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Subject[PATCH 4.17 51/63] net: ip6_gre: get ipv6hdr after skb_cow_head()
4.17-stable review patch.  If anyone has any objections, please let me know.


From: Prashant Bhole <>

[ Upstream commit b7ed879425be371905d856410d19e9a42a62bcf3 ]

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

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 e41c7c68ea77 ("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: 5a963eb61b7c ("ip6_gre: Add ERSPAN native tunnel support")
Signed-off-by: Prashant Bhole <>
Reviewed-by: Greg Rose <>
Acked-by: William Tu <>
Signed-off-by: David S. Miller <>
Signed-off-by: Greg Kroah-Hartman <>
net/ipv6/ip6_gre.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

--- a/net/ipv6/ip6_gre.c
+++ b/net/ipv6/ip6_gre.c
@@ -927,7 +927,6 @@ tx_err:
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;
@@ -998,6 +997,8 @@ static netdev_tx_t ip6erspan_tunnel_xmit
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));

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