Commit 60b1af33 authored Jan 24, 2017 by Eric Dumazet Committed by David S. Miller Jan 25, 2017

tcp: reduce skb overhead in selected places



tcp_add_backlog() can use skb_condense() helper to get better
gains and less SKB_TRUESIZE() magic. This only happens when socket
backlog has to be used.

Some attacks involve specially crafted out of order tiny TCP packets,
clogging the ofo queue of (many) sockets.
Then later, expensive collapse happens, trying to copy all these skbs
into single ones.
This unfortunately does not work if each skb has no neighbor in TCP
sequence order.

By using skb_condense() if the skb could not be coalesced to a prior
one, we defeat these kind of threats, potentially saving 4K per skb
(or more, since this is one page fragment).

A typical NAPI driver allocates gro packets with GRO_MAX_HEAD bytes
in skb->head, meaning the copy done by skb_condense() is limited to
about 200 bytes.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent 716dcaeb

net/ipv4/tcp_input.c

+1 −0

Original line number	Diff line number	Diff line
		@@ -4507,6 +4507,7 @@ add_sack:
		end:
		if (skb) {
		tcp_grow_window(sk, skb);
		skb_condense(skb);
		skb_set_owner_r(skb, sk);
		}
		}

net/ipv4/tcp_ipv4.c

+1 −2

Original line number	Diff line number	Diff line
		@@ -1556,8 +1556,7 @@ bool tcp_add_backlog(struct sock sk, struct sk_buff skb)
		* It has been noticed pure SACK packets were sometimes dropped
		* (if cooked by drivers without copybreak feature).
		*/
		if (!skb->data_len)
		skb->truesize = SKB_TRUESIZE(skb_end_offset(skb));
		skb_condense(skb);

		if (unlikely(sk_add_backlog(sk, skb, limit))) {
		bh_unlock_sock(sk);

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