[RFC PATCH ghak100 V1 2/2] audit: moar filter PATH records keyed on filesystem magic

[Richard Guy Briggs <rgb@redhat.com>]

Like 42d5e37654e4 ("audit: filter PATH records keyed on filesystem magic")

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.

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

Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
---
 kernel/auditsc.c | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/kernel/auditsc.c b/kernel/auditsc.c
index d39a7fbaf944..59d6d3fbc00e 100644
--- a/kernel/auditsc.c
+++ b/kernel/auditsc.c
@@ -1777,10 +1777,33 @@ 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) {
+					if (audit_comparator(inode->i_sb->s_magic,
+					    f->op, f->val)) {
+						if (e->rule.action == AUDIT_NEVER) {
+							rcu_read_unlock();
+							return;
+						}
+					}
+				}
+			}
+		}
+	}
+	rcu_read_unlock();
+
 	if (!name)
 		goto out_alloc;
 
-- 
1.8.3.1