Subject: batman-adv: Fix hang up with small MTU hard-interface
From: Shigeru Yoshida <[email-masked]>
Date: Sat, 20 Aug 2022 12:25:16 +0900

The system hangs up when batman-adv soft-interface is created on
hard-interface with small MTU.  For example, the following commands
create batman-adv soft-interface on dummy interface with zero MTU:

  # ip link add name dummy0 type dummy
  # ip link set mtu 0 dev dummy0
  # ip link set up dev dummy0
  # ip link add name bat0 type batadv
  # ip link set dev dummy0 master bat0

These commands cause the system hang up with the following messages:

  [   90.578925][ T6689] batman_adv: bat0: Adding interface: dummy0
  [   90.580884][ T6689] batman_adv: bat0: The MTU of interface dummy0 is too small (0) to handle the transport of batman-adv packets. Packets going over this interface will be fragmented on layer2 which could impact the performance. Setting the MTU to 1560 would solve the problem.
  [   90.586264][ T6689] batman_adv: bat0: Interface activated: dummy0
  [   90.590061][ T6689] batman_adv: bat0: Forced to purge local tt entries to fit new maximum fragment MTU (-320)
  [   90.595517][ T6689] batman_adv: bat0: Forced to purge local tt entries to fit new maximum fragment MTU (-320)
  [   90.598499][ T6689] batman_adv: bat0: Forced to purge local tt entries to fit new maximum fragment MTU (-320)

This patch fixes this issue by returning error when enabling
hard-interface with small MTU size.

Fixes: 29a2a3dc52f4 ("hard interface rewrite")
Signed-off-by: Shigeru Yoshida <[email-masked]>
Signed-off-by: Sven Eckelmann <[email-masked]>
Origin: upstream, https://git.open-mesh.org/batman-adv.git/commit/a124b52ebab91ac416cf7ba05e671cbb860f30e8

--- a/net/batman-adv/hard-interface.c
+++ b/net/batman-adv/hard-interface.c
@@ -697,6 +697,9 @@ int batadv_hardif_enable_interface(struc
 	int max_header_len = batadv_max_header_len();
 	int ret;
 
+	if (hard_iface->net_dev->mtu < ETH_MIN_MTU + max_header_len)
+		return -EINVAL;
+
 	if (hard_iface->if_status != BATADV_IF_NOT_IN_USE)
 		goto out;