Re: [PATCH v5 2/5] KEYS: trusted: allow users to use kernel RNG for key material

[Jarkko Sakkinen <jarkko@kernel.org>]

On Tue, Feb 22, 2022 at 08:58:16PM +0100, Ahmad Fatoum wrote:
> The two existing trusted key sources don't make use of the kernel RNG,
> but instead let the hardware doing the sealing/unsealing also
> generate the random key material. However, users may want to place
> less trust into the quality of the trust source's random number
> generator and instead use the kernel entropy pool, which can be
> seeded from multiple entropy sources.
> 
> Make this possible by adding a new trusted.kernel_rng parameter,
> that will force use of the kernel RNG. In its absence, it's up
> to the trust source to decide, which random numbers to use,
> maintaining the existing behavior.
> 
> Suggested-by: Jarkko Sakkinen <jarkko@kernel.org>
> Acked-by: Sumit Garg <sumit.garg@linaro.org>
> Reviewed-by: David Gstir <david@sigma-star.at>
> Reviewed-by: Pankaj Gupta <pankaj.gupta@nxp.com>
> Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
> ---
> To: James Bottomley <jejb@linux.ibm.com>
> To: Jarkko Sakkinen <jarkko@kernel.org>
> To: Mimi Zohar <zohar@linux.ibm.com>
> To: David Howells <dhowells@redhat.com>
> Cc: James Morris <jmorris@namei.org>
> Cc: "Serge E. Hallyn" <serge@hallyn.com>
> Cc: "Horia Geantă" <horia.geanta@nxp.com>
> Cc: Aymen Sghaier <aymen.sghaier@nxp.com>
> Cc: Herbert Xu <herbert@gondor.apana.org.au>
> Cc: "David S. Miller" <davem@davemloft.net>
> Cc: Jan Luebbe <j.luebbe@pengutronix.de>
> Cc: Eric Biggers <ebiggers@kernel.org>
> Cc: David Gstir <david@sigma-star.at>
> Cc: Richard Weinberger <richard@nod.at>
> Cc: Franck LENORMAND <franck.lenormand@nxp.com>
> Cc: Sumit Garg <sumit.garg@linaro.org>
> Cc: Tim Harvey <tharvey@gateworks.com>
> Cc: Matthias Schiffer <matthias.schiffer@ew.tq-group.com>
> Cc: Pankaj Gupta <pankaj.gupta@nxp.com>
> Cc: keyrings@vger.kernel.org
> Cc: linux-crypto@vger.kernel.org
> Cc: linux-integrity@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Cc: linux-security-module@vger.kernel.org
> ---
>  .../admin-guide/kernel-parameters.txt         | 10 ++++++
>  .../security/keys/trusted-encrypted.rst       | 20 ++++++-----
>  security/keys/trusted-keys/trusted_core.c     | 35 ++++++++++++++++++-
>  3 files changed, 56 insertions(+), 9 deletions(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index f5a27f067db9..844c883ca9d8 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -5880,6 +5880,16 @@
>  			first trust source as a backend which is initialized
>  			successfully during iteration.
>  
> +	trusted.rng=	[KEYS]
> +			Format: <string>
> +			The RNG used to generate key material for trusted keys.
> +			Can be one of:
> +			- "kernel"
> +			- the same value as trusted.source: "tpm" or "tee"
> +			- "default"
> +			If not specified, "default" is used. In this case,
> +			the RNG's choice is left to each individual trust source.
> +
>  	tsc=		Disable clocksource stability checks for TSC.
>  			Format: <string>
>  			[x86] reliable: mark tsc clocksource as reliable, this
> diff --git a/Documentation/security/keys/trusted-encrypted.rst b/Documentation/security/keys/trusted-encrypted.rst
> index 80d5a5af62a1..99cf34d7c025 100644
> --- a/Documentation/security/keys/trusted-encrypted.rst
> +++ b/Documentation/security/keys/trusted-encrypted.rst
> @@ -87,22 +87,26 @@ Key Generation
>  Trusted Keys
>  ------------
>  
> -New keys are created from random numbers generated in the trust source. They
> -are encrypted/decrypted using a child key in the storage key hierarchy.
> -Encryption and decryption of the child key must be protected by a strong
> -access control policy within the trust source.
> +New keys are created from random numbers. They are encrypted/decrypted using
> +a child key in the storage key hierarchy. Encryption and decryption of the
