ima: Add modsig appraise_type option for module-style appended signatures

Introduce the modsig keyword to the IMA policy syntax to specify that
a given hook should expect the file to have the IMA signature appended
to it. Here is how it can be used in a rule:

appraise func=KEXEC_KERNEL_CHECK appraise_type=imasig|modsig

With this rule, IMA will accept either a signature stored in the extended
attribute or an appended signature.

For now, the rule above will behave exactly the same as if
appraise_type=imasig was specified. The actual modsig implementation
will be introduced separately.

Suggested-by: Mimi Zohar <zohar@linux.ibm.com>
Signed-off-by: Thiago Jung Bauermann <bauerman@linux.ibm.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

7 files changed, 67 insertions, 3 deletions

diff --git a/Documentation/ABI/testing/ima_policy b/Documentation/ABI/testing/ima_policy
index fc376a323908..29ebe9afdac4 100644
--- a/Documentation/ABI/testing/ima_policy
+++ b/Documentation/ABI/testing/ima_policy
@@ -37,7 +37,7 @@ Description:
                         euid:= decimal value
                         fowner:= decimal value
                 lsm:    are LSM specific
-                option: appraise_type:= [imasig]
+                option: appraise_type:= [imasig] [imasig|modsig]
                         template:= name of a defined IMA template type
                         (eg, ima-ng). Only valid when action is "measure".
                         pcr:= decimal value
@@ -105,3 +105,7 @@ Description:
 
                         measure func=KEXEC_KERNEL_CHECK pcr=4
                         measure func=KEXEC_INITRAMFS_CHECK pcr=5
+
+                Example of appraise rule allowing modsig appended signatures:
+
+                        appraise func=KEXEC_KERNEL_CHECK appraise_type=imasig|modsig
diff --git a/security/integrity/ima/Kconfig b/security/integrity/ima/Kconfig
index 2ced99dde694..8bf46646b185 100644
--- a/security/integrity/ima/Kconfig
+++ b/security/integrity/ima/Kconfig
@@ -233,6 +233,16 @@ config IMA_APPRAISE_BOOTPARAM
           This option enables the different "ima_appraise=" modes
           (eg. fix, log) from the boot command line.
 
+config IMA_APPRAISE_MODSIG
+        bool "Support module-style signatures for appraisal"
+        depends on IMA_APPRAISE
+        default n
+        help
+           Adds support for signatures appended to files. The format of the
+           appended signature is the same used for signed kernel modules.
+           The modsig keyword can be used in the IMA policy to allow a hook
+           to accept such signatures.
+
 config IMA_TRUSTED_KEYRING
         bool "Require all keys on the .ima keyring be signed (deprecated)"
         depends on IMA_APPRAISE && SYSTEM_TRUSTED_KEYRING
diff --git a/security/integrity/ima/Makefile b/security/integrity/ima/Makefile
index d921dc4f9eb0..31d57cdf2421 100644
--- a/security/integrity/ima/Makefile
+++ b/security/integrity/ima/Makefile
@@ -9,5 +9,6 @@ obj-$(CONFIG_IMA) += ima.o
 ima-y := ima_fs.o ima_queue.o ima_init.o ima_main.o ima_crypto.o ima_api.o \
          ima_policy.o ima_template.o ima_template_lib.o
 ima-$(CONFIG_IMA_APPRAISE) += ima_appraise.o
+ima-$(CONFIG_IMA_APPRAISE_MODSIG) += ima_modsig.o
 ima-$(CONFIG_HAVE_IMA_KEXEC) += ima_kexec.o
 obj-$(CONFIG_IMA_BLACKLIST_KEYRING) += ima_mok.o
diff --git a/security/integrity/ima/ima.h b/security/integrity/ima/ima.h
index 011b91c79351..e21b06942858 100644
--- a/security/integrity/ima/ima.h
+++ b/security/integrity/ima/ima.h
@@ -302,6 +302,15 @@ static inline int ima_read_xattr(struct dentry *dentry,
 
