Re: [PATCH 4/4] integrity: CA enforcement in machine keyring

Stefan Berger <stefanb@xxxxxxxxxxxxx> · Fri, 4 Mar 2022 18:19:59 -0500

On 3/1/22 12:36, Eric Snowberg wrote:
When INTEGRITY_MACHINE_KEYRING is set, all Machine Owner Keys (MOK)
are loaded into the machine keyring.  Add a new
INTEGRITY_MACHINE_KEYRING_CA_ENFORCED option where only MOK CA keys are
added.

Set the restriction check to restrict_link_by_ca.  This will only allow
CA keys into the machine keyring. Unlike when INTEGRITY_MACHINE_KEYRING
is enabled, IMA_KEYRINGS_PERMIT_SIGNED_BY_BUILTIN_OR_SECONDARY may
also be enabled, allowing IMA to use keys in the machine keyring as
another trust anchor.

I tried to test this but could only do it by disabling the 
MokListTrustedRT variable check and then also the check for secure boot. 
It did load the expected keys onto the .machine keyring, enforcing the 
x509 indicating a self-signed CA if the compile time option 
CONFIG_INTEGRITY_MACHINE_KEYRING_CA_ENFORCED=y was set, loading all keys 
in the case of CONFIG_INTEGRITY_MACHINE_KEYRING=y.

I tried with this branch here from mokutils 
https://github.com/esnowberg/mokutil/tree/trust-mok but this seems to 
create an EFI variable with a different name. I guess this is the wrong 
branch?

   Stefan

Signed-off-by: Eric Snowberg <eric.snowberg@xxxxxxxxxx>
---
  certs/system_keyring.c                        |  9 +++++---
  include/keys/system_keyring.h                 |  3 ++-
  security/integrity/Kconfig                    | 21 +++++++++++++++++++
  security/integrity/Makefile                   |  1 +
  security/integrity/digsig.c                   | 14 ++++++++++---
  security/integrity/integrity.h                |  3 ++-
  .../platform_certs/keyring_handler.c          |  4 +++-
  7 files changed, 46 insertions(+), 9 deletions(-)

diff --git a/certs/system_keyring.c b/certs/system_keyring.c
index 05b66ce9d1c9..0811b44cf3bf 100644
--- a/certs/system_keyring.c
+++ b/certs/system_keyring.c
@@ -22,7 +22,8 @@ static struct key *builtin_trusted_keys;
  #ifdef CONFIG_SECONDARY_TRUSTED_KEYRING
  static struct key *secondary_trusted_keys;
  #endif
-#ifdef CONFIG_INTEGRITY_MACHINE_KEYRING
+#if defined(CONFIG_INTEGRITY_MACHINE_KEYRING) || \
+    defined(CONFIG_INTEGRITY_MACHINE_KEYRING_CA_ENFORCED)
  static struct key *machine_trusted_keys;
  #endif
  #ifdef CONFIG_INTEGRITY_PLATFORM_KEYRING
@@ -89,7 +90,8 @@ static __init struct key_restriction *get_builtin_and_secondary_restriction(void
  	if (!restriction)
  		panic("Can't allocate secondary trusted keyring restriction\n");
  
-	if (IS_ENABLED(CONFIG_INTEGRITY_MACHINE_KEYRING))
+	if (IS_ENABLED(CONFIG_INTEGRITY_MACHINE_KEYRING) ||
+	    IS_ENABLED(CONFIG_INTEGRITY_MACHINE_KEYRING_CA_ENFORCED))
  		restriction->check = restrict_link_by_builtin_secondary_and_machine;
  	else
  		restriction->check = restrict_link_by_builtin_and_secondary_trusted;
@@ -97,7 +99,8 @@ static __init struct key_restriction *get_builtin_and_secondary_restriction(void
  	return restriction;
  }
  #endif
-#ifdef CONFIG_INTEGRITY_MACHINE_KEYRING
+#if defined(CONFIG_INTEGRITY_MACHINE_KEYRING) || \
+    defined(CONFIG_INTEGRITY_MACHINE_KEYRING_CA_ENFORCED)
  void __init set_machine_trusted_keys(struct key *keyring)
  {
  	machine_trusted_keys = keyring;
diff --git a/include/keys/system_keyring.h b/include/keys/system_keyring.h
index 91e080efb918..e4a6574bbcb6 100644
--- a/include/keys/system_keyring.h
+++ b/include/keys/system_keyring.h
@@ -45,7 +45,8 @@ extern int restrict_link_by_builtin_and_secondary_trusted(
  #define restrict_link_by_builtin_and_secondary_trusted restrict_link_by_builtin_trusted
  #endif
  
