batman-adv: Don't accept TT entries for out-of-spec VIDs

The internal handling of VLAN IDs in batman-adv is only specified for
following encodings:

* VLAN is used
  - bit 15 is 1
  - bit 11 - bit 0 is the VLAN ID (0-4095)
  - remaining bits are 0
* No VLAN is used
  - bit 15 is 0
  - remaining bits are 0

batman-adv was only preparing new translation table entries (based on its
soft interface information) using this encoding format. But the receive
path was never checking if entries in the roam or TT TVLVs were also
following this encoding.

It was therefore possible to create more than the expected maximum of 4096
+ 1 entries in the originator VLAN list. Simply by setting the "remaining
bits" to "random" values in corresponding TVLV.

Cc: stable@vger.kernel.org
Fixes: 7ea7b4a142 ("batman-adv: make the TT CRC logic VLAN specific")
Reported-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>

net/batman-adv/originator.c

@@ -12,6 +12,7 @@
 #include <linux/errno.h>
 #include <linux/etherdevice.h>
 #include <linux/gfp.h>
+#include <linux/if_vlan.h>
 #include <linux/jiffies.h>
 #include <linux/kref.h>
 #include <linux/list.h>
@@ -131,6 +132,29 @@ batadv_orig_node_vlan_get(struct batadv_orig_node *orig_node,
 	return vlan;
 }
 
+/**
+ * batadv_vlan_id_valid() - check if vlan id is in valid batman-adv encoding
+ * @vid: the VLAN identifier
+ *
+ * Return: true when either no vlan is set or if VLAN is in correct range,
+ *  false otherwise
+ */
+static bool batadv_vlan_id_valid(unsigned short vid)
+{
+	unsigned short non_vlan = vid & ~(BATADV_VLAN_HAS_TAG | VLAN_VID_MASK);
+
+	if (vid == 0)
+		return true;
+
+	if (!(vid & BATADV_VLAN_HAS_TAG))
+		return false;
+
+	if (non_vlan)
+		return false;
+
+	return true;
+}
+
 /**
  * batadv_orig_node_vlan_new() - search and possibly create an orig_node_vlan
  *  object
@@ -149,6 +173,9 @@ batadv_orig_node_vlan_new(struct batadv_orig_node *orig_node,
 {
 	struct batadv_orig_node_vlan *vlan;
 
+	if (!batadv_vlan_id_valid(vid))
+		return NULL;
+
 	spin_lock_bh(&orig_node->vlan_list_lock);
 
 	/* first look if an object for this vid already exists */