tcp: reduce skb overhead in selected places
authorEric Dumazet <edumazet@google.com>
Tue, 24 Jan 2017 22:57:36 +0000 (14:57 -0800)
committerDavid S. Miller <davem@davemloft.net>
Wed, 25 Jan 2017 18:13:31 +0000 (13:13 -0500)
commit60b1af3300724d211bb0b420c1fbe6bf5b87b013
treecc0c1db3870b6b6360ad31fb8fe1f8cdbba6ba6f
parent716dcaebed99120cf5d3d9faa9720d6bceddae3d
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>
net/ipv4/tcp_input.c
net/ipv4/tcp_ipv4.c