lsm: add LSM_AUDIT_DATA_NS for namespace audit records

Add a new LSM audit data type LSM_AUDIT_DATA_NS that logs namespace
information in audit records.  Two fields are provided:

- namespace_type: the CLONE_NEW* flag identifying the namespace type,
  logged in hexadecimal.

- namespace_id: the unique 64-bit namespace identifier, retrievable
  from userspace via NS_GET_ID or listns(2).  Unlike the proc inode
  number (inum), namespace_id is never recycled.  For namespace creation
  denials, namespace_id is 0 because the namespace does not exist yet.

A new audit data type is needed because no existing LSM_AUDIT_DATA_*
type carries namespace information.  The closest alternatives (e.g.
LSM_AUDIT_DATA_TASK or LSM_AUDIT_DATA_NONE with custom strings) would
either lose the namespace type or require ad-hoc formatting that
bypasses the structured audit data union.

Cc: Günther Noack <gnoack@google.com>
Cc: Paul Moore <paul@paul-moore.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Reviewed-by: Günther Noack <gnoack@google.com>
Signed-off-by: Mickaël Salaün <mic@digikod.net>
[PM: corrected audit fields in the description, subject tweaks]
Signed-off-by: Paul Moore <paul@paul-moore.com>

include/linux/lsm_audit.h

@@ -78,6 +78,7 @@ struct common_audit_data {
 #define LSM_AUDIT_DATA_NOTIFICATION 16
 #define LSM_AUDIT_DATA_ANONINODE	17
 #define LSM_AUDIT_DATA_NLMSGTYPE	18
+#define LSM_AUDIT_DATA_NS		19
 	union 	{
 		struct path path;
 		struct dentry *dentry;
@@ -100,6 +101,10 @@ struct common_audit_data {
 		int reason;
 		const char *anonclass;
 		u16 nlmsg_type;
+		struct {
+			u32 ns_type;
+			u64 ns_id;
+		} ns;
 	} u;
 	/* this union contains LSM specific data */
 	union {

security/lsm_audit.c

@@ -403,6 +403,10 @@ void audit_log_lsm_data(struct audit_buffer *ab,
 	case LSM_AUDIT_DATA_NLMSGTYPE:
 		audit_log_format(ab, " nl-msgtype=%hu", a->u.nlmsg_type);
 		break;
+	case LSM_AUDIT_DATA_NS:
+		audit_log_format(ab, " namespace_type=0x%x namespace_id=%llu",
+				 a->u.ns.ns_type, a->u.ns.ns_id);
+		break;
 	} /* switch (a->type) */
 }