Re: [PATCH 2/4] PKCS#7: Check codeSigning EKU for kernel module and kexec pe verification

[Varad Gautam]

Hi Joey,

On 4/9/21 4:46 AM, Lee, Chun-Yi wrote:
> This patch adds the logic for checking the CodeSigning extended
> key usage when verifying signature of kernel module or
> kexec PE binary in PKCS#7.
> 
> Signed-off-by: "Lee, Chun-Yi" <jlee@xxxxxxxx>
> ---
>  certs/system_keyring.c               |  2 +-
>  crypto/asymmetric_keys/Kconfig       |  9 +++++++++
>  crypto/asymmetric_keys/pkcs7_trust.c | 37 +++++++++++++++++++++++++++++++++---
>  include/crypto/pkcs7.h               |  3 ++-
>  4 files changed, 46 insertions(+), 5 deletions(-)
> 
> diff --git a/certs/system_keyring.c b/certs/system_keyring.c
> index 4b693da488f1..c9f8bca0b0d3 100644
> --- a/certs/system_keyring.c
> +++ b/certs/system_keyring.c
> @@ -243,7 +243,7 @@ int verify_pkcs7_message_sig(const void *data, size_t len,
>  			goto error;
>  		}
>  	}
> -	ret = pkcs7_validate_trust(pkcs7, trusted_keys);
> +	ret = pkcs7_validate_trust(pkcs7, trusted_keys, usage);
>  	if (ret < 0) {
>  		if (ret == -ENOKEY)
>  			pr_devel("PKCS#7 signature not signed with a trusted key\n");
> diff --git a/crypto/asymmetric_keys/Kconfig b/crypto/asymmetric_keys/Kconfig
> index 1f1f004dc757..1754812df989 100644
> --- a/crypto/asymmetric_keys/Kconfig
> +++ b/crypto/asymmetric_keys/Kconfig
> @@ -96,4 +96,13 @@ config SIGNED_PE_FILE_VERIFICATION
>  	  This option provides support for verifying the signature(s) on a
>  	  signed PE binary.
>  
> +config CHECK_CODESIGN_EKU
> +	bool "Check codeSigning extended key usage"
> +	depends on PKCS7_MESSAGE_PARSER=y
> +	depends on SYSTEM_DATA_VERIFICATION
> +	help
> +	  This option provides support for checking the codeSigning extended
> +	  key usage when verifying the signature in PKCS#7. It affects kernel
> +	  module verification and kexec PE binary verification.
> +
>  endif # ASYMMETRIC_KEY_TYPE
> diff --git a/crypto/asymmetric_keys/pkcs7_trust.c b/crypto/asymmetric_keys/pkcs7_trust.c
> index b531df2013c4..077bfef928b6 100644
> --- a/crypto/asymmetric_keys/pkcs7_trust.c
> +++ b/crypto/asymmetric_keys/pkcs7_trust.c
> @@ -16,12 +16,36 @@
>  #include <crypto/public_key.h>
>  #include "pkcs7_parser.h"
>  
> +#ifdef CONFIG_CHECK_CODESIGN_EKU
> +static bool check_codesign_eku(struct key *key,
> +			     enum key_being_used_for usage)
> +{
> +	struct public_key *public_key = key->payload.data[asym_crypto];
> +
> +	switch (usage) {
> +	case VERIFYING_MODULE_SIGNATURE:
> +	case VERIFYING_KEXEC_PE_SIGNATURE:
> +		return !!(public_key->eku & EKU_codeSigning);
> +	default:
> +		break;
> +	}
> +	return true;
> +}
> +#else
> +static bool check_codesign_eku(struct key *key,
> +			     enum key_being_used_for usage)
> +{
> +	return true;
> +}
> +#endif
> +
>  /*
>   * Check the trust on one PKCS#7 SignedInfo block.
>   */
>  static int pkcs7_validate_trust_one(struct pkcs7_message *pkcs7,
>  				    struct pkcs7_signed_info *sinfo,
> -				    struct key *trust_keyring)
> +				    struct key *trust_keyring,
> +				    enum key_being_used_for usage)
>  {
>  	struct public_key_signature *sig = sinfo->sig;
>  	struct x509_certificate *x509, *last = NULL, *p;
> @@ -112,6 +136,12 @@ static int pkcs7_validate_trust_one(struct pkcs7_message *pkcs7,
>  	return -ENOKEY;
>  
>  matched:
> +	if (!check_codesign_eku(key, usage)) {

Perhaps this can be a generic check_eku_usage() call, with codesigning as one of the
things it can check for.

> +		pr_warn("sinfo %u: The signer %x key is not CodeSigning\n",
> +			sinfo->index, key_serial(key));
> +		key_put(key);
> +		return -ENOKEY;
> +	}
>  	ret = verify_signature(key, sig);
>  	key_put(key);
>  	if (ret < 0) {
> @@ -156,7 +186,8 @@ static int pkcs7_validate_trust_one(struct pkcs7_message *pkcs7,
>   * May also return -ENOMEM.
>   */
>  int pkcs7_validate_trust(struct pkcs7_message *pkcs7,
> -			 struct key *trust_keyring)
> +			 struct key *trust_keyring,
> +			 enum key_being_used_for usage)

Please also update the comment above to mention the `usage` parameter.

Regards,
Varad

>  {
>  	struct pkcs7_signed_info *sinfo;
>  	struct x509_certificate *p;
> @@ -167,7 +198,7 @@ int pkcs7_validate_trust(struct pkcs7_message *pkcs7,
>  		p->seen = false;
>  
>  	for (sinfo = pkcs7->signed_infos; sinfo; sinfo = sinfo->next) {
> -		ret = pkcs7_validate_trust_one(pkcs7, sinfo, trust_keyring);
> +		ret = pkcs7_validate_trust_one(pkcs7, sinfo, trust_keyring, usage);
>  		switch (ret) {
>  		case -ENOKEY:
>  			continue;
> diff --git a/include/crypto/pkcs7.h b/include/crypto/pkcs7.h
> index 38ec7f5f9041..b3b48240ba73 100644
> --- a/include/crypto/pkcs7.h
> +++ b/include/crypto/pkcs7.h
> @@ -30,7 +30,8 @@ extern int pkcs7_get_content_data(const struct pkcs7_message *pkcs7,
>   * pkcs7_trust.c
>   */
>  extern int pkcs7_validate_trust(struct pkcs7_message *pkcs7,
> -				struct key *trust_keyring);
> +				struct key *trust_keyring,
> +				enum key_being_used_for usage);
>  
>  /*
>   * pkcs7_verify.c
> 

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