batman-adv: tt: fix TOCTOU race for reported vlans

commit 94d27005016be15ffc638b2ecbc4d58805ad7b48 upstream.

The local TT based TVLV is generated by first checking the number of VLANs
which have at least one TT entry. A new buffer with the correct size for
the VLANs is then allocated. Only then, the list of VLANs s used to fill
the VLAN entries in the buffer. During this time, the meshif_vlan_list_lock
is held. But the actual number of TT entries of each VLAN can still
increase during this time - just not the number of VLANs in the list.

But the prefilter used in the buffer size calculation might still cause an
increase of the number of VLANs which need to be stored. Simply because a
VLAN might now suddenly have at least one entry when it had none in the
pre-alloc check - and then needs to occupy space which was not allocated.

It is better to overestimate the buffer size at the beginning and then fill
the buffer only with the VLANs which are not empty.

Cc: stable@kernel.org
Fixes: 16116dac23 ("batman-adv: prevent TT request storms by not sending inconsistent TT TLVLs")
[ Context, drop flex array dependency ]
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
[uli: backport to 4.19]
Signed-off-by: Ulrich Hecht <uli@kernel.org>
Reviewed-by: Pavel Machek <pavel@nabladev.com>

net/batman-adv/translation-table.c

@@ -960,11 +960,8 @@ batadv_tt_prepare_tvlv_local_data(struct batadv_priv *bat_priv,
 	spin_lock_bh(&bat_priv->softif_vlan_list_lock);
 	hlist_for_each_entry_rcu(vlan, &bat_priv->softif_vlan_list, list) {
 		vlan_entries = atomic_read(&vlan->tt.num_entries);
-		if (vlan_entries < 1)
-			continue;
-
-		num_vlan++;
 		total_entries += vlan_entries;
+		num_vlan++;
 	}
 
 	change_offset = sizeof(**tt_data);
@@ -990,6 +987,7 @@ batadv_tt_prepare_tvlv_local_data(struct batadv_priv *bat_priv,
 	(*tt_data)->num_vlan = htons(num_vlan);
 
 	tt_vlan = (struct batadv_tvlv_tt_vlan_data *)(*tt_data + 1);
+	num_vlan = 0;
 	hlist_for_each_entry_rcu(vlan, &bat_priv->softif_vlan_list, list) {
 		vlan_entries = atomic_read(&vlan->tt.num_entries);
 		if (vlan_entries < 1)
@@ -1000,8 +998,16 @@ batadv_tt_prepare_tvlv_local_data(struct batadv_priv *bat_priv,
 		tt_vlan->reserved = 0;
 
 		tt_vlan++;
+		num_vlan++;
 	}
 
+	/* recalculate in case number of VLANs reduced */
+	change_offset = sizeof(**tt_data);
+	change_offset += num_vlan * sizeof(*tt_vlan);
+	tvlv_len = *tt_len + change_offset;
+
+	(*tt_data)->num_vlan = htons(num_vlan);
+
 	tt_change_ptr = (u8 *)*tt_data + change_offset;
 	*tt_change = (struct batadv_tvlv_tt_change *)tt_change_ptr;