IMA: support for duplicate measurement records

IMA measures contents of a given file/buffer/critical-data record,
and properly re-measures it on change.  However, IMA does not measure
the duplicate value for a given record, since TPM extend is a very
expensive operation.  For example, if the record changes from value
'v#1' to 'v#2', and then back to 'v#1', IMA will not measure and log
the last change to 'v#1', since the hash of 'v#1' for that record is
already present in the IMA htable.  This limits the ability of an
external attestation service to accurately determine the current state
of the system.  The service would incorrectly conclude that the latest
value of the given record on the system is 'v#2', and act accordingly.

Define and use a new Kconfig option IMA_DISABLE_HTABLE to permit
duplicate records in the IMA measurement list.

In addition to the duplicate measurement records described above,
other duplicate file measurement records may be included in the log,
when CONFIG_IMA_DISABLE_HTABLE is enabled.  For example,
    - i_version is not enabled,
    - i_generation changed,
    - same file present on different filesystems,
    - an inode is evicted from dcache

Signed-off-by: Tushar Sugandhi <tusharsu@linux.microsoft.com>
Reviewed-by: Petr Vorel <pvorel@suse.cz>
[zohar@linux.ibm.com: updated list of duplicate measurement records]
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

security/integrity/ima/Kconfig

@@ -334,3 +334,10 @@ config IMA_SECURE_AND_OR_TRUSTED_BOOT
        help
           This option is selected by architectures to enable secure and/or
           trusted boot based on IMA runtime policies.
+
+config IMA_DISABLE_HTABLE
+	bool "Disable htable to allow measurement of duplicate records"
+	depends on IMA
+	default n
+	help
+	   This option disables htable to allow measurement of duplicate records.

security/integrity/ima/ima_queue.c

@@ -168,7 +168,7 @@ int ima_add_template_entry(struct ima_template_entry *entry, int violation,
 	int result = 0, tpmresult = 0;
 
 	mutex_lock(&ima_extend_list_mutex);
-	if (!violation) {
+	if (!violation && !IS_ENABLED(CONFIG_IMA_DISABLE_HTABLE)) {
 		if (ima_lookup_digest_entry(digest, entry->pcr)) {
 			audit_cause = "hash_exists";
 			result = -EEXIST;
@@ -176,7 +176,8 @@ int ima_add_template_entry(struct ima_template_entry *entry, int violation,
 		}
 	}
 
-	result = ima_add_digest_entry(entry, 1);
+	result = ima_add_digest_entry(entry,
+				      !IS_ENABLED(CONFIG_IMA_DISABLE_HTABLE));
 	if (result < 0) {
 		audit_cause = "ENOMEM";
 		audit_info = 0;