apparmor: fix: limit the number of levels of policy namespaces

Currently the number of policy namespaces is not bounded relying on
the user namespace limit. However policy namespaces aren't strictly
tied to user namespaces and it is possible to create them and nest
them arbitrarily deep which can be used to exhaust system resource.

Hard cap policy namespaces to the same depth as user namespaces.

Fixes: c88d4c7b04 ("AppArmor: core policy routines")
Reported-by: Qualys Security Advisory <qsa@qualys.com>
Reviewed-by: Ryan Lee <ryan.lee@canonical.com>
Reviewed-by: Cengiz Can <cengiz.can@canonical.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>

security/apparmor/include/policy_ns.h

@@ -18,6 +18,8 @@
 #include "label.h"
 #include "policy.h"
 
+/* Match max depth of user namespaces */
+#define MAX_NS_DEPTH 32
 
 /* struct aa_ns_acct - accounting of profiles in namespace
  * @max_size: maximum space allowed for all profiles in namespace

security/apparmor/policy_ns.c

@@ -223,6 +223,8 @@ static struct aa_ns *__aa_create_ns(struct aa_ns *parent, const char *name,
 	AA_BUG(!name);
 	AA_BUG(!mutex_is_locked(&parent->lock));
 
+	if (parent->level > MAX_NS_DEPTH)
+		return ERR_PTR(-ENOSPC);
 	ns = alloc_ns(parent->base.hname, name);
 	if (!ns)
 		return ERR_PTR(-ENOMEM);