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

[joeyli]

Hi Varad,

Thanks for your review!

On Thu, Apr 15, 2021 at 02:08:32PM +0200, Varad Gautam wrote:
> 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.
>

Because only codesign EKU be checked now. So I prefer to keep it
as my current implementation until there have other EKU requirement. 
 
> > +		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

I will update it to the comment. Thanks!

Joey Lee

> 
> >  {
> >  	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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