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Subject[ 48/66] net: loopback: fix a dst refcounting issue
3.2-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Eric Dumazet <edumazet@google.com>

[ Upstream commit 794ed393b707f01858f5ebe2ae5eabaf89d00022 ]

Ben Greear reported crashes in ip_rcv_finish() on a stress
test involving many macvlans.

We tracked the bug to a dst use after free. ip_rcv_finish()
was calling dst->input() and got garbage for dst->input value.

It appears the bug is in loopback driver, lacking
a skb_dst_force() before calling netif_rx().

As a result, a non refcounted dst, normally protected by a
RCU read_lock section, was escaping this section and could
be freed before the packet being processed.

[<ffffffff813a3c4d>] loopback_xmit+0x64/0x83
[<ffffffff81477364>] dev_hard_start_xmit+0x26c/0x35e
[<ffffffff8147771a>] dev_queue_xmit+0x2c4/0x37c
[<ffffffff81477456>] ? dev_hard_start_xmit+0x35e/0x35e
[<ffffffff8148cfa6>] ? eth_header+0x28/0xb6
[<ffffffff81480f09>] neigh_resolve_output+0x176/0x1a7
[<ffffffff814ad835>] ip_finish_output2+0x297/0x30d
[<ffffffff814ad6d5>] ? ip_finish_output2+0x137/0x30d
[<ffffffff814ad90e>] ip_finish_output+0x63/0x68
[<ffffffff814ae412>] ip_output+0x61/0x67
[<ffffffff814ab904>] dst_output+0x17/0x1b
[<ffffffff814adb6d>] ip_local_out+0x1e/0x23
[<ffffffff814ae1c4>] ip_queue_xmit+0x315/0x353
[<ffffffff814adeaf>] ? ip_send_unicast_reply+0x2cc/0x2cc
[<ffffffff814c018f>] tcp_transmit_skb+0x7ca/0x80b
[<ffffffff814c3571>] tcp_connect+0x53c/0x587
[<ffffffff810c2f0c>] ? getnstimeofday+0x44/0x7d
[<ffffffff810c2f56>] ? ktime_get_real+0x11/0x3e
[<ffffffff814c6f9b>] tcp_v4_connect+0x3c2/0x431
[<ffffffff814d6913>] __inet_stream_connect+0x84/0x287
[<ffffffff814d6b38>] ? inet_stream_connect+0x22/0x49
[<ffffffff8108d695>] ? _local_bh_enable_ip+0x84/0x9f
[<ffffffff8108d6c8>] ? local_bh_enable+0xd/0x11
[<ffffffff8146763c>] ? lock_sock_nested+0x6e/0x79
[<ffffffff814d6b38>] ? inet_stream_connect+0x22/0x49
[<ffffffff814d6b49>] inet_stream_connect+0x33/0x49
[<ffffffff814632c6>] sys_connect+0x75/0x98

This bug was introduced in linux-2.6.35, in commit
7fee226ad2397b (net: add a noref bit on skb dst)

skb_dst_force() is enforced in dev_queue_xmit() for devices having a
qdisc.

Reported-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Tested-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
drivers/net/loopback.c | 5 +++++
1 file changed, 5 insertions(+)

diff --git a/drivers/net/loopback.c b/drivers/net/loopback.c
index 4ce9e5f..d0893e4 100644
--- a/drivers/net/loopback.c
+++ b/drivers/net/loopback.c
@@ -78,6 +78,11 @@ static netdev_tx_t loopback_xmit(struct sk_buff *skb,

skb_orphan(skb);

+ /* Before queueing this packet to netif_rx(),
+ * make sure dst is refcounted.
+ */
+ skb_dst_force(skb);
+
skb->protocol = eth_type_trans(skb, dev);

/* it's OK to use per_cpu_ptr() because BHs are off */



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