 #endif /* CONFIG_IMA_APPRAISE */
 
+#ifdef CONFIG_IMA_APPRAISE_MODSIG
+bool ima_hook_supports_modsig(enum ima_hooks func);
+#else
+static inline bool ima_hook_supports_modsig(enum ima_hooks func)
+{
+        return false;
+}
+#endif /* CONFIG_IMA_APPRAISE_MODSIG */
+
 /* LSM based policy rules require audit */
 #ifdef CONFIG_IMA_LSM_RULES
 
diff --git a/security/integrity/ima/ima_modsig.c b/security/integrity/ima/ima_modsig.c
new file mode 100644
index 000000000000..87503bfe8c8b
--- /dev/null
+++ b/security/integrity/ima/ima_modsig.c
@@ -0,0 +1,31 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * IMA support for appraising module-style appended signatures.
+ *
+ * Copyright (C) 2019  IBM Corporation
+ *
+ * Author:
+ * Thiago Jung Bauermann <bauerman@linux.ibm.com>
+ */
+
+#include "ima.h"
+
+/**
+ * ima_hook_supports_modsig - can the policy allow modsig for this hook?
+ *
+ * modsig is only supported by hooks using ima_post_read_file(), because only
+ * they preload the contents of the file in a buffer. FILE_CHECK does that in
+ * some cases, but not when reached from vfs_open(). POLICY_CHECK can support
+ * it, but it's not useful in practice because it's a text file so deny.
+ */
+bool ima_hook_supports_modsig(enum ima_hooks func)
+{
+        switch (func) {
+        case KEXEC_KERNEL_CHECK:
+        case KEXEC_INITRAMFS_CHECK:
+        case MODULE_CHECK:
+                return true;
+        default:
+                return false;
+        }
+}
diff --git a/security/integrity/ima/ima_policy.c b/security/integrity/ima/ima_policy.c
index 36a0727f1d7a..5b6061d6bce0 100644
--- a/security/integrity/ima/ima_policy.c
+++ b/security/integrity/ima/ima_policy.c
@@ -1130,6 +1130,10 @@ static int ima_parse_rule(char *rule, struct ima_rule_entry *entry)
                         ima_log_string(ab, "appraise_type", args[0].from);
                         if ((strcmp(args[0].from, "imasig")) == 0)
                                 entry->flags |= IMA_DIGSIG_REQUIRED;
+                        else if (ima_hook_supports_modsig(entry->func) &&
+                                 strcmp(args[0].from, "imasig|modsig") == 0)
+                                entry->flags |= IMA_DIGSIG_REQUIRED |
+                                                IMA_MODSIG_ALLOWED;
                         else
                                 result = -EINVAL;
                         break;
@@ -1449,8 +1453,12 @@ int ima_policy_show(struct seq_file *m, void *v)
         }
         if (entry->template)
                 seq_printf(m, "template=%s ", entry->template->name);
-        if (entry->flags & IMA_DIGSIG_REQUIRED)
-                seq_puts(m, "appraise_type=imasig ");
+        if (entry->flags & IMA_DIGSIG_REQUIRED) {
+                if (entry->flags & IMA_MODSIG_ALLOWED)
+                        seq_puts(m, "appraise_type=imasig|modsig ");
+                else
+                        seq_puts(m, "appraise_type=imasig ");
+        }
         if (entry->flags & IMA_PERMIT_DIRECTIO)
                 seq_puts(m, "permit_directio ");
         rcu_read_unlock();
diff --git a/security/integrity/integrity.h b/security/integrity/integrity.h
index ed12d8e13d04..8c5736b68156 100644
--- a/security/integrity/integrity.h
+++ b/security/integrity/integrity.h
@@ -31,6 +31,7 @@
 #define IMA_NEW_FILE            0x04000000
 #define EVM_IMMUTABLE_DIGSIG    0x08000000
 #define IMA_FAIL_UNVERIFIABLE_SIGS      0x10000000
+#define IMA_MODSIG_ALLOWED      0x20000000
 
 #define IMA_DO_MASK             (IMA_MEASURE | IMA_APPRAISE | IMA_AUDIT | \
                                  IMA_HASH | IMA_APPRAISE_SUBMASK)