ipv4: tcp: send zero IPID for RST and ACK sent in SYN-RECV and TIME-WAIT state
authorEric Dumazet <edumazet@google.com>
Wed, 22 Aug 2018 20:30:45 +0000 (13:30 -0700)
committerDavid S. Miller <davem@davemloft.net>
Thu, 23 Aug 2018 04:42:58 +0000 (21:42 -0700)
commit431280eebed9f5079553daf003011097763e71fd
tree13f7bee9436e2ab1ca9b6d2b14b8b7fe66da4f28
parente500c6d349f7f36120886841c6f057ce248b48b2
ipv4: tcp: send zero IPID for RST and ACK sent in SYN-RECV and TIME-WAIT state

tcp uses per-cpu (and per namespace) sockets (net->ipv4.tcp_sk) internally
to send some control packets.

1) RST packets, through tcp_v4_send_reset()
2) ACK packets in SYN-RECV and TIME-WAIT state, through tcp_v4_send_ack()

These packets assert IP_DF, and also use the hashed IP ident generator
to provide an IPv4 ID number.

Geoff Alexander reported this could be used to build off-path attacks.

These packets should not be fragmented, since their size is smaller than
IPV4_MIN_MTU. Only some tunneled paths could eventually have to fragment,
regardless of inner IPID.

We really can use zero IPID, to address the flaw, and as a bonus,
avoid a couple of atomic operations in ip_idents_reserve()

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Geoff Alexander <alexandg@cs.unm.edu>
Tested-by: Geoff Alexander <alexandg@cs.unm.edu>
Signed-off-by: David S. Miller <davem@davemloft.net>
net/ipv4/tcp_ipv4.c