batman-adv: Fix double free during fragment merge error

BugLink: https://bugs.launchpad.net/bugs/1868629

commit 248e23b5 upstream.

The function batadv_frag_skb_buffer was supposed not to consume the skbuff
on errors. This was followed in the helper function
batadv_frag_insert_packet when the skb would potentially be inserted in the
fragment queue. But it could happen that the next helper function
batadv_frag_merge_packets would try to merge the fragments and fail. This
results in a kfree_skb of all the enqueued fragments (including the just
inserted one). batadv_recv_frag_packet would detect the error in
batadv_frag_skb_buffer and try to free the skb again.

The behavior of batadv_frag_skb_buffer (and its helper
batadv_frag_insert_packet) must therefore be changed to always consume the
skbuff to have a common behavior and avoid the double kfree_skb.

Fixes: 610bfc6b ("batman-adv: Receive fragmented packets and merge")
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>
Signed-off-by: Kelsey Skunberg <kelsey.skunberg@canonical.com>

net/batman-adv/fragmentation.c

@@ -233,8 +233,10 @@ static bool batadv_frag_insert_packet(struct batadv_orig_node *orig_node,
 	spin_unlock_bh(&chain->lock);
 
 err:
-	if (!ret)
+	if (!ret) {
 		kfree(frag_entry_new);
+		kfree_skb(skb);
+	}
 
 	return ret;
 }
@@ -329,9 +331,9 @@ bool batadv_frag_skb_buffer(struct sk_buff **skb,
 		goto out_err;
 
 out:
-	*skb = skb_out;
 	ret = true;
 out_err:
+	*skb = skb_out;
 	return ret;
 }
 

net/batman-adv/routing.c

@@ -1053,6 +1053,12 @@ int batadv_recv_frag_packet(struct sk_buff *skb,
 	batadv_inc_counter(bat_priv, BATADV_CNT_FRAG_RX);
 	batadv_add_counter(bat_priv, BATADV_CNT_FRAG_RX_BYTES, skb->len);
 
+	/* batadv_frag_skb_buffer will always consume the skb and
+	 * the caller should therefore never try to free the
+	 * skb after this point
+	 */
+	ret = NET_RX_SUCCESS;
+
 	/* Add fragment to buffer and merge if possible. */
 	if (!batadv_frag_skb_buffer(&skb, orig_node_src))
 		goto out;