> +child key must be protected by a strong access control policy within the
> +trust source. The random number generator in use differs according to the
> +selected trust source:
>  
> -  *  TPM (hardware device) based RNG
> +  *  TPM: hardware device based RNG
>  
> -     Strength of random numbers may vary from one device manufacturer to
> -     another.
> +     Keys are generated within the TPM. Strength of random numbers may vary
> +     from one device manufacturer to another.
>  
> -  *  TEE (OP-TEE based on Arm TrustZone) based RNG
> +  *  TEE: OP-TEE based on Arm TrustZone based RNG
>  
>       RNG is customizable as per platform needs. It can either be direct output
>       from platform specific hardware RNG or a software based Fortuna CSPRNG
>       which can be seeded via multiple entropy sources.
>  
> +Users may override this by specifying ``trusted.rng=kernel`` on the kernel
> +command-line to override the used RNG with the kernel's random number pool.
> +
>  Encrypted Keys
>  --------------
>  
> diff --git a/security/keys/trusted-keys/trusted_core.c b/security/keys/trusted-keys/trusted_core.c
> index 7cdbd16aed30..9235fb7d0ec9 100644
> --- a/security/keys/trusted-keys/trusted_core.c
> +++ b/security/keys/trusted-keys/trusted_core.c
> @@ -16,12 +16,17 @@
>  #include <linux/key-type.h>
>  #include <linux/module.h>
>  #include <linux/parser.h>
> +#include <linux/random.h>
>  #include <linux/rcupdate.h>
>  #include <linux/slab.h>
>  #include <linux/static_call.h>
>  #include <linux/string.h>
>  #include <linux/uaccess.h>
>  
> +static char *trusted_rng = "default";
> +module_param_named(rng, trusted_rng, charp, 0);
> +MODULE_PARM_DESC(rng, "Select trusted key RNG");
> +
>  static char *trusted_key_source;
>  module_param_named(source, trusted_key_source, charp, 0);
>  MODULE_PARM_DESC(source, "Select trusted keys source (tpm or tee)");
> @@ -312,8 +317,14 @@ struct key_type key_type_trusted = {
>  };
>  EXPORT_SYMBOL_GPL(key_type_trusted);
>  
> +static int kernel_get_random(unsigned char *key, size_t key_len)
> +{
> +	return get_random_bytes_wait(key, key_len) ?: key_len;
> +}
> +
>  static int __init init_trusted(void)
>  {
> +	int (*get_random)(unsigned char *key, size_t key_len);
>  	int i, ret = 0;
>  
>  	for (i = 0; i < ARRAY_SIZE(trusted_key_sources); i++) {
> @@ -322,6 +333,28 @@ static int __init init_trusted(void)
>  			    strlen(trusted_key_sources[i].name)))
>  			continue;
>  
> +		/*
> +		 * We always support trusted.rng="kernel" and "default" as
> +		 * well as trusted.rng=$trusted.source if the trust source
> +		 * defines its own get_random callback.
> +		 */
> +		get_random = trusted_key_sources[i].ops->get_random;
> +		if (trusted_rng && strcmp(trusted_rng, "default")) {
> +			if (!strcmp(trusted_rng, "kernel")) {
> +				get_random = kernel_get_random;
> +			} else if (strcmp(trusted_rng, trusted_key_sources[i].name) ||
> +				   !get_random) {
> +				pr_warn("Unsupported RNG. Supported: kernel");
> +				if (get_random)
> +					pr_cont(", %s", trusted_key_sources[i].name);
> +				pr_cont(", default\n");
> +				return -EINVAL;
> +			}
> +		}
> +
> +		if (!get_random)
> +			get_random = kernel_get_random;
> +
>  		static_call_update(trusted_key_init,
>  				   trusted_key_sources[i].ops->init);
>  		static_call_update(trusted_key_seal,
> @@ -329,7 +362,7 @@ static int __init init_trusted(void)
>  		static_call_update(trusted_key_unseal,
>  				   trusted_key_sources[i].ops->unseal);
>  		static_call_update(trusted_key_get_random,
> -				   trusted_key_sources[i].ops->get_random);
> +				   get_random);
>  		static_call_update(trusted_key_exit,
>  				   trusted_key_sources[i].ops->exit);
>  		migratable = trusted_key_sources[i].ops->migratable;
> -- 
> 2.30.2
> 

Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>

BR, Jarkko