkml.kernel.org/r/20211122074727.25988-4-hare@suse.de
> 
> Signed-off-by: Nicolai Stange <nstange@...e.de>
> ---
>   crypto/dh_helper.c  | 88 +++++++++++++++++++++++++++++++++++----------
>   crypto/testmgr.h    | 16 +++++----
>   include/crypto/dh.h |  6 ++++
>   3 files changed, 86 insertions(+), 24 deletions(-)
> 
> diff --git a/crypto/dh_helper.c b/crypto/dh_helper.c
> index aabc91e4f63f..a6c9389d8219 100644
> --- a/crypto/dh_helper.c
> +++ b/crypto/dh_helper.c
> @@ -10,7 +10,32 @@
>   #include <crypto/dh.h>
>   #include <crypto/kpp.h>
>   
> -#define DH_KPP_SECRET_MIN_SIZE (sizeof(struct kpp_secret) + 3 * sizeof(int))
> +#define DH_KPP_SECRET_MIN_SIZE (sizeof(struct kpp_secret) + \
> +				sizeof(enum dh_group_id) + 3 * sizeof(int))

That is not a good practise; 'enum' doesn't have a defined size, and 
will typically default to 'unsigned int'.
But this might well be compiler dependent, so I suggest using a fixes 
size here.

> +
> +static const struct safe_prime_group
> +{
> +	enum dh_group_id group_id;
> +	unsigned int max_strength;
> +	unsigned int p_size;
> +	const char *p;
> +} safe_prime_groups[] = {};
> +
> +/* 2 is used as a generator for all safe-prime groups. */
> +static const char safe_prime_group_g[]  = { 2 };
> +
> +static inline const struct safe_prime_group *
> +get_safe_prime_group(enum dh_group_id group_id)
> +{
> +	int i;
> +
> +	for (i = 0; i < ARRAY_SIZE(safe_prime_groups); ++i) {
> +		if (safe_prime_groups[i].group_id == group_id)
> +			return &safe_prime_groups[i];
> +	}
> +
> +	return NULL;
> +}
>   
>   static inline u8 *dh_pack_data(u8 *dst, u8 *end, const void *src, size_t size)
>   {
> @@ -28,7 +53,10 @@ static inline const u8 *dh_unpack_data(void *dst, const void *src, size_t size)
>   
>   static inline unsigned int dh_data_size(const struct dh *p)
>   {
> -	return p->key_size + p->p_size + p->g_size;
> +	if (p->group_id == dh_group_id_unknown)
> +		return p->key_size + p->p_size + p->g_size;
> +	else
> +		return p->key_size;
>   }
>   
>   unsigned int crypto_dh_key_len(const struct dh *p)
> @@ -50,13 +78,18 @@ int crypto_dh_encode_key(char *buf, unsigned int len, const struct dh *params)
>   		return -EINVAL;
>   
>   	ptr = dh_pack_data(ptr, end, &secret, sizeof(secret));
> +	ptr = dh_pack_data(ptr, end, &params->group_id,
> +			   sizeof(params->group_id));
>   	ptr = dh_pack_data(ptr, end, &params->key_size,
>   			   sizeof(params->key_size));
>   	ptr = dh_pack_data(ptr, end, &params->p_size, sizeof(params->p_size));
>   	ptr = dh_pack_data(ptr, end, &params->g_size, sizeof(params->g_size));
>   	ptr = dh_pack_data(ptr, end, params->key, params->key_size);
> -	ptr = dh_pack_data(ptr, end, params->p, params->p_size);
> -	ptr = dh_pack_data(ptr, end, params->g, params->g_size);
> +	if (params->group_id == dh_group_id_unknown) {
> +		ptr = dh_pack_data(ptr, end, params->p, params->p_size);
> +		ptr = dh_pack_data(ptr, end, params->g, params->g_size);
> +	}
> +
>   	if (ptr != end)
>   		return -EINVAL;
>   	return 0;
> @@ -75,12 +108,45 @@ int crypto_dh_decode_key(const char *buf, unsigned int len, struct dh *params)
>   	if (secret.type != CRYPTO_KPP_SECRET_TYPE_DH)
>   		return -EINVAL;
>   
> +	ptr = dh_unpack_data(&params->group_id, ptr, sizeof(params->group_id));
>   	ptr = dh_unpack_data(&params->key_size, ptr, sizeof(params->key_size));
>   	ptr = dh_unpack_data(&params->p_size, ptr, sizeof(params->p_size));
>   	ptr = dh_unpack_data(&params->g_size, ptr, sizeof(params->g_size));
>   	if (secret.len != crypto_dh_key_len(params))
>   		return -EINVAL;
>   
> +	if (params->group_id == dh_group_id_unknown) {
> +		/* Don't allocate memory. Set pointers to data within
> +		 * the given buffer
> +		 */
> +		params->key = (void *)ptr;
> +		params->p = (void *)(ptr + params->key_size);
> +		params->g = (void *)(ptr + params->key_size + params->p_size);
> +
> +		/*
> +		 * Don't permit 'p' to be 0.  It's not a prime number,
> +		 * and it's subject to corner cases such as 'mod 0'
> +		 * being undefined or crypto_kpp_maxsize() returning
> +		 * 0.
> +		 */
> +		if (memchr_inv(params->p, 0, params->p_size) == NULL)
> +			return -EINVAL;
> +
> +	} else {
> +		const struct safe_prime_group *g;
> +
> +		g = get_safe_prime_group(params->group_id);
> +		if (!g)
> +			return -EINVAL;
