bonding: avoid defaulting hard_header_len to ETH_HLEN on slave removal

On slave list updates, the bonding driver computes its hard_header_len
as the maximum of all enslaved devices's hard_header_len.
If the slave list is empty, e.g. on last enslaved device removal,
ETH_HLEN is used.

Since the bonding header_ops are set only when the first enslaved
device is attached, the above can lead to header_ops->create()
being called with the wrong skb headroom in place.

If bond0 is configured on top of ipoib devices, with the
following commands:

ifup bond0
for slave in $BOND_SLAVES_LIST; do
	ip link set dev $slave nomaster
done
ping -c 1 <ip on bond0 subnet>

we will obtain a skb_under_panic() with a similar call trace:
	skb_push+0x3d/0x40
	push_pseudo_header+0x17/0x30 [ib_ipoib]
	ipoib_hard_header+0x4e/0x80 [ib_ipoib]
	arp_create+0x12f/0x220
	arp_send_dst.part.19+0x28/0x50
	arp_solicit+0x115/0x290
	neigh_probe+0x4d/0x70
	__neigh_event_send+0xa7/0x230
	neigh_resolve_output+0x12e/0x1c0
	ip_finish_output2+0x14b/0x390
	ip_finish_output+0x136/0x1e0
	ip_output+0x76/0xe0
	ip_local_out+0x35/0x40
	ip_send_skb+0x19/0x40
	ip_push_pending_frames+0x33/0x40
	raw_sendmsg+0x7d3/0xb50
	inet_sendmsg+0x31/0xb0
	sock_sendmsg+0x38/0x50
	SYSC_sendto+0x102/0x190
	SyS_sendto+0xe/0x10
	do_syscall_64+0x67/0x180
	entry_SYSCALL64_slow_path+0x25/0x25

This change addresses the issue avoiding updating the bonding device
hard_header_len when the slaves list become empty, forbidding to
shrink it below the value used by header_ops->create().

The bug is there since commit 54ef31371407 ("[PATCH] bonding: Handle large
hard_header_len") but the panic can be triggered only since
commit fc791b633515 ("IB/ipoib: move back IB LL address into the hard
header").

Reported-by: Norbert P <noe@physik.uzh.ch>
Fixes: 54ef31371407 ("[PATCH] bonding: Handle large hard_header_len")
Fixes: fc791b633515 ("IB/ipoib: move back IB LL address into the hard header")
Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Jay Vosburgh <jay.vosburgh@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/bonding/bond_main.c

@@ -1104,11 +1104,11 @@ static void bond_compute_features(struct bonding *bond)
 		gso_max_size = min(gso_max_size, slave->dev->gso_max_size);
 		gso_max_segs = min(gso_max_segs, slave->dev->gso_max_segs);
 	}
+	bond_dev->hard_header_len = max_hard_header_len;
 
 done:
 	bond_dev->vlan_features = vlan_features;
 	bond_dev->hw_enc_features = enc_features | NETIF_F_GSO_ENCAP_ALL;
-	bond_dev->hard_header_len = max_hard_header_len;
 	bond_dev->gso_max_segs = gso_max_segs;
 	netif_set_gso_max_size(bond_dev, gso_max_size);