Commit b7ed8794 authored Jul 13, 2018 by Prashant Bhole Committed by David S. Miller Jul 16, 2018

net: ip6_gre: get ipv6hdr after skb_cow_head()



A KASAN:use-after-free bug was found related to ip6-erspan
while running selftests/net/ip6_gre_headroom.sh

It happens because of following sequence:
- ipv6hdr pointer is obtained from skb
- skb_cow_head() is called, skb->head memory is reallocated
- old data is accessed using ipv6hdr pointer

skb_cow_head() call was added in e41c7c68 ("ip6erspan: make sure
enough headroom at xmit."), but looking at the history there was a
chance of similar bug because gre_handle_offloads() and pskb_trim()
can also reallocate skb->head memory. Fixes tag points to commit
which introduced possibility of this bug.

This patch moves ipv6hdr pointer assignment after skb_cow_head() call.

Fixes: 5a963eb6 ("ip6_gre: Add ERSPAN native tunnel support")
Signed-off-by: Prashant Bhole <bhole_prashant_q7@lab.ntt.co.jp>
Reviewed-by: Greg Rose <gvrose8192@gmail.com>
Acked-by: William Tu <u9012063@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent 6e8cfd6d

net/ipv6/ip6_gre.c

+2 −1

Original line number	Diff line number	Diff line
		@@ -927,7 +927,6 @@ static netdev_tx_t ip6gre_tunnel_xmit(struct sk_buff *skb,
		static netdev_tx_t ip6erspan_tunnel_xmit(struct sk_buff *skb,
		struct net_device *dev)
		{
		struct ipv6hdr *ipv6h = ipv6_hdr(skb);
		struct ip6_tnl *t = netdev_priv(dev);
		struct dst_entry *dst = skb_dst(skb);
		struct net_device_stats *stats;
		@@ -1010,6 +1009,8 @@ static netdev_tx_t ip6erspan_tunnel_xmit(struct sk_buff *skb,
		goto tx_err;
		}
		} else {
		struct ipv6hdr *ipv6h = ipv6_hdr(skb);

		switch (skb->protocol) {
		case htons(ETH_P_IP):
		memset(&(IPCB(skb)->opt), 0, sizeof(IPCB(skb)->opt));