integrity: Define a trusted platform keyring

On secure boot enabled systems, a verified kernel may need to kexec
additional kernels. For example, it may be used as a bootloader needing
to kexec a target kernel or it may need to kexec a crashdump kernel. In
such cases, it may want to verify the signature of the next kernel
image.

It is further possible that the kernel image is signed with third party
keys which are stored as platform or firmware keys in the 'db' variable.
The kernel, however, can not directly verify these platform keys, and an
administrator may therefore not want to trust them for arbitrary usage.
In order to differentiate platform keys from other keys and provide the
necessary separation of trust, the kernel needs an additional keyring to
store platform keys.

This patch creates the new keyring called ".platform" to isolate keys
provided by platform from keys by kernel. These keys are used to
facilitate signature verification during kexec. Since the scope of this
keyring is only the platform/firmware keys, it cannot be updated from
userspace.

This keyring can be enabled by setting CONFIG_INTEGRITY_PLATFORM_KEYRING.

Signed-off-by: Nayna Jain <[e-mail]>
Reviewed-by: Mimi Zohar <[e-mail]>
Acked-by: Serge Hallyn <[e-mail]>
Reviewed-by: James Morris <[e-mail]>
Reviewed-by: Thiago Jung Bauermann <[e-mail]>
Signed-off-by: Mimi Zohar <[e-mail]>

security/integrity/Kconfig

···
 	   .evm keyrings be signed by a key on the system trusted
 	   keyring.
 
+config INTEGRITY_PLATFORM_KEYRING
+        bool "Provide keyring for platform/firmware trusted keys"
+        depends on INTEGRITY_ASYMMETRIC_KEYS
+        depends on SYSTEM_BLACKLIST_KEYRING
+        depends on EFI
+        help
+         Provide a separate, distinct keyring for platform trusted keys, which
+         the kernel automatically populates during initialization from values
+         provided by the platform for verifying the kexec'ed kerned image
+         and, possibly, the initramfs signature.
+
 config INTEGRITY_AUDIT
 	bool "Enables integrity auditing support "
 	depends on AUDIT

security/integrity/Makefile

···
 integrity-$(CONFIG_INTEGRITY_AUDIT) += integrity_audit.o
 integrity-$(CONFIG_INTEGRITY_SIGNATURE) += digsig.o
 integrity-$(CONFIG_INTEGRITY_ASYMMETRIC_KEYS) += digsig_asymmetric.o
+integrity-$(CONFIG_INTEGRITY_PLATFORM_KEYRING) += platform_certs/platform_keyring.o
 
 subdir-$(CONFIG_IMA)			+= ima
 obj-$(CONFIG_IMA)			+= ima/

security/integrity/digsig.c

···
 	".ima",
 #endif
 	"_module",
+	".platform",
 };
 
 #ifdef CONFIG_IMA_KEYRINGS_PERMIT_SIGNED_BY_BUILTIN_OR_SECONDARY
···
 	return -EOPNOTSUPP;
 }
 
-int __init integrity_init_keyring(const unsigned int id)
+static int __integrity_init_keyring(const unsigned int id, key_perm_t perm,
+				    struct key_restriction *restriction)
 {
 	const struct cred *cred = current_cred();
-	struct key_restriction *restriction;
 	int err = 0;
+
+	keyring[id] = keyring_alloc(keyring_name[id], KUIDT_INIT(0),
+				    KGIDT_INIT(0), cred, perm,
+				    KEY_ALLOC_NOT_IN_QUOTA,
+				    restriction, NULL);
+	if (IS_ERR(keyring[id])) {
+		err = PTR_ERR(keyring[id]);
+		pr_info("Can't allocate %s keyring (%d)\n",
+			keyring_name[id], err);
+		keyring[id] = NULL;
+	}
+
+	return err;
+}
+
+int __init integrity_init_keyring(const unsigned int id)
+{
+	struct key_restriction *restriction;
+	key_perm_t perm;
+
+	perm = (KEY_POS_ALL & ~KEY_POS_SETATTR) | KEY_USR_VIEW
+		| KEY_USR_READ | KEY_USR_SEARCH;
+
+	if (id == INTEGRITY_KEYRING_PLATFORM) {
+		restriction = NULL;
+		goto out;
+	}
 
 	if (!IS_ENABLED(CONFIG_INTEGRITY_TRUSTED_KEYRING))
 		return 0;
···
 		return -ENOMEM;
 
 	restriction->check = restrict_link_to_ima;
+	perm |= KEY_USR_WRITE;
 
-	keyring[id] = keyring_alloc(keyring_name[id], KUIDT_INIT(0),
-				    KGIDT_INIT(0), cred,
-				    ((KEY_POS_ALL & ~KEY_POS_SETATTR) |
-				     KEY_USR_VIEW | KEY_USR_READ |
-				     KEY_USR_WRITE | KEY_USR_SEARCH),
-				    KEY_ALLOC_NOT_IN_QUOTA,
-				    restriction, NULL);
-	if (IS_ERR(keyring[id])) {
-		err = PTR_ERR(keyring[id]);
-		pr_info("Can't allocate %s keyring (%d)\n",
-			keyring_name[id], err);
-		keyring[id] = NULL;
-	}
-	return err;
+out:
+	return __integrity_init_keyring(id, perm, restriction);
 }
 
 int __init integrity_load_x509(const unsigned int id, const char *path)

security/integrity/integrity.h

···
 #define INTEGRITY_KEYRING_EVM		0
 #define INTEGRITY_KEYRING_IMA		1
 #define INTEGRITY_KEYRING_MODULE	2
-#define INTEGRITY_KEYRING_MAX		3
+#define INTEGRITY_KEYRING_PLATFORM	3
+#define INTEGRITY_KEYRING_MAX		4
 
 extern struct dentry *integrity_dir;
 

security/integrity/platform_certs/platform_keyring.c

···
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Platform keyring for firmware/platform keys
+ *
+ * Copyright IBM Corporation, 2018
+ * Author(s): Nayna Jain <[e-mail]>
+ */
+
+#include <linux/export.h>
+#include <linux/kernel.h>
+#include <linux/sched.h>
+#include <linux/cred.h>
+#include <linux/err.h>
+#include <linux/slab.h>
+#include "../integrity.h"
+
+/*
+ * Create the trusted keyrings.
+ */
+static __init int platform_keyring_init(void)
+{
+	int rc;
+
+	rc = integrity_init_keyring(INTEGRITY_KEYRING_PLATFORM);
+	if (rc)
+		return rc;
+
+	pr_notice("Platform Keyring initialized\n");
+	return 0;
+}
+
+/*
+ * Must be initialised before we try and load the keys into the keyring.
+ */
+device_initcall(platform_keyring_init);