From b1cb8a71f1eaec4eb77051590f7f561f25b15e32 Mon Sep 17 00:00:00 2001
From: Shigeru Yoshida <syoshida@redhat.com>
Date: Sat, 20 Aug 2022 12:25:16 +0900
Subject: [PATCH] batman-adv: Fix hang up with small MTU hard-interface
Git-commit: b1cb8a71f1eaec4eb77051590f7f561f25b15e32
Patch-mainline: v6.0-rc7
References: git-fixes

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: c6c8fea29769 ("net: Add batman-adv meshing protocol")
Signed-off-by: Shigeru Yoshida <syoshida@redhat.com>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>
Acked-by: Takashi Iwai <tiwai@suse.de>

---
 net/batman-adv/hard-interface.c |    4 ++++
 1 file changed, 4 insertions(+)

--- a/net/batman-adv/hard-interface.c
+++ b/net/batman-adv/hard-interface.c
@@ -9,6 +9,7 @@
 
 #include <linux/atomic.h>
 #include <linux/byteorder/generic.h>
+#include <linux/errno.h>
 #include <linux/gfp.h>
 #include <linux/if.h>
 #include <linux/if_arp.h>
@@ -702,6 +703,9 @@ 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;