Re: [PATCH v5 1/4] certs: Add EFI_CERT_X509_GUID support for dbx entries

[Eric Snowberg <eric.snowberg@oracle.com>]

> On Jan 27, 2021, at 8:54 PM, Nayna <nayna@linux.vnet.ibm.com> wrote:
> 
> 
> On 1/22/21 1:10 PM, Eric Snowberg wrote:
>> This fixes CVE-2020-26541.
>> 
>> The Secure Boot Forbidden Signature Database, dbx, contains a list of now
>> revoked signatures and keys previously approved to boot with UEFI Secure
>> Boot enabled.  The dbx is capable of containing any number of
>> EFI_CERT_X509_SHA256_GUID, EFI_CERT_SHA256_GUID, and EFI_CERT_X509_GUID
>> entries.
>> 
>> Currently when EFI_CERT_X509_GUID are contained in the dbx, the entries are
>> skipped.
>> 
>> Add support for EFI_CERT_X509_GUID dbx entries. When a EFI_CERT_X509_GUID
>> is found, it is added as an asymmetrical key to the .blacklist keyring.
>> Anytime the .platform keyring is used, the keys in the .blacklist keyring
>> are referenced, if a matching key is found, the key will be rejected.
>> 
>> Signed-off-by: Eric Snowberg <eric.snowberg@oracle.com>
>> Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
>> Signed-off-by: David Howells <dhowells@redhat.com>
>> ---
>> v5: Function name changes done by David Howells
>> ---
>>  certs/blacklist.c                             | 32 +++++++++++++++++++
>>  certs/blacklist.h                             | 12 +++++++
>>  certs/system_keyring.c                        |  6 ++++
>>  include/keys/system_keyring.h                 | 11 +++++++
>>  .../platform_certs/keyring_handler.c          | 11 +++++++
>>  5 files changed, 72 insertions(+)
>> 
>> diff --git a/certs/blacklist.c b/certs/blacklist.c
>> index 6514f9ebc943..a7f021878a4b 100644
>> --- a/certs/blacklist.c
>> +++ b/certs/blacklist.c
>> @@ -100,6 +100,38 @@ int mark_hash_blacklisted(const char *hash)
>>  	return 0;
>>  }
>> 
>> +int add_key_to_revocation_list(const char *data, size_t size)
>> +{
>> +	key_ref_t key;
>> +
>> +	key = key_create_or_update(make_key_ref(blacklist_keyring, true),
>> +				   "asymmetric",
>> +				   NULL,
>> +				   data,
>> +				   size,
>> +				   ((KEY_POS_ALL & ~KEY_POS_SETATTR) | KEY_USR_VIEW),
>> +				   KEY_ALLOC_NOT_IN_QUOTA | KEY_ALLOC_BUILT_IN);
>> +
>> +	if (IS_ERR(key)) {
>> +		pr_err("Problem with revocation key (%ld)\n", PTR_ERR(key));
>> +		return PTR_ERR(key);
>> +	}
>> +
>> +	return 0;
>> +}
>> +
>> +int is_key_on_revocation_list(struct pkcs7_message *pkcs7)
>> +{
>> +	int ret;
>> +
>> +	ret = validate_trust(pkcs7, blacklist_keyring);
>> +
>> +	if (ret == 0)
>> +		return -EKEYREJECTED;
>> +
>> +	return -ENOKEY;
>> +}
>> +
>>  /**
>>   * is_hash_blacklisted - Determine if a hash is blacklisted
>>   * @hash: The hash to be checked as a binary blob
>> diff --git a/certs/blacklist.h b/certs/blacklist.h
>> index 1efd6fa0dc60..420bb7c86e07 100644
>> --- a/certs/blacklist.h
>> +++ b/certs/blacklist.h
>> @@ -1,3 +1,15 @@
>>  #include <linux/kernel.h>
>> +#include <linux/errno.h>
>> +#include <crypto/pkcs7.h>
>> 
>>  extern const char __initconst *const blacklist_hashes[];
>> +
>> +#ifdef CONFIG_INTEGRITY_PLATFORM_KEYRING
