Re: [PATCH v9,2/4] PKCS#7: Check codeSigning EKU for kernel module and kexec pe verification

[joeyli <jlee@suse.com>]

Hi Jarkko,

On Sun, Aug 28, 2022 at 06:34:26AM +0300, Jarkko Sakkinen wrote:
> On Thu, Aug 25, 2022 at 10:23:12PM +0800, 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.
> 
> Pretty much the same feedback as for 1/4.
>

I will also change this description.

Thanks
Joey Lee
 
> > 
> > Signed-off-by: "Lee, Chun-Yi" <jlee@suse.com>
> > ---
> >  certs/blacklist.c                    |  5 ++--
> >  certs/system_keyring.c               |  4 +--
> >  crypto/asymmetric_keys/Kconfig       |  9 ++++++
> >  crypto/asymmetric_keys/pkcs7_trust.c | 43 ++++++++++++++++++++++++++--
> >  crypto/asymmetric_keys/selftest.c    |  2 +-
> >  include/crypto/pkcs7.h               |  4 ++-
> >  include/keys/system_keyring.h        |  7 +++--
> >  7 files changed, 63 insertions(+), 11 deletions(-)
> > 
> > diff --git a/certs/blacklist.c b/certs/blacklist.c
> > index 41f10601cc72..fa41454055be 100644
> > --- a/certs/blacklist.c
> > +++ b/certs/blacklist.c
> > @@ -282,11 +282,12 @@ int add_key_to_revocation_list(const char *data, size_t size)
> >   * is_key_on_revocation_list - Determine if the key for a PKCS#7 message is revoked
> >   * @pkcs7: The PKCS#7 message to check
> >   */
> > -int is_key_on_revocation_list(struct pkcs7_message *pkcs7)
> > +int is_key_on_revocation_list(struct pkcs7_message *pkcs7,
> > +			      enum key_being_used_for usage)
> >  {
> >  	int ret;
> >  
> > -	ret = pkcs7_validate_trust(pkcs7, blacklist_keyring);
> > +	ret = pkcs7_validate_trust(pkcs7, blacklist_keyring, usage, false);
> >  
> >  	if (ret == 0)
> >  		return -EKEYREJECTED;
> > diff --git a/certs/system_keyring.c b/certs/system_keyring.c
> > index 5042cc54fa5e..66737bfb26de 100644
> > --- a/certs/system_keyring.c
> > +++ b/certs/system_keyring.c
> > @@ -263,13 +263,13 @@ int verify_pkcs7_message_sig(const void *data, size_t len,
> >  			goto error;
> >  		}
> >  
> > -		ret = is_key_on_revocation_list(pkcs7);
> > +		ret = is_key_on_revocation_list(pkcs7, usage);
> >  		if (ret != -ENOKEY) {
> >  			pr_devel("PKCS#7 platform key is on revocation list\n");
> >  			goto error;
> >  		}
> >  	}
> > -	ret = pkcs7_validate_trust(pkcs7, trusted_keys);
> > +	ret = pkcs7_validate_trust(pkcs7, trusted_keys, usage, true);
> >  	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 3df3fe4ed95f..189536bd0f9a 100644
> > --- a/crypto/asymmetric_keys/Kconfig
> > +++ b/crypto/asymmetric_keys/Kconfig
> > @@ -85,4 +85,13 @@ config FIPS_SIGNATURE_SELFTEST
> >  	depends on ASYMMETRIC_KEY_TYPE
> >  	depends on PKCS7_MESSAGE_PARSER
> >  
> > +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 9a87c34ed173..087d3761d9a8 100644
> > --- a/crypto/asymmetric_keys/pkcs7_trust.c
> > +++ b/crypto/asymmetric_keys/pkcs7_trust.c
> > @@ -16,12 +16,40 @@
> >  #include <crypto/public_key.h>
> >  #include "pkcs7_parser.h"
> >  
> > +#ifdef CONFIG_CHECK_CODESIGN_EKU
> > +static bool check_eku_by_usage(struct key *key, enum key_being_used_for usage)
> > +{
> > +	struct public_key *public_key = key->payload.data[asym_crypto];
> > +	bool ret = true;
> > +
> > +	switch (usage) {
> > +	case VERIFYING_MODULE_SIGNATURE:
> > +	case VERIFYING_KEXEC_PE_SIGNATURE:
> > +		ret = !!(public_key->ext_key_usage & EKU_codeSigning);
> > +		if (!ret)
> > +			pr_warn("The signer '%s' key is not CodeSigning\n",
> > +				key->description);
> > +		break;
> > +	default:
> > +		break;
> > +	}
> > +	return ret;
> > +}
> > +#else
> > +static bool check_eku_by_usage(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,
