[PATCH v3 14/30] inet: frags: do not clone skb in ip_expire()

[Stephen Hemminger <stephen@networkplumber.org>]

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)
---
 net/ipv4/ip_fragment.c | 16 ++++++----------
 1 file changed, 6 insertions(+), 10 deletions(-)

diff --git a/net/ipv4/ip_fragment.c b/net/ipv4/ip_fragment.c
index dc3ed0ac4c58..88fa8ffc5558 100644
--- a/net/ipv4/ip_fragment.c
+++ b/net/ipv4/ip_fragment.c
@@ -143,8 +143,8 @@ static bool frag_expire_skip_icmp(u32 user)
 static void ip_expire(struct timer_list *t)
 {
 	struct inet_frag_queue *frag = from_timer(frag, t, timer);
-	struct sk_buff *clone, *head;
 	const struct iphdr *iph;
+	struct sk_buff *head;
 	struct net *net;
 	struct ipq *qp;
 	int err;
@@ -187,16 +187,12 @@ static void ip_expire(struct timer_list *t)
 	    (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:
-- 
2.18.0