Re: [PATCH v3] certs: Add option to disallow non-CA certificates in secondary trusted keying

["Jarkko Sakkinen" <jarkko@kernel.org>]

On Mon Oct 2, 2023 at 1:46 PM EEST, Denis Glazkov wrote:
> The Linux kernel has an IMA (Integrity Measurement Architecture)
> subsystem to check the integrity of the file system based on digital
> signatures. IMA uses certificates in `.ima` keying to check integrity.
>
> Only certificates issued by one of the trusted CA (Certificate Authority)
> certificates can be added to the `.ima` keying.
>
> The Linux kernel now has a secondary trusted keying to which trusted
> certificates from user space can be added if you have superuser
> privileges. Previously, all trusted certificates were in the built-in
> trusted keying, which could not be modified from user space.
> Trusted certificates were placed in the built-in trusted keying at
> kernel compile time.
>
> The secondary trusted keying is designed so that any certificates that
> are signed by one of the trusted CA certificates in the built-in or
> secondary trusted keyring can be added to it.
>
> Let's imagine that we have the following certificate trust chain:
>
>              ┌───────────────────────────┬─────────────────────┐
>              │                           │     ┌───────┐       │
>              │                           │     │       │       │
> ┌────────────▼────────┐    ┌─────────────▼─────▼────┐  │ ┌─────┴─────┐
> │.builtin_trusted_keys│◄───┤.secondary_trusted_keys ├──┘ │   .ima    │
> ├─────────────────────┤    ├────────────────────────┤    ├───────────┤
> │     Root CA Cert    │-----► Intermediate CA Cert  │-----► IMA Cert │
> └─────────────────────┘    └────────────────────────┘    └───────────┘
>
>                 Issues                  Restricted by
>             -------------►             ──────────────►
>
> Since the IMA certificate is signed by a CA certificate from a secondary
> trusted keying, an attacker with superuser privileges will be able to
> add the IMA certificate to the secondary trusted keying. That is, the IMA
> certificate will become trusted.
>
> Since, with `CONFIG_MODULE_SIG` option enabled, modules can only be
> loaded into kernel space if they are signed with one of the trusted
> certificates, an attacker could sign untrusted kernel modules with
> the private key corresponding to the IMA certificate and successfully
> load the untrusted modules into kernel space.
>
> This patch was created not to solve only the problem of loading
> untrusted kernel modules, but to make it possible to use a secondary
> trusted keying only as a part of a chain of trust containing only
> CA certificates with no digital signature capability. This will
> help avoid similar problems when new features appear in the linux
> kernel that are similar to kernel modules in terms of their impact
> on system security, which will also use trusted certificates for
> signature verification.
>
> This patch adds the configuration that once enabled, only
> certificates that meet the following requirements can be added
> to the secondary trusted keying:
>
> 1. The certificate is a CA (Certificate Authority)
> 2. The certificate must be used for verifying a CA's signatures
> 3. The certificate must not be used for digital signatures
>
> Signed-off-by: Denis Glazkov <d.glazkov@omp.ru>
> ---
> v1 -> v2:
>  - Rebase the patch from `linux-next` to the main `linux` repo master branch
>  - Make the commit message more detailed
>  - Move the variable declaration to the `if` block
>  - Replace `#ifdef` with `IS_ENABLED` macro
>
> v2 -> v3:
>  - Add the purpose and goal of the patch to the commit message
> ---
>  certs/Kconfig          |  9 +++++++++
>  certs/system_keyring.c | 16 ++++++++++++++++
>  2 files changed, 25 insertions(+)
>
> diff --git a/certs/Kconfig b/certs/Kconfig
> index 1f109b070877..4a4dc8aab892 100644
> --- a/certs/Kconfig
> +++ b/certs/Kconfig
> @@ -90,6 +90,15 @@ config SECONDARY_TRUSTED_KEYRING
>  	  those keys are not blacklisted and are vouched for by a key built
>  	  into the kernel or already in the secondary trusted keyring.
>  
> +config SECONDARY_TRUSTED_KEYRING_FOR_CA_CERTIFICATES_ONLY
> +	bool "Allow only CA certificates to be added to the secondary trusted keyring"
> +	depends on SECONDARY_TRUSTED_KEYRING
> +	help
> +	  If set, only CA certificates can be added to the secondary trusted keyring.
> +	  An acceptable CA certificate must include the `keyCertSign` value in
> +	  the `keyUsage` field. CA certificates that include the `digitalSignature`
> +	  value in the `keyUsage` field will not be accepted.
> +
>  config SYSTEM_BLACKLIST_KEYRING
>  	bool "Provide system-wide ring of blacklisted keys"
>  	depends on KEYS
> diff --git a/certs/system_keyring.c b/certs/system_keyring.c
> index 9de610bf1f4b..ee14447374e7 100644
> --- a/certs/system_keyring.c
> +++ b/certs/system_keyring.c
> @@ -99,6 +99,22 @@ int restrict_link_by_builtin_and_secondary_trusted(
>  		/* Allow the builtin keyring to be added to the secondary */
>  		return 0;
>  
> +	if (IS_ENABLED(CONFIG_SECONDARY_TRUSTED_KEYRING_FOR_CA_CERTIFICATES_ONLY) &&
> +	    dest_keyring == secondary_trusted_keys) {
> +		const struct public_key *pub = payload->data[asym_crypto];
> +
> +		if (type != &key_type_asymmetric)
> +			return -EOPNOTSUPP;
> +		if (!pub)
> +			return -ENOPKG;
> +		if (!test_bit(KEY_EFLAG_CA, &pub->key_eflags))
> +			return -EPERM;
> +		if (!test_bit(KEY_EFLAG_KEYCERTSIGN, &pub->key_eflags))
> +			return -EPERM;
> +		if (test_bit(KEY_EFLAG_DIGITALSIG, &pub->key_eflags))
> +			return -EPERM;
> +	}
> +
>  	return restrict_link_by_signature(dest_keyring, type, payload,
>  					  secondary_trusted_keys);
>  }
> -- 
> 2.34.1

I don't think this does any harm. What do you think Mimi?

BR, Jarkko