-#ifdef CONFIG_INTEGRITY_MACHINE_KEYRING
+#if defined(CONFIG_INTEGRITY_MACHINE_KEYRING) || \
+    defined(CONFIG_INTEGRITY_MACHINE_KEYRING_CA_ENFORCED)
  extern int restrict_link_by_builtin_secondary_and_machine(
  	struct key *dest_keyring,
  	const struct key_type *type,
diff --git a/security/integrity/Kconfig b/security/integrity/Kconfig
index 599429f99f99..14c927eea5ee 100644
--- a/security/integrity/Kconfig
+++ b/security/integrity/Kconfig
@@ -62,6 +62,14 @@ config INTEGRITY_PLATFORM_KEYRING
           provided by the platform for verifying the kexec'ed kerned image
           and, possibly, the initramfs signature.
  
+
+choice
+	prompt "Machine keyring"
+	default INTEGRITY_MACHINE_NONE
+
+config INTEGRITY_MACHINE_NONE
+	bool "Do not enable the Machine Owner Keyring"
+
  config INTEGRITY_MACHINE_KEYRING
  	bool "Provide a keyring to which Machine Owner Keys may be added"
  	depends on SECONDARY_TRUSTED_KEYRING
@@ -75,6 +83,19 @@ config INTEGRITY_MACHINE_KEYRING
  	 in the platform keyring, keys contained in the .machine keyring will
  	 be trusted within the kernel.
  
+config INTEGRITY_MACHINE_KEYRING_CA_ENFORCED
+	bool "Provide a keyring to which Machine Owner CA Keys may be added"
+	depends on SECONDARY_TRUSTED_KEYRING
+	depends on INTEGRITY_ASYMMETRIC_KEYS
+	depends on SYSTEM_BLACKLIST_KEYRING
+	depends on LOAD_UEFI_KEYS
+	help
+	 If set, provide a keyring to which CA Machine Owner Keys (MOK) may
+	 be added. This keyring shall contain just CA MOK keys.  Unlike keys
+	 in the platform keyring, keys contained in the .machine keyring will
+	 be trusted within the kernel.
+endchoice
+
  config LOAD_UEFI_KEYS
         depends on INTEGRITY_PLATFORM_KEYRING
         depends on EFI
diff --git a/security/integrity/Makefile b/security/integrity/Makefile
index d0ffe37dc1d6..370ee63774c3 100644
--- a/security/integrity/Makefile
+++ b/security/integrity/Makefile
@@ -11,6 +11,7 @@ integrity-$(CONFIG_INTEGRITY_SIGNATURE) += digsig.o
  integrity-$(CONFIG_INTEGRITY_ASYMMETRIC_KEYS) += digsig_asymmetric.o
  integrity-$(CONFIG_INTEGRITY_PLATFORM_KEYRING) += platform_certs/platform_keyring.o
  integrity-$(CONFIG_INTEGRITY_MACHINE_KEYRING) += platform_certs/machine_keyring.o
+integrity-$(CONFIG_INTEGRITY_MACHINE_KEYRING_CA_ENFORCED) += platform_certs/machine_keyring.o
  integrity-$(CONFIG_LOAD_UEFI_KEYS) += platform_certs/efi_parser.o \
  				      platform_certs/load_uefi.o \
  				      platform_certs/keyring_handler.o
diff --git a/security/integrity/digsig.c b/security/integrity/digsig.c
index c8c8a4a4e7a0..041edd9744db 100644
--- a/security/integrity/digsig.c
+++ b/security/integrity/digsig.c
@@ -34,7 +34,11 @@ static const char * const keyring_name[INTEGRITY_KEYRING_MAX] = {
  };
  
