Re: [PATCH v6 2/3] IMA: Define an IMA hook to measure keys

[Mimi Zohar <zohar@linux.ibm.com>]

On Wed, 2019-11-13 at 10:46 -0800, Lakshmi Ramasubramanian wrote:
> Measure asymmetric keys used for verifying file signatures,
> certificates, etc.
> 
> This patch defines a new IMA hook namely ima_post_key_create_or_update()
> to measure asymmetric keys.
> 
> Note that currently IMA subsystem can be enabled without
> enabling KEYS subsystem.
> 
> Adding support for measuring asymmetric keys in IMA requires KEYS
> subsystem to be enabled. To handle this dependency a new config
> namely CONFIG_IMA_MEASURE_ASYMMETRIC_KEYS has been added. Enabling
> this config requires the following configs to be enabled:
>     CONFIG_IMA, CONFIG_KEYS, CONFIG_ASYMMETRIC_KEY_TYPE, and
>     CONFIG_ASYMMETRIC_PUBLIC_KEY_SUBTYPE.
> 
> CONFIG_IMA_MEASURE_ASYMMETRIC_KEYS is off by default.
> 
> The IMA hook is defined in a new file namely ima_asymmetric_keys.c
> which is built only if CONFIG_IMA_MEASURE_ASYMMETRIC_KEYS is enabled.
> 
> Signed-off-by: Lakshmi Ramasubramanian <nramas@linux.microsoft.com>

All that is is needed is the key and public_key structures, which are
defined in include/linux/keys.h and include/crypto/public_key.h.  If
the keys subsystem is disabled, then the new IMA hook won't be called.
 There's no need for a new Kconfig option or a new file.

Please move the hook to just after ima_kexec_cmdline().

Mimi

> ---
>  security/integrity/ima/Kconfig               | 14 ++++++
>  security/integrity/ima/Makefile              |  1 +
>  security/integrity/ima/ima_asymmetric_keys.c | 51 ++++++++++++++++++++
>  3 files changed, 66 insertions(+)
>  create mode 100644 security/integrity/ima/ima_asymmetric_keys.c
> 
> diff --git a/security/integrity/ima/Kconfig b/security/integrity/ima/Kconfig
> index 838476d780e5..c6d14884bc19 100644
> --- a/security/integrity/ima/Kconfig
> +++ b/security/integrity/ima/Kconfig
> @@ -310,3 +310,17 @@ config IMA_APPRAISE_SIGNED_INIT
>  	default n
>  	help
>  	   This option requires user-space init to be signed.
> +
> +config IMA_MEASURE_ASYMMETRIC_KEYS
> +	bool "Enable measuring asymmetric keys on key create or update"
> +	depends on IMA
> +	depends on KEYS
> +	depends on ASYMMETRIC_KEY_TYPE
> +	depends on ASYMMETRIC_PUBLIC_KEY_SUBTYPE
> +	default n
> +	help
> +	   This option enables measuring asymmetric keys when
> +	   the key is created or updated. Additionally, IMA policy
> +	   needs to be configured to either measure keys linked to
> +	   any keyring or only measure keys linked to the keyrings
> +	   specified in the IMA policy through the keyrings= option.
> diff --git a/security/integrity/ima/Makefile b/security/integrity/ima/Makefile
> index 31d57cdf2421..3e9d0ad68c7b 100644
> --- a/security/integrity/ima/Makefile
> +++ b/security/integrity/ima/Makefile
> @@ -12,3 +12,4 @@ ima-$(CONFIG_IMA_APPRAISE) += ima_appraise.o
>  ima-$(CONFIG_IMA_APPRAISE_MODSIG) += ima_modsig.o
>  ima-$(CONFIG_HAVE_IMA_KEXEC) += ima_kexec.o
>  obj-$(CONFIG_IMA_BLACKLIST_KEYRING) += ima_mok.o
> +obj-$(CONFIG_IMA_MEASURE_ASYMMETRIC_KEYS) += ima_asymmetric_keys.o
> diff --git a/security/integrity/ima/ima_asymmetric_keys.c b/security/integrity/ima/ima_asymmetric_keys.c
> new file mode 100644
> index 000000000000..f6884641a622
> --- /dev/null
> +++ b/security/integrity/ima/ima_asymmetric_keys.c
> @@ -0,0 +1,51 @@
> +// SPDX-License-Identifier: GPL-2.0+
> +/*
> + * Copyright (C) 2019 Microsoft Corporation
> + *
> + * Author: Lakshmi Ramasubramanian (nramas@linux.microsoft.com)
> + *
> + * File: ima_asymmetric_keys.c
> + *       Defines an IMA hook to measure asymmetric keys on key
> + *       create or update.
> + */
> +
> +#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
> +
> +#include <crypto/public_key.h>
> +#include <keys/asymmetric-type.h>
> +#include "ima.h"
> +
> +/**
> + * ima_post_key_create_or_update - measure asymmetric keys
> + * @keyring: keyring to which the key is linked to
> + * @key: created or updated key
> + * @flags: key flags
> + * @create: flag indicating whether the key was created or updated
> + *
> + * Keys can only be measured, not appraised.
> + */
> +void ima_post_key_create_or_update(struct key *keyring, struct key *key,
> +				   unsigned long flags, bool create)
> +{
> +	const struct public_key *pk;
> +
> +	/* Only asymmetric keys are handled by this hook. */
> +	if (key->type != &key_type_asymmetric)
> +		return;
> +
> +	/* Get the public_key of the given asymmetric key to measure. */
> +	pk = key->payload.data[asym_crypto];
> +
> +	/*
> +	 * keyring->description points to the name of the keyring
> +	 * (such as ".builtin_trusted_keys", ".ima", etc.) to
> +	 * which the given key is linked to.
> +	 *
> +	 * The name of the keyring is passed in the "eventname"
> +	 * parameter to process_buffer_measurement() and is set
> +	 * in the "eventname" field in ima_event_data for
> +	 * the key measurement IMA event.
> +	 */
> +	process_buffer_measurement(pk->key, pk->keylen,
> +				   keyring->description, KEY_CHECK, 0);
> +}