[PATCH 4.9 55/71] inet: frags: do not clone skb in ip_expire()

4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Eric Dumazet <edumazet@google.com>

An skb_clone() was added in commit ec4fbd64751d ("inet: frag: release
spinlock before calling icmp_send()")

While fixing the bug at that time, it also added a very high cost
for DDOS frags, as the ICMP rate limit is applied after this
expensive operation (skb_clone() + consume_skb(), implying memory
allocations, copy, and freeing)

We can use skb_get(head) here, all we want is to make sure skb wont
be freed by another cpu.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 1eec5d5670084ee644597bd26c25e22c69b9f748)
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 net/ipv4/ip_fragment.c |   16 ++++++----------
 1 file changed, 6 insertions(+), 10 deletions(-)

--- a/net/ipv4/ip_fragment.c
+++ b/net/ipv4/ip_fragment.c

@@ -141,8 +141,8 @@ static bool frag_expire_skip_icmp(u32 us
  */
 static void ip_expire(unsigned long arg)
 {
-	struct sk_buff *clone, *head;
 	const struct iphdr *iph;
+	struct sk_buff *head;
 	struct net *net;
 	struct ipq *qp;
 	int err;

@@ -185,16 +185,12 @@ static void ip_expire(unsigned long arg)
 	    (skb_rtable(head)->rt_type != RTN_LOCAL))
 		goto out;
 
-	clone = skb_clone(head, GFP_ATOMIC);
+	skb_get(head);
+	spin_unlock(&qp->q.lock);
+	icmp_send(head, ICMP_TIME_EXCEEDED, ICMP_EXC_FRAGTIME, 0);
+	kfree_skb(head);
+	goto out_rcu_unlock;
 
-	/* Send an ICMP "Fragment Reassembly Timeout" message. */
-	if (clone) {
-		spin_unlock(&qp->q.lock);
-		icmp_send(clone, ICMP_TIME_EXCEEDED,
-			  ICMP_EXC_FRAGTIME, 0);
-		consume_skb(clone);
-		goto out_rcu_unlock;
-	}
 out:
 	spin_unlock(&qp->q.lock);
 out_rcu_unlock: