ima: add fs_subtype condition for distinguishing FUSE instances

Linux systems often use FUSE for several different purposes, where the
contents of some FUSE instances can be of more interest for auditing
than others.

Allow distinguishing between them based on the filesystem subtype
(s_subtype) using the new condition "fs_subtype".

The subtype string is supplied by userspace FUSE daemons
when a FUSE connection is initialized, so policy authors who want to
filter based on subtype need to ensure that FUSE mount operations are
sufficiently audited or restricted.

Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

Author: thejh

Documentation/ABI/testing/ima_policy

@@ -23,6 +23,7 @@ Description:
 			  audit | dont_audit | hash | dont_hash
 		  condition:= base | lsm  [option]
 			base:	[[func=] [mask=] [fsmagic=] [fsuuid=] [fsname=]
+				[fs_subtype=]
 				[uid=] [euid=] [gid=] [egid=]
 				[fowner=] [fgroup=]]
 			lsm:	[[subj_user=] [subj_role=] [subj_type=]

security/integrity/ima/ima_policy.c

@@ -38,6 +38,7 @@
 #define IMA_GID		0x2000
 #define IMA_EGID	0x4000
 #define IMA_FGROUP	0x8000
+#define IMA_FS_SUBTYPE	0x10000
 
 #define UNKNOWN		0
 #define MEASURE		0x0001	/* same as IMA_MEASURE */
@@ -120,6 +121,7 @@ struct ima_rule_entry {
 		int type;	/* audit type */
 	} lsm[MAX_LSM_RULES];
 	char *fsname;
+	char *fs_subtype;
 	struct ima_rule_opt_list *keyrings; /* Measure keys added to these keyrings */
 	struct ima_rule_opt_list *label; /* Measure data grouped under this label */
 	struct ima_template_desc *template;
@@ -398,6 +400,7 @@ static void ima_free_rule(struct ima_rule_entry *entry)
 	 * the defined_templates list and cannot be freed here
 	 */
 	kfree(entry->fsname);
+	kfree(entry->fs_subtype);
 	ima_free_rule_opt_list(entry->keyrings);
 	ima_lsm_free_rule(entry);
 	kfree(entry);
@@ -602,6 +605,12 @@ static bool ima_match_rules(struct ima_rule_entry *rule,
 	if ((rule->flags & IMA_FSNAME)
 	    && strcmp(rule->fsname, inode->i_sb->s_type->name))
 		return false;
+	if (rule->flags & IMA_FS_SUBTYPE) {
+		if (!inode->i_sb->s_subtype)
+			return false;
+		if (strcmp(rule->fs_subtype, inode->i_sb->s_subtype))
+			return false;
+	}
 	if ((rule->flags & IMA_FSUUID) &&
 	    !uuid_equal(&rule->fsuuid, &inode->i_sb->s_uuid))
 		return false;
@@ -1068,7 +1077,7 @@ enum policy_opt {
 	Opt_audit, Opt_dont_audit, Opt_hash, Opt_dont_hash,
 	Opt_obj_user, Opt_obj_role, Opt_obj_type,
 	Opt_subj_user, Opt_subj_role, Opt_subj_type,
-	Opt_func, Opt_mask, Opt_fsmagic, Opt_fsname, Opt_fsuuid,
+	Opt_func, Opt_mask, Opt_fsmagic, Opt_fsname, Opt_fs_subtype, Opt_fsuuid,
 	Opt_uid_eq, Opt_euid_eq, Opt_gid_eq, Opt_egid_eq,
 	Opt_fowner_eq, Opt_fgroup_eq,
 	Opt_uid_gt, Opt_euid_gt, Opt_gid_gt, Opt_egid_gt,
@@ -1100,6 +1109,7 @@ static const match_table_t policy_tokens = {
 	{Opt_mask, "mask=%s"},
 	{Opt_fsmagic, "fsmagic=%s"},
 	{Opt_fsname, "fsname=%s"},
+	{Opt_fs_subtype, "fs_subtype=%s"},
 	{Opt_fsuuid, "fsuuid=%s"},
 	{Opt_uid_eq, "uid=%s"},
 	{Opt_euid_eq, "euid=%s"},
@@ -1284,7 +1294,8 @@ static bool ima_validate_rule(struct ima_rule_entry *entry)
 		if (entry->flags & ~(IMA_FUNC | IMA_MASK | IMA_FSMAGIC |
 				     IMA_UID | IMA_FOWNER | IMA_FSUUID |
 				     IMA_INMASK | IMA_EUID | IMA_PCR |
-				     IMA_FSNAME | IMA_GID | IMA_EGID |
+				     IMA_FSNAME | IMA_FS_SUBTYPE |
+				     IMA_GID | IMA_EGID |
 				     IMA_FGROUP | IMA_DIGSIG_REQUIRED |
 				     IMA_PERMIT_DIRECTIO | IMA_VALIDATE_ALGOS |
 				     IMA_CHECK_BLACKLIST | IMA_VERITY_REQUIRED))
@@ -1297,7 +1308,8 @@ static bool ima_validate_rule(struct ima_rule_entry *entry)
 		if (entry->flags & ~(IMA_FUNC | IMA_MASK | IMA_FSMAGIC |
 				     IMA_UID | IMA_FOWNER | IMA_FSUUID |
 				     IMA_INMASK | IMA_EUID | IMA_PCR |
-				     IMA_FSNAME | IMA_GID | IMA_EGID |
+				     IMA_FSNAME | IMA_FS_SUBTYPE |
+				     IMA_GID | IMA_EGID |
 				     IMA_FGROUP | IMA_DIGSIG_REQUIRED |
 				     IMA_PERMIT_DIRECTIO | IMA_MODSIG_ALLOWED |
 				     IMA_CHECK_BLACKLIST | IMA_VALIDATE_ALGOS))
@@ -1310,7 +1322,8 @@ static bool ima_validate_rule(struct ima_rule_entry *entry)
 
