[PATCH batadv v2 3/5] batman-adv: frag: disallow unicast fragment in fragment

batadv_frag_skb_buffer() is called by batadv_batman_skb_recv() when a
BATADV_UNICAST_FRAG packet is received. Once all fragments are collected
and the packet is reassembled, batadv_recv_frag_packet() calls
batadv_batman_skb_recv() again to process the defragmented payload.

A malicious sender can craft a BATADV_UNICAST_FRAG packet whose reassembled
payload is itself a BATADV_UNICAST_FRAG packet (matryoshka-style nesting).
Each nesting level recurses through batadv_batman_skb_recv() without bound,
growing the kernel stack until it is exhausted.

Since refragmentation or fragments in fragments are not actually allowed,
discard all packets which are still BATADV_UNICAST_FRAG packets after the
defragmentation process.

Cc: [email protected]
Fixes: 610bfc6bc99b ("batman-adv: Receive fragmented packets and merge")
Reported-by: Yuan Tan <[email protected]>
Reported-by: Yifan Wu <[email protected]>
Reported-by: Juefei Pu <[email protected]>
Reported-by: Xin Liu <[email protected]>
Signed-off-by: Sven Eckelmann <[email protected]>
---
 net/batman-adv/fragmentation.c | 35 +++++++++++++++++++++++++++++++++++
 1 file changed, 35 insertions(+)

diff --git a/net/batman-adv/fragmentation.c b/net/batman-adv/fragmentation.c
index f4e45cc2..42f3d4ee 100644
--- a/net/batman-adv/fragmentation.c
+++ b/net/batman-adv/fragmentation.c
@@ -293,6 +293,31 @@ batadv_frag_merge_packets(struct hlist_head *chain)
 	return skb_out;
 }
 
+/**
+ * batadv_skb_is_frag() - check if newly merged skb is gain a unicast packet
+ * @skb: newly merged skb
+ *
+ * Return: if newly skb is of type BATADV_UNICAST_FRAG
+ */
+static bool batadv_skb_is_frag(struct sk_buff *skb)
+{
+	struct batadv_ogm_packet *batadv_ogm_packet;
+
+	/* packet should hold at least type and version */
+	if (unlikely(!pskb_may_pull(skb, 2)))
+		return false;
+
+	batadv_ogm_packet = (struct batadv_ogm_packet *)skb->data;
+
+	if (batadv_ogm_packet->version != BATADV_COMPAT_VERSION)
+		return false;
+
+	if (batadv_ogm_packet->packet_type != BATADV_UNICAST_FRAG)
+		return false;
+
+	return true;
+}
+
 /**
  * batadv_frag_skb_buffer() - buffer fragment for later merge
  * @skb: skb to buffer
@@ -326,6 +351,16 @@ bool batadv_frag_skb_buffer(struct sk_buff **skb,
 	if (!skb_out)
 		goto out_err;
 
+	/* fragment in fragment is not allowed. otherwise it is possible
+	 * to exhaust the stack when receiving a matryoshka-style
+	 * "fragments in a fragment packet"
+	 */
+	if (batadv_skb_is_frag(skb_out)) {
+		kfree_skb(skb_out);
+		skb_out = NULL;
+		goto out_err;
+	}
+
 out:
 	ret = true;
 out_err:

-- 
2.47.3