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


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