The automatic recalculation of the maximum allowed MTU is usually triggered by code sections which are already rtnl lock protected by callers outside of batman-adv. But when the fragmentation setting is changed via batman-adv's own batadv genl family, then the rtnl lock is not yet taken.
But dev_set_mtu requires that the caller holds the rtnl lock because it uses netdevice notifiers. And this code will then fail the check for this lock:
RTNL: assertion failed at net/core/dev.c (1953)
Cc: stable@vger.kernel.org Reported-by: syzbot+f8812454d9b3ac00d282@syzkaller.appspotmail.com Fixes: c6a953cce8d0 ("batman-adv: Trigger events for auto adjusted MTU") Signed-off-by: Sven Eckelmann sven@narfation.org --- This problem was just identified by syzbot [1]. I hope it is ok to directly send this patch to netdev instead of creating a single-patch PR from the batadv/net branch. If you still prefer a PR then we can also prepare it.
[1] https://lore.kernel.org/r/0000000000009bbb4b0603717cde@google.com --- net/batman-adv/netlink.c | 3 +++ 1 file changed, 3 insertions(+)
diff --git a/net/batman-adv/netlink.c b/net/batman-adv/netlink.c index ad5714f737be..6efbc9275aec 100644 --- a/net/batman-adv/netlink.c +++ b/net/batman-adv/netlink.c @@ -495,7 +495,10 @@ static int batadv_netlink_set_mesh(struct sk_buff *skb, struct genl_info *info) attr = info->attrs[BATADV_ATTR_FRAGMENTATION_ENABLED];
atomic_set(&bat_priv->fragmentation, !!nla_get_u8(attr)); + + rtnl_lock(); batadv_update_min_mtu(bat_priv->soft_iface); + rtnl_unlock(); }
if (info->attrs[BATADV_ATTR_GW_BANDWIDTH_DOWN]) {
--- base-commit: 421d467dc2d483175bad4fb76a31b9e5a3d744cf change-id: 20230821-batadv-missing-mtu-rtnl-lock-bc4cee67731d
Best regards,