[PATCH net] batman-adv: Close OGM aggregation before transmission

[Chengfeng Ye <nicoyip.dev@gmail.com>]

The OGM send worker leaves its forwarding packet on forw_bat_list until
after batadv_iv_ogm_emit() returns. Aggregation holds forw_bat_list_lock,
but emission reads and clones the packet without that lock.

CPU 0 can therefore start emitting a queued packet while CPU 1 takes the
list lock, finds the same packet and appends another OGM. The sender can
observe the new packet length before the corresponding direct-link flag
is set, or clone the skb while its length and payload are being updated.
This can transmit an OGM with incorrect flags or inconsistent data.

KCSAN reported:

  BUG: KCSAN: data-race in batadv_iv_ogm_queue_add / batadv_iv_send_outstanding_bat_ogm_packet
  write to 0xffff888100fedcf8 of 2 bytes by interrupt on cpu 1:
  read to 0xffff888100fedcf8 of 2 bytes by task 70 on cpu 2:
  value changed: 0x00c0 -> 0x00d8

Set num_packets to BATADV_MAX_AGGREGATION_PACKETS under the list lock
before emission. This waits for any ongoing append and makes the existing
aggregation limit check reject further appends before inspecting mutable
OGM flags. Emission walks packet_len rather than num_packets, so the
queued contents are still sent normally. Keep the packet on the list so
interface purging can still wait for the worker and retain its existing
ownership of the packet when freeing it.

Backports before Linux 6.15 need count-handling adaptation.

Fixes: 9b4aec647a92 ("batman-adv: fix rare race conditions on interface removal")
Cc: stable@vger.kernel.org
Signed-off-by: Chengfeng Ye <nicoyip.dev@gmail.com>
---
 net/batman-adv/bat_iv_ogm.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/batman-adv/bat_iv_ogm.c b/net/batman-adv/bat_iv_ogm.c
index 53fbdbbe8f4f..59847299a123 100644
--- a/net/batman-adv/bat_iv_ogm.c
+++ b/net/batman-adv/bat_iv_ogm.c
@@ -1909,6 +1909,10 @@ static void batadv_iv_send_outstanding_bat_ogm_packet(struct work_struct *work)
 		goto out;
 	}
 
+	spin_lock_bh(&bat_priv->forw_bat_list_lock);
+	forw_packet->num_packets = BATADV_MAX_AGGREGATION_PACKETS;
+	spin_unlock_bh(&bat_priv->forw_bat_list_lock);
+
 	batadv_iv_ogm_emit(forw_packet);
 
 	/* we have to have at least one packet in the queue to determine the

base-commit: 165768bb70265b5c38cf0b73fafd75be235f8b14
-- 
2.43.0