 		if (entry->flags & ~(IMA_FUNC | IMA_FSMAGIC | IMA_UID |
 				     IMA_FOWNER | IMA_FSUUID | IMA_EUID |
-				     IMA_PCR | IMA_FSNAME | IMA_GID | IMA_EGID |
+				     IMA_PCR | IMA_FSNAME | IMA_FS_SUBTYPE |
+				     IMA_GID | IMA_EGID |
 				     IMA_FGROUP))
 			return false;
 
@@ -1597,6 +1610,22 @@ static int ima_parse_rule(char *rule, struct ima_rule_entry *entry)
 			result = 0;
 			entry->flags |= IMA_FSNAME;
 			break;
+		case Opt_fs_subtype:
+			ima_log_string(ab, "fs_subtype", args[0].from);
+
+			if (entry->fs_subtype) {
+				result = -EINVAL;
+				break;
+			}
+
+			entry->fs_subtype = kstrdup(args[0].from, GFP_KERNEL);
+			if (!entry->fs_subtype) {
+				result = -ENOMEM;
+				break;
+			}
+			result = 0;
+			entry->flags |= IMA_FS_SUBTYPE;
+			break;
 		case Opt_keyrings:
 			ima_log_string(ab, "keyrings", args[0].from);
 
@@ -2145,6 +2174,12 @@ int ima_policy_show(struct seq_file *m, void *v)
 		seq_puts(m, " ");
 	}
 
+	if (entry->flags & IMA_FS_SUBTYPE) {
+		snprintf(tbuf, sizeof(tbuf), "%s", entry->fs_subtype);
+		seq_printf(m, pt(Opt_fs_subtype), tbuf);
+		seq_puts(m, " ");
+	}
+
 	if (entry->flags & IMA_KEYRINGS) {
 		seq_puts(m, "keyrings=");
 		ima_show_rule_opt_list(m, entry->keyrings);