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Message-ID: <YhZV1g0kvXfBOZ06@iki.fi>
Date: Wed, 23 Feb 2022 16:42:14 +0100
From: Jarkko Sakkinen <jarkko@...nel.org>
To: Ahmad Fatoum <a.fatoum@...gutronix.de>
Cc: James Bottomley <jejb@...ux.ibm.com>,
Mimi Zohar <zohar@...ux.ibm.com>,
David Howells <dhowells@...hat.com>, kernel@...gutronix.de,
Sumit Garg <sumit.garg@...aro.org>,
David Gstir <david@...ma-star.at>,
Pankaj Gupta <pankaj.gupta@....com>,
James Morris <jmorris@...ei.org>,
"Serge E. Hallyn" <serge@...lyn.com>,
Horia Geantă <horia.geanta@....com>,
Aymen Sghaier <aymen.sghaier@....com>,
Herbert Xu <herbert@...dor.apana.org.au>,
"David S. Miller" <davem@...emloft.net>,
Jan Luebbe <j.luebbe@...gutronix.de>,
Eric Biggers <ebiggers@...nel.org>,
Richard Weinberger <richard@....at>,
Franck LENORMAND <franck.lenormand@....com>,
Tim Harvey <tharvey@...eworks.com>,
Matthias Schiffer <matthias.schiffer@...tq-group.com>,
keyrings@...r.kernel.org, linux-crypto@...r.kernel.org,
linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-security-module@...r.kernel.org
Subject: Re: [PATCH v5 2/5] KEYS: trusted: allow users to use kernel RNG for
key material
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@...nel.org>
> Acked-by: Sumit Garg <sumit.garg@...aro.org>
> Reviewed-by: David Gstir <david@...ma-star.at>
> Reviewed-by: Pankaj Gupta <pankaj.gupta@....com>
> Signed-off-by: Ahmad Fatoum <a.fatoum@...gutronix.de>
> ---
> To: James Bottomley <jejb@...ux.ibm.com>
> To: Jarkko Sakkinen <jarkko@...nel.org>
> To: Mimi Zohar <zohar@...ux.ibm.com>
> To: David Howells <dhowells@...hat.com>
> Cc: James Morris <jmorris@...ei.org>
> Cc: "Serge E. Hallyn" <serge@...lyn.com>
> Cc: "Horia Geantă" <horia.geanta@....com>
> Cc: Aymen Sghaier <aymen.sghaier@....com>
> Cc: Herbert Xu <herbert@...dor.apana.org.au>
> Cc: "David S. Miller" <davem@...emloft.net>
> Cc: Jan Luebbe <j.luebbe@...gutronix.de>
> Cc: Eric Biggers <ebiggers@...nel.org>
> Cc: David Gstir <david@...ma-star.at>
> Cc: Richard Weinberger <richard@....at>
> Cc: Franck LENORMAND <franck.lenormand@....com>
> Cc: Sumit Garg <sumit.garg@...aro.org>
> Cc: Tim Harvey <tharvey@...eworks.com>
> Cc: Matthias Schiffer <matthias.schiffer@...tq-group.com>
> Cc: Pankaj Gupta <pankaj.gupta@....com>
> Cc: keyrings@...r.kernel.org
> Cc: linux-crypto@...r.kernel.org
> Cc: linux-integrity@...r.kernel.org
> Cc: linux-kernel@...r.kernel.org
> Cc: linux-security-module@...r.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@...nel.org>
BR, Jarkko
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