audit: more filter PATH records keyed on filesystem magic

Like commit 42d5e37654 ("audit: filter PATH records keyed on
filesystem magic") that addresses
https://github.com/linux-audit/audit-kernel/issues/8

Any user or remote filesystem could become unavailable and effectively
block on a forced unmount.

    -a always,exit -S umount2 -F key=umount2

Provide a method to ignore these user and remote filesystems to prevent
them from being impossible to unmount.

Extend the "AUDIT_FILTER_FS" filter that uses the field type
AUDIT_FSTYPE keying off the filesystem 4-octet hexadecimal magic
identifier to filter specific filesystems to cover audit_inode() to address
this blockage.

An example rule would look like:
    -a never,filesystem -F fstype=0x517B -F key=ignore_smb
    -a never,filesystem -F fstype=0x6969 -F key=ignore_nfs

Arguably the better way to address this issue is to disable auditing
processes that touch removable filesystems.

Note: refactor __audit_inode_child() to remove two levels of if
indentation.

Please see the github issue tracker
https://github.com/linux-audit/audit-kernel/issues/100

Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

kernel/auditsc.c

@@ -1766,10 +1766,31 @@ void __audit_inode(struct filename *name, const struct dentry *dentry,
 	struct inode *inode = d_backing_inode(dentry);
 	struct audit_names *n;
 	bool parent = flags & AUDIT_INODE_PARENT;
+	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
+				    && audit_comparator(inode->i_sb->s_magic,
+							f->op, f->val)
+				    && e->rule.action == AUDIT_NEVER) {
+					rcu_read_unlock();
+					return;
+				}
+			}
+		}
+	}
+	rcu_read_unlock();
+
 	if (!name)
 		goto out_alloc;
 
@@ -1878,14 +1899,12 @@ void __audit_inode_child(struct inode *parent,
 			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;
-						}
-					}
+				if (f->type == AUDIT_FSTYPE
+				    && audit_comparator(parent->i_sb->s_magic,
+							f->op, f->val)
+				    && e->rule.action == AUDIT_NEVER) {
+					rcu_read_unlock();
+					return;
 				}
 			}
 		}