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

Eric Snowberg
2021-01-22T18:49:09Z

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);
+}
+
/*
* Return the appropriate handler for particular signature list types found in
* the UEFI db and MokListRT tables.
@@ -76,5 +85,7 @@ __init efi_element_handler_t get_handler_for_dbx(const efi_guid_t *sig_type)
return uefi_blacklist_x509_tbs;
if (efi_guidcmp(*sig_type, efi_cert_sha256_guid) == 0)
return uefi_blacklist_binary;
+ if (efi_guidcmp(*sig_type, efi_cert_x509_guid) == 0)
+ return uefi_revocation_list_x509;
return 0;
}
--
2.18.4