Re: [PATCH 01/20] KEYS: verify a certificate is signed by a 'trusted' key

[Mimi Zohar]

On Wed, 2014-04-23 at 16:30 +0300, Dmitry Kasatkin wrote: 
> From: Mimi Zohar <zohar@xxxxxxxxxxxxxxxxxx>
> 
> Only public keys, with certificates signed by an existing
> 'trusted' key on the system trusted keyring, should be added
> to a trusted keyring.  This patch adds support for verifying
> a certificate's signature.
> 
> This is derived from David Howells pkcs7_request_asymmetric_key() patch.
> 
> Changes:
> - Flaged out the code to prevent build break if system keyring
>   is not enabled (Dmitry).

An updated version of this patch was posted, which resolves the Kconfig
issues.  There are a number of other issues which need to be addressed,
before this patch can be upstreamed.  Please refer to the patch
description for more details -
http://marc.info/?l=linux-security-module&m=138672063109662&w=2

Reminder, as per Documentation/SubmittingPatches: "#ifdefs are ugly",
please no ifdefs in C code.

thanks,

Mimi

> 
> Signed-off-by: Mimi Zohar <zohar@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
> Signed-off-by: Dmitry Kasatkin <d.kasatkin@xxxxxxxxxxx>
> ---
>  crypto/asymmetric_keys/x509_public_key.c | 85 +++++++++++++++++++++++++++++++-
>  1 file changed, 84 insertions(+), 1 deletion(-)
> 
> diff --git a/crypto/asymmetric_keys/x509_public_key.c b/crypto/asymmetric_keys/x509_public_key.c
> index 382ef0d..d279f43 100644
> --- a/crypto/asymmetric_keys/x509_public_key.c
> +++ b/crypto/asymmetric_keys/x509_public_key.c
> @@ -18,6 +18,7 @@
>  #include <linux/asn1_decoder.h>
>  #include <keys/asymmetric-subtype.h>
>  #include <keys/asymmetric-parser.h>
> +#include <keys/system_keyring.h>
>  #include <crypto/hash.h>
>  #include "asymmetric_keys.h"
>  #include "public_key.h"
> @@ -102,6 +103,82 @@ int x509_check_signature(const struct public_key *pub,
>  }
>  EXPORT_SYMBOL_GPL(x509_check_signature);
> 
> +#ifdef CONFIG_SYSTEM_TRUSTED_KEYRING
> +/*
> + * Find a key in the given keyring by issuer and authority.
> + */
> +static struct key *x509_request_asymmetric_key(
> +	struct key *keyring,
> +	const char *signer, size_t signer_len,
> +	const char *authority, size_t auth_len)
> +{
> +	key_ref_t key;
> +	char *id;
> +
> +	/* Construct an identifier. */
> +	id = kmalloc(signer_len + 2 + auth_len + 1, GFP_KERNEL);
> +	if (!id)
> +		return ERR_PTR(-ENOMEM);
> +
> +	memcpy(id, signer, signer_len);
> +	id[signer_len + 0] = ':';
> +	id[signer_len + 1] = ' ';
> +	memcpy(id + signer_len + 2, authority, auth_len);
> +	id[signer_len + 2 + auth_len] = 0;
> +
> +	pr_debug("Look up: \"%s\"\n", id);
> +
> +	key = keyring_search(make_key_ref(keyring, 1),
> +			     &key_type_asymmetric, id);
> +	if (IS_ERR(key))
> +		pr_debug("Request for module key '%s' err %ld\n",
> +			 id, PTR_ERR(key));
> +	kfree(id);
> +
> +	if (IS_ERR(key)) {
> +		switch (PTR_ERR(key)) {
> +			/* Hide some search errors */
> +		case -EACCES:
> +		case -ENOTDIR:
> +		case -EAGAIN:
> +			return ERR_PTR(-ENOKEY);
> +		default:
> +			return ERR_CAST(key);
> +		}
> +	}
> +
> +	pr_devel("<==%s() = 0 [%x]\n", __func__, key_serial(key_ref_to_ptr(key)));
> +	return key_ref_to_ptr(key);
> +}
> +
> +/*
> + * Check the new certificate against the ones in the trust keyring.  If one of
> + * those is the signing key and validates the new certificate, then mark the
> + * new certificate as being trusted.
> + *
> + * Return 0 if the new certificate was successfully validated, 1 if we couldn't
> + * find a matching parent certificate in the trusted list and an error if there
> + * is a matching certificate but the signature check fails.
> + */
> +static int x509_validate_trust(struct x509_certificate *cert,
> +			       struct key *trust_keyring)
> +{
> +	const struct public_key *pk;
> +	struct key *key;
> +	int ret = 1;
> +
> +	key = x509_request_asymmetric_key(trust_keyring,
> +					  cert->issuer, strlen(cert->issuer),
> +					  cert->authority,
> +					  strlen(cert->authority));
> +	if (!IS_ERR(key))  {
> +		pk = key->payload.data;
> +		ret = x509_check_signature(pk, cert);
> +	}
> +	return ret;
> +}
> +#endif
> +
>  /*
>   * Attempt to parse a data blob for a key as an X509 certificate.
>   */
> @@ -155,9 +232,15 @@ static int x509_key_preparse(struct key_preparsed_payload *prep)
>  	/* Check the signature on the key if it appears to be self-signed */
>  	if (!cert->authority ||
>  	    strcmp(cert->fingerprint, cert->authority) == 0) {
> -		ret = x509_check_signature(cert->pub, cert);
> +		ret = x509_check_signature(cert->pub, cert); /* self-signed */
>  		if (ret < 0)
>  			goto error_free_cert;
> +	} else {
> +#ifdef CONFIG_SYSTEM_TRUSTED_KEYRING
> +		ret = x509_validate_trust(cert, system_trusted_keyring);
> +		if (!ret)
> +			prep->trusted = 1;
> +#endif
>  	}
> 
>  	/* Propose a description */


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