Re: [PATCH net-next] net: ip6_gre: get ipv6hdr after skb_cow_head()

[Gregory Rose <gvrose8192@gmail.com>]

On 7/12/2018 10:40 PM, Prashant Bhole wrote:
> 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 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 <bhole_prashant_q7@lab.ntt.co.jp>

Good catch.

LGTM

Reviewed-by: Greg Rose <gvrose8192@gmail.com>

> ---
>   net/ipv6/ip6_gre.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/net/ipv6/ip6_gre.c b/net/ipv6/ip6_gre.c
> index 367177786e34..fc7dd3a04360 100644
> --- a/net/ipv6/ip6_gre.c
> +++ b/net/ipv6/ip6_gre.c
> @@ -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;
> @@ -1012,6 +1011,8 @@ 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));