> +
> +		params->key = (void *)ptr;
> +
> +		params->p = g->p;
> +		params->p_size = g->p_size;
> +		params->g = safe_prime_group_g;
> +		params->g_size = sizeof(safe_prime_group_g);
> +	}
> +
>   	/*
>   	 * Don't permit the buffer for 'key' or 'g' to be larger than 'p', since
>   	 * some drivers assume otherwise.
> @@ -89,20 +155,6 @@ int crypto_dh_decode_key(const char *buf, unsigned int len, struct dh *params)
>   	    params->g_size > params->p_size)
>   		return -EINVAL;
>   
> -	/* Don't allocate memory. Set pointers to data within
> -	 * the given buffer
> -	 */
> -	params->key = (void *)ptr;
> -	params->p = (void *)(ptr + params->key_size);
> -	params->g = (void *)(ptr + params->key_size + params->p_size);
> -
> -	/*
> -	 * Don't permit 'p' to be 0.  It's not a prime number, and it's subject
> -	 * to corner cases such as 'mod 0' being undefined or
> -	 * crypto_kpp_maxsize() returning 0.
> -	 */
> -	if (memchr_inv(params->p, 0, params->p_size) == NULL)
> -		return -EINVAL;
>   
>   	return 0;
>   }
> diff --git a/crypto/testmgr.h b/crypto/testmgr.h
> index 7f7d5ae48721..a8983c8626fa 100644
> --- a/crypto/testmgr.h
> +++ b/crypto/testmgr.h
> @@ -1244,13 +1244,15 @@ static const struct kpp_testvec dh_tv_template[] = {
>   	.secret =
>   #ifdef __LITTLE_ENDIAN
>   	"\x01\x00" /* type */
> -	"\x11\x02" /* len */
> +	"\x15\x02" /* len */
> +	"\x00\x00\x00\x00" /* group_id == dh_group_id_unknown */
>   	"\x00\x01\x00\x00" /* key_size */
>   	"\x00\x01\x00\x00" /* p_size */
>   	"\x01\x00\x00\x00" /* g_size */
>   #else
>   	"\x00\x01" /* type */
> -	"\x02\x11" /* len */
> +	"\x02\x15" /* len */
> +	"\x00\x00\x00\x00" /* group_id == dh_group_id_unknown */
>   	"\x00\x00\x01\x00" /* key_size */
>   	"\x00\x00\x01\x00" /* p_size */
>   	"\x00\x00\x00\x01" /* g_size */
> @@ -1342,7 +1344,7 @@ static const struct kpp_testvec dh_tv_template[] = {
>   	"\xd3\x34\x49\xad\x64\xa6\xb1\xc0\x59\x28\x75\x60\xa7\x8a\xb0\x11"
>   	"\x56\x89\x42\x74\x11\xf5\xf6\x5e\x6f\x16\x54\x6a\xb1\x76\x4d\x50"
>   	"\x8a\x68\xc1\x5b\x82\xb9\x0d\x00\x32\x50\xed\x88\x87\x48\x92\x17",
> -	.secret_size = 529,
> +	.secret_size = 533,
>   	.b_public_size = 256,
>   	.expected_a_public_size = 256,
>   	.expected_ss_size = 256,
> @@ -1351,13 +1353,15 @@ static const struct kpp_testvec dh_tv_template[] = {
>   	.secret =
>   #ifdef __LITTLE_ENDIAN
>   	"\x01\x00" /* type */
> -	"\x11\x02" /* len */
> +	"\x15\x02" /* len */
> +	"\x00\x00\x00\x00" /* group_id == dh_group_id_unknown */
>   	"\x00\x01\x00\x00" /* key_size */
>   	"\x00\x01\x00\x00" /* p_size */
>   	"\x01\x00\x00\x00" /* g_size */
>   #else
>   	"\x00\x01" /* type */
> -	"\x02\x11" /* len */
> +	"\x02\x15" /* len */
> +	"\x00\x00\x00\x00" /* group_id == dh_group_id_unknown */
>   	"\x00\x00\x01\x00" /* key_size */
>   	"\x00\x00\x01\x00" /* p_size */
>   	"\x00\x00\x00\x01" /* g_size */
> @@ -1449,7 +1453,7 @@ static const struct kpp_testvec dh_tv_template[] = {
>   	"\x5e\x5a\x64\xbd\xf6\x85\x04\xe8\x28\x6a\xac\xef\xce\x19\x8e\x9a"
>   	"\xfe\x75\xc0\x27\x69\xe3\xb3\x7b\x21\xa7\xb1\x16\xa4\x85\x23\xee"
>   	"\xb0\x1b\x04\x6e\xbd\xab\x16\xde\xfd\x86\x6b\xa9\x95\xd7\x0b\xfd",
> -	.secret_size = 529,
> +	.secret_size = 533,
>   	.b_public_size = 256,
>   	.expected_a_public_size = 256,
>   	.expected_ss_size = 256,
> diff --git a/include/crypto/dh.h b/include/crypto/dh.h
> index 67f3f6bca527..15d8b2dfe4a2 100644
> --- a/include/crypto/dh.h
> +++ b/include/crypto/dh.h
> @@ -19,6 +19,11 @@
>    * the KPP API function call of crypto_kpp_set_secret.
>    */
>   
> +/** enum dh_group_id - identify well-known domain parameter sets */
> +enum dh_group_id {
> +	dh_group_id_unknown = 0,
> +};
> +

Shouldn't the enum definitions be in uppercase?

>   /**
>    * struct dh - define a DH private key
>    *
> @@ -30,6 +35,7 @@
>    * @g_size:	Size of DH generator G
>    */
>   struct dh {
> +	enum dh_group_id group_id;
>   	const void *key;
>   	const void *p;
>   	const void *g;
> 

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
hare@...e.de                              +49 911 74053 688
SUSE Software Solutions GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 36809 (AG Nürnberg), Geschäftsführer: Felix Imendörffer

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