[PATCH net-next-2.6] net/ipv4: push IP options to CB in ip_fragment

[Bandan Das <bandan.das@stratus.com>]

The bridge layer can overwrite the IPCB using the
BR_INPUT_SKB_CB macro. In br_nf_dev_queue_xmit, 
if we recieve a packet greater in size than the bridge 
device MTU, we call ip_fragment which in turn will lead to 
icmp_send calling ip_options_echo if the DF flag is set. 
ip_options_echo will then incorrectly try to parse the IPCB as 
IP options resulting in a buffer overflow. 
This change refills the CB area back with IP 
options before ip_fragment calls icmp_send. If we fail parsing, 
we zero out the IPCB area to guarantee that the stack does 
not get corrupted.

Test setup that can exhibit this behavior:
<Remote Machine>--<Host Machine>--<Bridge>--<Tun/Tap>--<KVM Guest>
  1500 mtu          576 mtu        576 mtu   576 mtu    1500 mtu

Example of crash :
Kernel panic - not syncing: stack-protector: Kernel stack is corrupted in: ffffffff8146dff3
Pid: 3173, comm: qemu-kvm Not tainted 2.6.32-63.el6.x86_64 #1
Call Trace:
Message from <IRQ>  syslogd@kvm at  [<ffffffff814c8285>] panic+0x78/0x137
Aug 30 14:09:50  [<ffffffff8146dff3>] ? icmp_send+0x743/0x780

Signed-off-by: Bandan Das <bandan.das@stratus.com>
---
 net/ipv4/ip_output.c |   11 +++++++++++
 1 files changed, 11 insertions(+), 0 deletions(-)

diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index e427620..5fef4a9 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -445,6 +445,7 @@ int ip_fragment(struct sk_buff *skb, int (*output)(struct sk_buff *))
 	struct iphdr *iph;
 	int ptr;
 	struct net_device *dev;
+	struct ip_options *opt;
 	struct sk_buff *skb2;
 	unsigned int mtu, hlen, left, len, ll_rs;
 	int offset;
@@ -462,6 +463,16 @@ int ip_fragment(struct sk_buff *skb, int (*output)(struct sk_buff *))
 
 	if (unlikely((iph->frag_off & htons(IP_DF)) && !skb->local_df)) {
 		IP_INC_STATS(dev_net(dev), IPSTATS_MIB_FRAGFAILS);
+
+		/* Refill CB with IP options */
+		opt = &(IPCB(skb)->opt);
+		opt->optlen = iph->ihl*4 - sizeof(struct iphdr);
+		if (ip_options_compile(dev_net(dev), opt, skb)) {
+			IP_INC_STATS(dev_net(dev), IPSTATS_MIB_INHDRERRORS);
+			/* If we fail, zero out IPCB and continue */
+			memset(IPCB(skb), 0, sizeof(struct inet_skb_parm));
+		}
+
 		icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED,
 			  htonl(ip_skb_dst_mtu(skb)));
 		kfree_skb(skb);
-- 
1.6.6.1