Date:	Fri, 12 Sep 2014 14:22:36 +0200
From:	Nikolay Aleksandrov <nikolay@...hat.com>
To:	netdev@...r.kernel.org
Cc:	Nikolay Aleksandrov <nikolay@...hat.com>,
	Jiri Pirko <jiri@...nulli.us>,
	Andy Gospodarek <andy@...yhouse.net>,
	Jay Vosburgh <j.vosburgh@...il.com>,
	Veaceslav Falico <vfalico@...il.com>
Subject: [PATCH net] bonding: fix div by zero while enslaving and transmitting

The problem is that the slave is first linked and slave_cnt is
incremented afterwards leading to a div by zero in the modes that use it
as a modulus. What happens is that in bond_start_xmit() bond_has_slaves()
is used to evaluate further transmission and it becomes true after the
slave is linked in, but when slave_cnt is used in the xmit path it is
still 0. The problem was introduced in commit:
5378c2e6ea236d ("bonding: move bond-specific init after enslave happens")
when the slave_cnt increment was moved after the linking.

Call trace (took it out of net-next kernel, but it's the same with net):
[46934.330038] divide error: 0000 [#1] SMP
[46934.330041] Modules linked in: bonding(O) 9p fscache
snd_hda_codec_generic crct10dif_pclmul
[46934.330041] bond0: Enslaving eth1 as an active interface with an up
link
[46934.330051]  ppdev joydev crc32_pclmul crc32c_intel 9pnet_virtio
ghash_clmulni_intel snd_hda_intel 9pnet snd_hda_controller parport_pc
serio_raw pcspkr snd_hda_codec parport virtio_balloon virtio_console
snd_hwdep snd_pcm pvpanic i2c_piix4 snd_timer i2ccore snd soundcore
virtio_blk virtio_net virtio_pci virtio_ring virtio ata_generic
pata_acpi floppy [last unloaded: bonding]
[46934.330053] CPU: 1 PID: 3382 Comm: ping Tainted: G           O
3.17.0-rc4+ #27
[46934.330053] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
[46934.330054] task: ffff88005aebf2c0 ti: ffff88005b728000 task.ti:
ffff88005b728000
[46934.330059] RIP: 0010:[<ffffffffa0198c33>]  [<ffffffffa0198c33>]
bond_start_xmit+0x1c3/0x450 [bonding]
[46934.330060] RSP: 0018:ffff88005b72b7f8  EFLAGS: 00010246
[46934.330060] RAX: 0000000000000679 RBX: ffff88004b077000 RCX:
000000000000002a
[46934.330061] RDX: 0000000000000000 RSI: ffff88004b3f0500 RDI:
ffff88004b077940
[46934.330061] RBP: ffff88005b72b830 R08: 00000000000000c0 R09:
ffff88004a83e000
[46934.330062] R10: 000000000000ffff R11: ffff88004b1f12c0 R12:
ffff88004b3f0500
[46934.330062] R13: ffff88004b3f0500 R14: 000000000000002a R15:
ffff88004b077940
[46934.330063] FS:  00007fbd91a4c740(0000) GS:ffff88005f080000(0000)
knlGS:0000000000000000
[46934.330064] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[46934.330064] CR2: 00007f803a8bb000 CR3: 000000004b2c9000 CR4:
00000000000406e0
[46934.330069] Stack:
[46934.330071]  ffffffff811e6169 00000000e772fa05 ffff88004b077000
ffff88004b3f0500
[46934.330072]  ffffffff81d17d18 000000000000002a 0000000000000000
ffff88005b72b8a0
[46934.330073]  ffffffff81620108 ffffffff8161fe0e ffff88005b72b8c4
ffff88005b302000
[46934.330073] Call Trace:
[46934.330077]  [<ffffffff811e6169>] ?
__kmalloc_node_track_caller+0x119/0x300
[46934.330084]  [<ffffffff81620108>] dev_hard_start_xmit+0x188/0x410
[46934.330086]  [<ffffffff8161fe0e>] ? harmonize_features+0x2e/0x90
[46934.330088]  [<ffffffff81620b06>] __dev_queue_xmit+0x456/0x590
[46934.330089]  [<ffffffff81620c50>] dev_queue_xmit+0x10/0x20
[46934.330090]  [<ffffffff8168f022>] arp_xmit+0x22/0x60
[46934.330091]  [<ffffffff8168f090>] arp_send.part.16+0x30/0x40
[46934.330092]  [<ffffffff8168f1e5>] arp_solicit+0x115/0x2b0
[46934.330094]  [<ffffffff8160b5d7>] ? copy_skb_header+0x17/0xa0
[46934.330096]  [<ffffffff8162875a>] neigh_probe+0x4a/0x70
[46934.330097]  [<ffffffff8162979c>] __neigh_event_send+0xac/0x230
[46934.330098]  [<ffffffff8162a00b>] neigh_resolve_output+0x13b/0x220
[46934.330100]  [<ffffffff8165f120>] ? ip_forward_options+0x1c0/0x1c0
[46934.330101]  [<ffffffff81660478>] ip_finish_output+0x1f8/0x860
[46934.330102]  [<ffffffff81661f08>] ip_output+0x58/0x90
[46934.330103]  [<ffffffff81661602>] ? __ip_local_out+0xa2/0xb0
[46934.330104]  [<ffffffff81661640>] ip_local_out_sk+0x30/0x40
[46934.330105]  [<ffffffff81662a66>] ip_send_skb+0x16/0x50
[46934.330106]  [<ffffffff81662ad3>] ip_push_pending_frames+0x33/0x40
[46934.330107]  [<ffffffff8168854c>] raw_sendmsg+0x88c/0xa30
[46934.330110]  [<ffffffff81612b31>] ? skb_recv_datagram+0x41/0x60
[46934.330111]  [<ffffffff816875a9>] ? raw_recvmsg+0xa9/0x1f0
[46934.330113]  [<ffffffff816978d4>] inet_sendmsg+0x74/0xc0
[46934.330114]  [<ffffffff81697a9b>] ? inet_recvmsg+0x8b/0xb0
[46934.330115] bond0: Adding slave eth2
[46934.330116]  [<ffffffff8160357c>] sock_sendmsg+0x9c/0xe0
[46934.330118]  [<ffffffff81603248>] ?
move_addr_to_kernel.part.20+0x28/0x80
[46934.330121]  [<ffffffff811b4477>] ? might_fault+0x47/0x50
[46934.330122]  [<ffffffff816039b9>] ___sys_sendmsg+0x3a9/0x3c0
[46934.330125]  [<ffffffff8144a14a>] ? n_tty_write+0x3aa/0x530
[46934.330127]  [<ffffffff810d1ae4>] ? __wake_up+0x44/0x50
[46934.330129]  [<ffffffff81242b38>] ? fsnotify+0x238/0x310
[46934.330130]  [<ffffffff816048a1>] __sys_sendmsg+0x51/0x90
[46934.330131]  [<ffffffff816048f2>] SyS_sendmsg+0x12/0x20
[46934.330134]  [<ffffffff81738b29>] system_call_fastpath+0x16/0x1b
[46934.330144] Code: 48 8b 10 4c 89 ee 4c 89 ff e8 aa bc ff ff 31 c0 e9
1a ff ff ff 0f 1f 00 4c 89 ee 4c 89 ff e8 65 fb ff ff 31 d2 4c 89 ee 4c
89 ff <f7> b3 64 09 00 00 e8 02 bd ff ff 31 c0 e9 f2 fe ff ff 0f 1f 00
[46934.330146] RIP  [<ffffffffa0198c33>] bond_start_xmit+0x1c3/0x450
[bonding]
[46934.330146]  RSP <ffff88005b72b7f8>

