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 <rgb@redhat.com>
[PM: fixed the whitespace damage in kernel/auditsc.c]
Signed-off-by: Paul Moore <paul@paul-moore.com>

1 files changed, 23 insertions, 0 deletions

diff --git a/kernel/auditsc.c b/kernel/auditsc.c
index aac1a41f82bd..c9bb29e17335 100644
--- a/kernel/auditsc.c
+++ b/kernel/auditsc.c
@@ -1869,10 +1869,33 @@ void __audit_inode_child(struct inode *parent,
 	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);