ipv4: fix a potential use after free in gre_offload.c

pskb_may_pull() may change skb->data and make greh pointer oboslete;
so need to reassign greh;
but since first calling pskb_may_pull already ensured that skb->data
has enough space for greh, so move the reference of greh before second
calling pskb_may_pull(), to avoid reassign greh.

Fixes: 7a7ffbabf9("ipv4: fix tunneled VM traffic over hw VXLAN/GRE GSO NIC")
Cc: Wei-Chun Chao <weichunc@plumgrid.com>
Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 3 insertions, 3 deletions

diff --git a/net/ipv4/gre_offload.c b/net/ipv4/gre_offload.c
index a77729503071..ccda09628de7 100644
--- a/net/ipv4/gre_offload.c
+++ b/net/ipv4/gre_offload.c
@@ -55,13 +55,13 @@ static struct sk_buff *gre_gso_segment(struct sk_buff *skb,
         if (csum)
                 skb->encap_hdr_csum = 1;
 
-        if (unlikely(!pskb_may_pull(skb, ghl)))
-                goto out;
-
         /* setup inner skb. */
         skb->protocol = greh->protocol;
         skb->encapsulation = 0;
 
+        if (unlikely(!pskb_may_pull(skb, ghl)))
+                goto out;
+
         __skb_pull(skb, ghl);
         skb_reset_mac_header(skb);
         skb_set_network_header(skb, skb_inner_network_offset(skb));