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

Nayna
2021-01-28T15:36:54Z

On 1/27/21 11:11 PM, Eric Snowberg wrote:
>> 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
>
>
Thanks Eric for clarifying. I was confusing it with with the broader
meaning of revocation i.e. certificate revocation list. To avoid similar
confusion in the future, I wonder if we should call it as 'blocklist' or
'denylist' as suggested in the document. This is to avoid conflicts with
actual CRL support if added in the future. I also wonder if we should
add the clarification in the patch description.

Thanks & Regards,

- Nayna