Commit a16405ad authored by Eric Dumazet's avatar Eric Dumazet Committed by Greg Kroah-Hartman

ipv4: tcp: send zero IPID for RST and ACK sent in SYN-RECV and TIME-WAIT state

[ Upstream commit 431280ee ]

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: default avatarEric Dumazet <>
Reported-by: default avatarGeoff Alexander <>
Tested-by: default avatarGeoff Alexander <>
Signed-off-by: default avatarDavid S. Miller <>
Signed-off-by: default avatarGreg Kroah-Hartman <>
parent a08d7ea1
......@@ -2468,6 +2468,12 @@ static int __net_init tcp_sk_init(struct net *net)
if (res)
goto fail;
sock_set_flag(sk, SOCK_USE_WRITE_QUEUE);
/* Please enforce IP_DF and IPID==0 for RST and
* ACK sent in SYN-RECV and TIME-WAIT state.
inet_sk(sk)->pmtudisc = IP_PMTUDISC_DO;
*per_cpu_ptr(net->ipv4.tcp_sk, cpu) = sk;
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