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

Stephen Hemminger
2018-09-13T20:09:22Z

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