  #ifdef CONFIG_IMA_KEYRINGS_PERMIT_SIGNED_BY_BUILTIN_OR_SECONDARY
+#ifdef CONFIG_INTEGRITY_MACHINE_KEYRING_CA_ENFORCED
+#define restrict_link_to_ima restrict_link_by_builtin_secondary_and_machine
+#else
  #define restrict_link_to_ima restrict_link_by_builtin_and_secondary_trusted
+#endif
  #else
  #define restrict_link_to_ima restrict_link_by_builtin_trusted
  #endif
@@ -130,19 +134,23 @@ int __init integrity_init_keyring(const unsigned int id)
  		| KEY_USR_READ | KEY_USR_SEARCH;
  
  	if (id == INTEGRITY_KEYRING_PLATFORM ||
-	    id == INTEGRITY_KEYRING_MACHINE) {
+	   (IS_ENABLED(CONFIG_INTEGRITY_MACHINE_KEYRING))) {
  		restriction = NULL;
  		goto out;
  	}
  
-	if (!IS_ENABLED(CONFIG_INTEGRITY_TRUSTED_KEYRING))
+	if (!IS_ENABLED(CONFIG_INTEGRITY_TRUSTED_KEYRING) &&
+	    id != INTEGRITY_KEYRING_MACHINE)
  		return 0;
  
  	restriction = kzalloc(sizeof(struct key_restriction), GFP_KERNEL);
  	if (!restriction)
  		return -ENOMEM;
  
-	restriction->check = restrict_link_to_ima;
+	if (id == INTEGRITY_KEYRING_MACHINE)
+		restriction->check = restrict_link_by_ca;
+	else
+		restriction->check = restrict_link_to_ima;
  
  	/*
  	 * MOK keys can only be added through a read-only runtime services
diff --git a/security/integrity/integrity.h b/security/integrity/integrity.h
index 2e214c761158..ca4d72fbd045 100644
--- a/security/integrity/integrity.h
+++ b/security/integrity/integrity.h
@@ -285,7 +285,8 @@ static inline void __init add_to_platform_keyring(const char *source,
  }
  #endif
  
-#ifdef CONFIG_INTEGRITY_MACHINE_KEYRING
+#if defined(CONFIG_INTEGRITY_MACHINE_KEYRING) || \
+    defined(CONFIG_INTEGRITY_MACHINE_KEYRING_CA_ENFORCED)
  void __init add_to_machine_keyring(const char *source, const void *data, size_t len);
  bool __init trust_moklist(void);
  #else
diff --git a/security/integrity/platform_certs/keyring_handler.c b/security/integrity/platform_certs/keyring_handler.c
index a2464f3e66cc..9c456ad0ab67 100644
--- a/security/integrity/platform_certs/keyring_handler.c
+++ b/security/integrity/platform_certs/keyring_handler.c
@@ -61,7 +61,9 @@ __init efi_element_handler_t get_handler_for_db(const efi_guid_t *sig_type)
  __init efi_element_handler_t get_handler_for_mok(const efi_guid_t *sig_type)
  {
  	if (efi_guidcmp(*sig_type, efi_cert_x509_guid) == 0) {
-		if (IS_ENABLED(CONFIG_INTEGRITY_MACHINE_KEYRING) && trust_moklist())
+		if ((IS_ENABLED(CONFIG_INTEGRITY_MACHINE_KEYRING) ||
+		     IS_ENABLED(CONFIG_INTEGRITY_MACHINE_KEYRING_CA_ENFORCED)) &&
+		     trust_moklist())
  			return add_to_machine_keyring;
  		else
  			return add_to_platform_keyring;






