ip_gre: fix possible use after free

Once skb_realloc_headroom() is called, tiph might point to freed memory.

Cache tiph->ttl value before the reallocation, to avoid unexpected
behavior.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 5 insertions, 1 deletions

diff --git a/net/ipv4/ip_gre.c b/net/ipv4/ip_gre.c
index 8fcf0ed76548..58cb627560dd 100644
--- a/net/ipv4/ip_gre.c
+++ b/net/ipv4/ip_gre.c
@@ -750,6 +750,7 @@ static netdev_tx_t ipgre_tunnel_xmit(struct sk_buff *skb, struct net_device *dev
         int    gre_hlen;
         __be32 dst;
         int    mtu;
+        u8     ttl;
 
         if (skb->ip_summed == CHECKSUM_PARTIAL &&
             skb_checksum_help(skb))
@@ -815,6 +816,7 @@ static netdev_tx_t ipgre_tunnel_xmit(struct sk_buff *skb, struct net_device *dev
                         goto tx_error;
         }
 
+        ttl = tiph->ttl;
         tos = tiph->tos;
         if (tos == 1) {
                 tos = 0;
@@ -907,6 +909,7 @@ static netdev_tx_t ipgre_tunnel_xmit(struct sk_buff *skb, struct net_device *dev
                 dev_kfree_skb(skb);
                 skb = new_skb;
                 old_iph = ip_hdr(skb);
+                /* Warning : tiph value might point to freed memory */
         }
 
         skb_reset_transport_header(skb);
@@ -930,8 +933,9 @@ static netdev_tx_t ipgre_tunnel_xmit(struct sk_buff *skb, struct net_device *dev
         iph->tos                =       ipgre_ecn_encapsulate(tos, old_iph, skb);
         iph->daddr              =       fl4.daddr;
         iph->saddr              =       fl4.saddr;
+        iph->ttl                =       ttl;
 
-        if ((iph->ttl = tiph->ttl) == 0) {
+        if (ttl == 0) {
                 if (skb->protocol == htons(ETH_P_IP))
                         iph->ttl = old_iph->ttl;
 #if IS_ENABLED(CONFIG_IPV6)