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    Subject[PATCH 4.14 002/115] ipv4: tcp: send zero IPID for RST and ACK sent in SYN-RECV and TIME-WAIT state
    Date
    4.14-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Eric Dumazet <edumazet@google.com>

    [ 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 <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>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    net/ipv4/tcp_ipv4.c | 6 ++++++
    1 file changed, 6 insertions(+)

    --- a/net/ipv4/tcp_ipv4.c
    +++ b/net/ipv4/tcp_ipv4.c
    @@ -2468,6 +2468,12 @@ 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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