CC: Jiri Pirko <jiri@...nulli.us>
CC: Andy Gospodarek <andy@...yhouse.net>
CC: Jay Vosburgh <j.vosburgh@...il.com>
CC: Veaceslav Falico <vfalico@...il.com>
Fixes: 5378c2e6ea236d ("bonding: move bond-specific init after enslave happens")
Signed-off-by: Nikolay Aleksandrov <nikolay@...hat.com>
---
 drivers/net/bonding/bond_main.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
index 57912ee231cb..10ad434ea184 100644
--- a/drivers/net/bonding/bond_main.c
+++ b/drivers/net/bonding/bond_main.c
@@ -1552,6 +1552,10 @@ int bond_enslave(struct net_device *bond_dev, struct net_device *slave_dev)
 		goto err_detach;
 	}
 
+	/* Increment slave_cnt before linking in the slave so we won't end up in
+	 * bond_start_xmit with bond_has_slaves() true and slave_cnt == 0.
+	 */
+	bond->slave_cnt++;
 	res = bond_master_upper_dev_link(bond_dev, slave_dev, new_slave);
 	if (res) {
 		netdev_dbg(bond_dev, "Error %d calling bond_master_upper_dev_link\n", res);
@@ -1564,7 +1568,6 @@ int bond_enslave(struct net_device *bond_dev, struct net_device *slave_dev)
 		goto err_upper_unlink;
 	}
 
-	bond->slave_cnt++;
 	bond_compute_features(bond);
 	bond_set_carrier(bond);
 
@@ -1590,6 +1593,7 @@ err_upper_unlink:
 
 err_unregister:
 	netdev_rx_handler_unregister(slave_dev);
+	bond->slave_cnt--;
 
 err_detach:
 	if (!bond_uses_primary(bond))
-- 
1.9.3

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