From: Linus Lüssing linus.luessing@c0d3.blue

We currently have two code paths for broadcast packets:

A) self-generated, via batadv_interface_tx()-> batadv_send_bcast_packet(). B) received/forwarded, via batadv_recv_bcast_packet()-> batadv_forw_bcast_packet().

For A), self-generated broadcast packets:

the only modifications to the skb data is the ethernet header which is added/pushed to the skb in batadv_send_broadcast_skb()->batadv_send_skb_packet(). However before doing so, batadv_skb_head_push() is called which calls skb_cow_head() to unshare the space for the to be pushed ethernet header. So for this case, it is safe to use skb clones.

For B), received/forwarded packets:

the same applies as in A) for the to be forwarded packets. Only the ethernet header is added. However after (queueing for) forwarding the packet in batadv_recv_bcast_packet()->batadv_forw_bcast_packet(), a packet is additionally decapsulated and is sent up the stack through batadv_recv_bcast_packet()->batadv_interface_rx().

Protocols higher up the stack are already required to check if the packet is shared and create a copy for further modifications. When the next (protocol) layer works correctly, it cannot happen that ot tries to operate on the data behind the skb clone which is still queued up for forwarding.

Signed-off-by: Linus Lüssing linus.luessing@c0d3.blue Co-authored-by: Sven Eckelmann sven@narfation.org Signed-off-by: Sven Eckelmann sven@narfation.org --- v3: * newly added this patch, to move skb_copy()->skb_clone() changes from PATCH 01/03 to a separate patch with its own explanation v4: * dropped skb_cow call in __batadv_forw_bcast_packet and adjusted the text for B) to explain the reasoning behind it

net/batman-adv/send.c | 12 ++++++++++-- 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/net/batman-adv/send.c b/net/batman-adv/send.c index 0b9dd29d..b1cb9eb3 100644 --- a/net/batman-adv/send.c +++ b/net/batman-adv/send.c @@ -748,6 +748,10 @@ void batadv_forw_packet_ogmv1_queue(struct batadv_priv *bat_priv, * Adds a broadcast packet to the queue and sets up timers. Broadcast packets * are sent multiple times to increase probability for being received. * + * This call clones the given skb, hence the caller needs to take into + * account that the data segment of the original skb might not be + * modifiable anymore. + * * Return: NETDEV_TX_OK on success and NETDEV_TX_BUSY on errors. */ static int batadv_forw_bcast_packet_to_list(struct batadv_priv *bat_priv, @@ -761,7 +765,7 @@ static int batadv_forw_bcast_packet_to_list(struct batadv_priv *bat_priv, unsigned long send_time = jiffies; struct sk_buff *newskb;

- newskb = skb_copy(skb, GFP_ATOMIC); + newskb = skb_clone(skb, GFP_ATOMIC); if (!newskb) goto err;

@@ -800,6 +804,10 @@ static int batadv_forw_bcast_packet_to_list(struct batadv_priv *bat_priv, * or if a delay is given after that. Furthermore, queues additional * retransmissions if this interface is a wireless one. * + * This call clones the given skb, hence the caller needs to take into + * account that the data segment of the original skb might not be + * modifiable anymore. + * * Return: NETDEV_TX_OK on success and NETDEV_TX_BUSY on errors. */ static int batadv_forw_bcast_packet_if(struct batadv_priv *bat_priv, @@ -814,7 +822,7 @@ static int batadv_forw_bcast_packet_if(struct batadv_priv *bat_priv, int ret = NETDEV_TX_OK;

if (!delay) { - newskb = skb_copy(skb, GFP_ATOMIC); + newskb = skb_clone(skb, GFP_ATOMIC); if (!newskb) return NETDEV_TX_BUSY;