>> +#define validate_trust pkcs7_validate_trust
>> +#else
>> +static inline int validate_trust(struct pkcs7_message *pkcs7,
>> +				 struct key *trust_keyring)
>> +{
>> +	return -ENOKEY;
>> +}
>> +#endif
>> diff --git a/certs/system_keyring.c b/certs/system_keyring.c
>> index 798291177186..cc165b359ea3 100644
>> --- a/certs/system_keyring.c
>> +++ b/certs/system_keyring.c
>> @@ -241,6 +241,12 @@ int verify_pkcs7_message_sig(const void *data, size_t len,
>>  			pr_devel("PKCS#7 platform keyring is not available\n");
>>  			goto error;
>>  		}
>> +
>> +		ret = is_key_on_revocation_list(pkcs7);
>> +		if (ret != -ENOKEY) {
>> +			pr_devel("PKCS#7 platform key is on revocation list\n");
>> +			goto error;
>> +		}
>>  	}
>>  	ret = pkcs7_validate_trust(pkcs7, trusted_keys);
>>  	if (ret < 0) {
>> diff --git a/include/keys/system_keyring.h b/include/keys/system_keyring.h
>> index fb8b07daa9d1..61f98739e8b1 100644
>> --- a/include/keys/system_keyring.h
>> +++ b/include/keys/system_keyring.h
>> @@ -31,11 +31,14 @@ extern int restrict_link_by_builtin_and_secondary_trusted(
>>  #define restrict_link_by_builtin_and_secondary_trusted restrict_link_by_builtin_trusted
>>  #endif
>> 
>> +extern struct pkcs7_message *pkcs7;
>>  #ifdef CONFIG_SYSTEM_BLACKLIST_KEYRING
>>  extern int mark_hash_blacklisted(const char *hash);
>> +extern int add_key_to_revocation_list(const char *data, size_t size);
>>  extern int is_hash_blacklisted(const u8 *hash, size_t hash_len,
>>  			       const char *type);
>>  extern int is_binary_blacklisted(const u8 *hash, size_t hash_len);
>> +extern int is_key_on_revocation_list(struct pkcs7_message *pkcs7);
>>  #else
>>  static inline int is_hash_blacklisted(const u8 *hash, size_t hash_len,
>>  				      const char *type)
>> @@ -47,6 +50,14 @@ static inline int is_binary_blacklisted(const u8 *hash, size_t hash_len)
>>  {
>>  	return 0;
>>  }
>> +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)
>> +{
>> +	return -ENOKEY;
>> +}
>>  #endif
>> 
>>  #ifdef CONFIG_IMA_BLACKLIST_KEYRING
>> diff --git a/security/integrity/platform_certs/keyring_handler.c b/security/integrity/platform_certs/keyring_handler.c
>> index c5ba695c10e3..5604bd57c990 100644
>> --- a/security/integrity/platform_certs/keyring_handler.c
>> +++ b/security/integrity/platform_certs/keyring_handler.c
>> @@ -55,6 +55,15 @@ static __init void uefi_blacklist_binary(const char *source,
>>  	uefi_blacklist_hash(source, data, len, "bin:", 4);
>>  }
>> 
>> +/*
>> + * Add an X509 cert to the revocation list.
>> + */
>> +static __init void uefi_revocation_list_x509(const char *source,
>> +					     const void *data, size_t len)
>> +{
>> +	add_key_to_revocation_list(data, len);
>> +}
> 
> In keeping the naming convention with other functions that blacklist hashes, why can't we call these functions:
> 
> * uefi_revocation_list_x509() -> uefi_blacklist_x509_cert()
> * add_key_to_revocation_list() -> uefi_blacklist_cert()
> * is_key_on_revocation_list() -> is_cert_blacklisted()

The word revocation was used do to the updated Linux coding style:

https://lkml.org/lkml/2020/7/4/229