ipsec: be careful of non existing mac headers

Niccolo Belli reported ipsec crashes in case we handle a frame without
mac header (atm in his case)

Before copying mac header, better make sure it is present.

Bugzilla reference:  https://bugzilla.kernel.org/show_bug.cgi?id=42809

Reported-by: Niccolò Belli <darkbasic@linuxsystems.it>
Tested-by: Niccolò Belli <darkbasic@linuxsystems.it>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 2 insertions, 4 deletions

diff --git a/net/ipv4/xfrm4_mode_tunnel.c b/net/ipv4/xfrm4_mode_tunnel.c
index 534972e114a..ed4bf11ef9f 100644
--- a/net/ipv4/xfrm4_mode_tunnel.c
+++ b/net/ipv4/xfrm4_mode_tunnel.c
@@ -66,7 +66,6 @@ static int xfrm4_mode_tunnel_output(struct xfrm_state *x, struct sk_buff *skb)
 
 static int xfrm4_mode_tunnel_input(struct xfrm_state *x, struct sk_buff *skb)
 {
-        const unsigned char *old_mac;
         int err = -EINVAL;
 
         if (XFRM_MODE_SKB_CB(skb)->protocol != IPPROTO_IPIP)
@@ -84,10 +83,9 @@ static int xfrm4_mode_tunnel_input(struct xfrm_state *x, struct sk_buff *skb)
         if (!(x->props.flags & XFRM_STATE_NOECN))
                 ipip_ecn_decapsulate(skb);
 
-        old_mac = skb_mac_header(skb);
-        skb_set_mac_header(skb, -skb->mac_len);
-        memmove(skb_mac_header(skb), old_mac, skb->mac_len);
         skb_reset_network_header(skb);
+        skb_mac_header_rebuild(skb);
+
         err = 0;
 
 out: