audit: filter PATH records keyed on filesystem magic

Tracefs or debugfs were causing hundreds to thousands of PATH records to
be associated with the init_module and finit_module SYSCALL records on a
few modules when the following rule was in place for startup:
-a always,exit -F arch=x86_64 -S init_module -F key=mod-load

Provide a method to ignore these large number of PATH records from
overwhelming the logs if they are not of interest. Introduce a new
filter list "AUDIT_FILTER_FS", with a new field type AUDIT_FSTYPE,
which keys off the filesystem 4-octet hexadecimal magic identifier to
filter specific filesystem PATH records.

An example rule would look like:
-a never,filesystem -F fstype=0x74726163 -F key=ignore_tracefs
-a never,filesystem -F fstype=0x64626720 -F key=ignore_debugfs

Arguably the better way to address this issue is to disable tracefs and
debugfs on boot from production systems.

See: https://github.com/linux-audit/audit-kernel/issues/16
See: https://github.com/linux-audit/audit-userspace/issues/8
Test case: https://github.com/linux-audit/audit-testsuite/issues/42

Signed-off-by: Richard Guy Briggs <[e-mail]>
[PM: fixed the whitespace damage in kernel/auditsc.c]
Signed-off-by: Paul Moore <[e-mail]>

include/uapi/linux/audit.h

···
 #define AUDIT_FILTER_WATCH	0x03	/* Apply rule to file system watches */
 #define AUDIT_FILTER_EXIT	0x04	/* Apply rule at syscall exit */
 #define AUDIT_FILTER_TYPE	0x05	/* Apply rule at audit_log_start */
+#define AUDIT_FILTER_FS		0x06	/* Apply rule at __audit_inode_child */
 
-#define AUDIT_NR_FILTERS	6
+#define AUDIT_NR_FILTERS	7
 
 #define AUDIT_FILTER_PREPEND	0x10	/* Prepend to front of list */
 
···
 #define AUDIT_OBJ_LEV_HIGH	23
 #define AUDIT_LOGINUID_SET	24
 #define AUDIT_SESSIONID	25	/* Session ID */
+#define AUDIT_FSTYPE	26	/* FileSystem Type */
 
 				/* These are ONLY useful when checking
 				 * at syscall exit time (AUDIT_AT_EXIT). */
···
 #define AUDIT_FEATURE_BITMAP_EXCLUDE_EXTEND	0x00000008
 #define AUDIT_FEATURE_BITMAP_SESSIONID_FILTER	0x00000010
 #define AUDIT_FEATURE_BITMAP_LOST_RESET		0x00000020
+#define AUDIT_FEATURE_BITMAP_FILTER_FS		0x00000040
 
 #define AUDIT_FEATURE_BITMAP_ALL (AUDIT_FEATURE_BITMAP_BACKLOG_LIMIT | \
 				  AUDIT_FEATURE_BITMAP_BACKLOG_WAIT_TIME | \
 				  AUDIT_FEATURE_BITMAP_EXECUTABLE_PATH | \
 				  AUDIT_FEATURE_BITMAP_EXCLUDE_EXTEND | \
 				  AUDIT_FEATURE_BITMAP_SESSIONID_FILTER | \
-				  AUDIT_FEATURE_BITMAP_LOST_RESET)
+				  AUDIT_FEATURE_BITMAP_LOST_RESET | \
+				  AUDIT_FEATURE_BITMAP_FILTER_FS)
 
 /* deprecated: AUDIT_VERSION_* */
 #define AUDIT_VERSION_LATEST 		AUDIT_FEATURE_BITMAP_ALL

kernel/auditfilter.c

···
 	LIST_HEAD_INIT(audit_filter_list[3]),
 	LIST_HEAD_INIT(audit_filter_list[4]),
 	LIST_HEAD_INIT(audit_filter_list[5]),
-#if AUDIT_NR_FILTERS != 6
+	LIST_HEAD_INIT(audit_filter_list[6]),
+#if AUDIT_NR_FILTERS != 7
 #error Fix audit_filter_list initialiser
 #endif
 };
···
 	LIST_HEAD_INIT(audit_rules_list[3]),
 	LIST_HEAD_INIT(audit_rules_list[4]),
 	LIST_HEAD_INIT(audit_rules_list[5]),
+	LIST_HEAD_INIT(audit_rules_list[6]),
 };
 
