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Subject[PATCH 4.18 002/197] ipv4: tcp: send zero IPID for RST and ACK sent in SYN-RECV and TIME-WAIT state
4.18-stable review patch.  If anyone has any objections, please let me know.


From: Eric Dumazet <>

[ Upstream commit 431280eebed9f5079553daf003011097763e71fd ]

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 <>
Reported-by: Geoff Alexander <>
Tested-by: Geoff Alexander <>
Signed-off-by: David S. Miller <>
Signed-off-by: Greg Kroah-Hartman <>
net/ipv4/tcp_ipv4.c | 6 ++++++
1 file changed, 6 insertions(+)

--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -2516,6 +2516,12 @@ static int __net_init tcp_sk_init(struct
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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