Subject: [PATCH stable 4.9 13/29] inet: frags: do not clone skb in ip_expire()

From: Florian Fainelli <[email-masked]>
Date: Tue, 9 Oct 2018 15:49:08 -0700

From: Eric Dumazet <[email-masked]>

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 <[email-masked]>
Signed-off-by: David S. Miller <[email-masked]>
(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 3dd19bebeb55..e235f62dab58 100644
--- a/net/ipv4/ip_fragment.c
+++ b/net/ipv4/ip_fragment.c
@@ -141,8 +141,8 @@ static bool frag_expire_skip_icmp(u32 user)
  */
 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:
-- 
2.17.1