 DEFINE_MUTEX(audit_filter_mutex);
···
 #endif
 	case AUDIT_FILTER_USER:
 	case AUDIT_FILTER_TYPE:
+	case AUDIT_FILTER_FS:
 		;
 	}
 	if (unlikely(rule->action == AUDIT_POSSIBLE)) {
···
 		    entry->rule.listnr != AUDIT_FILTER_USER)
 			return -EINVAL;
 		break;
+	case AUDIT_FSTYPE:
+		if (entry->rule.listnr != AUDIT_FILTER_FS)
+			return -EINVAL;
+		break;
+	}
+
+	switch(entry->rule.listnr) {
+	case AUDIT_FILTER_FS:
+		switch(f->type) {
+		case AUDIT_FSTYPE:
+		case AUDIT_FILTERKEY:
+			break;
+		default:
+			return -EINVAL;
+		}
 	}
 
 	switch(f->type) {
···
 			return -EINVAL;
 	/* FALL THROUGH */
 	case AUDIT_ARCH:
+	case AUDIT_FSTYPE:
 		if (f->op != Audit_not_equal && f->op != Audit_equal)
 			return -EINVAL;
 		break;
···
 #ifdef CONFIG_AUDITSYSCALL
 	int dont_count = 0;
 
-	/* If either of these, don't count towards total */
-	if (entry->rule.listnr == AUDIT_FILTER_USER ||
-		entry->rule.listnr == AUDIT_FILTER_TYPE)
+	/* If any of these, don't count towards total */
+	switch(entry->rule.listnr) {
+	case AUDIT_FILTER_USER:
+	case AUDIT_FILTER_TYPE:
+	case AUDIT_FILTER_FS:
 		dont_count = 1;
+	}
 #endif
 
 	mutex_lock(&audit_filter_mutex);
···
 #ifdef CONFIG_AUDITSYSCALL
 	int dont_count = 0;
 
-	/* If either of these, don't count towards total */
-	if (entry->rule.listnr == AUDIT_FILTER_USER ||
-		entry->rule.listnr == AUDIT_FILTER_TYPE)
+	/* If any of these, don't count towards total */
+	switch(entry->rule.listnr) {
+	case AUDIT_FILTER_USER:
+	case AUDIT_FILTER_TYPE:
+	case AUDIT_FILTER_FS:
 		dont_count = 1;
+	}
 #endif
 
 	mutex_lock(&audit_filter_mutex);

kernel/auditsc.c

···
 	struct inode *inode = d_backing_inode(dentry);
 	const char *dname = dentry->d_name.name;
 	struct audit_names *n, *found_parent = NULL, *found_child = NULL;
+	struct audit_entry *e;
+	struct list_head *list = &audit_filter_list[AUDIT_FILTER_FS];
+	int i;
 
 	if (!context->in_syscall)
 		return;
+
+	rcu_read_lock();
+	if (!list_empty(list)) {
+		list_for_each_entry_rcu(e, list, list) {
+			for (i = 0; i < e->rule.field_count; i++) {
+				struct audit_field *f = &e->rule.fields[i];
+
+				if (f->type == AUDIT_FSTYPE) {
+					if (audit_comparator(parent->i_sb->s_magic,
+					    f->op, f->val)) {
+						if (e->rule.action == AUDIT_NEVER) {
+							rcu_read_unlock();
+							return;
+						}
+					}
+				}
+			}
+		}
+	}
+	rcu_read_unlock();
 
 	if (inode)
 		handle_one(inode);