> > +				    bool check_eku)
> >  {
> >  	struct public_key_signature *sig = sinfo->sig;
> >  	struct x509_certificate *x509, *last = NULL, *p;
> > @@ -112,6 +140,10 @@ static int pkcs7_validate_trust_one(struct pkcs7_message *pkcs7,
> >  	return -ENOKEY;
> >  
> >  matched:
> > +	if (check_eku && !check_eku_by_usage(key, usage)) {
> > +		key_put(key);
> > +		return -ENOKEY;
> > +	}
> 
> Why you don't just open code this to those rare call
> sites where it is needed?
> 
> I counted that there is exactly one call site.
> 
> >  	ret = verify_signature(key, sig);
> >  	key_put(key);
> >  	if (ret < 0) {
> > @@ -135,6 +167,8 @@ static int pkcs7_validate_trust_one(struct pkcs7_message *pkcs7,
> >   * pkcs7_validate_trust - Validate PKCS#7 trust chain
> >   * @pkcs7: The PKCS#7 certificate to validate
> >   * @trust_keyring: Signing certificates to use as starting points
> > + * @usage: The use to which the key is being put.
> > + * @check_eku: Check EKU (Extended Key Usage)
> >   *
> >   * Validate that the certificate chain inside the PKCS#7 message intersects
> >   * keys we already know and trust.
> > @@ -156,7 +190,9 @@ 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,
> > +			 bool check_eku)
> >  {
> >  	struct pkcs7_signed_info *sinfo;
> >  	struct x509_certificate *p;
> > @@ -167,7 +203,8 @@ 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, check_eku);
> >  		switch (ret) {
> >  		case -ENOKEY:
> >  			continue;
> > diff --git a/crypto/asymmetric_keys/selftest.c b/crypto/asymmetric_keys/selftest.c
> > index fa0bf7f24284..756e9f224d8a 100644
> > --- a/crypto/asymmetric_keys/selftest.c
> > +++ b/crypto/asymmetric_keys/selftest.c
> > @@ -212,7 +212,7 @@ int __init fips_signature_selftest(void)
> >  		if (ret < 0)
> >  			panic("Certs selftest %d: pkcs7_verify() = %d\n", i, ret);
> >  
> > -		ret = pkcs7_validate_trust(pkcs7, keyring);
> > +		ret = pkcs7_validate_trust(pkcs7, keyring, VERIFYING_MODULE_SIGNATURE, false);
> >  		if (ret < 0)
> >  			panic("Certs selftest %d: pkcs7_validate_trust() = %d\n", i, ret);
> >  
> > diff --git a/include/crypto/pkcs7.h b/include/crypto/pkcs7.h
> > index 38ec7f5f9041..5d87b8a02f79 100644
> > --- a/include/crypto/pkcs7.h
> > +++ b/include/crypto/pkcs7.h
> > @@ -30,7 +30,9 @@ 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,
> > +				bool check_eku);
> >  
> >  /*
> >   * pkcs7_verify.c
> > diff --git a/include/keys/system_keyring.h b/include/keys/system_keyring.h
> > index 91e080efb918..bb33b527240e 100644
> > --- a/include/keys/system_keyring.h
> > +++ b/include/keys/system_keyring.h
> > @@ -9,6 +9,7 @@
> >  #define _KEYS_SYSTEM_KEYRING_H
> >  
> >  #include <linux/key.h>
> > +#include <keys/asymmetric-type.h>
> >  
> >  enum blacklist_hash_type {
> >  	/* TBSCertificate hash */
> > @@ -81,13 +82,15 @@ static inline int is_binary_blacklisted(const u8 *hash, size_t hash_len)
> >  
> >  #ifdef CONFIG_SYSTEM_REVOCATION_LIST
> >  extern int add_key_to_revocation_list(const char *data, size_t size);
> > -extern int is_key_on_revocation_list(struct pkcs7_message *pkcs7);
> > +extern int is_key_on_revocation_list(struct pkcs7_message *pkcs7,
> > +				     enum key_being_used_for usage);
> >  #else
> >  static inline int add_key_to_revocation_list(const char *data, size_t size)
> >  {
> >  	return 0;
> >  }
> > -static inline int is_key_on_revocation_list(struct pkcs7_message *pkcs7)
> > +static inline int is_key_on_revocation_list(struct pkcs7_message *pkcs7,
> > +					    enum key_being_used_for usage)
> >  {
> >  	return -ENOKEY;
> >  }
> > -- 
> > 2.26.2
> > 
> 
> BR, Jarkko