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    Subject[ 63/80] tcp: refine SYN handling in tcp_validate_incoming
    Date
    3.4-stable review patch.  If anyone has any objections, please let me know.

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


    From: Eric Dumazet <edumazet@google.com>

    [ Upstream commit e371589917011efe6ff8c7dfb4e9e81934ac5855 ]

    Followup of commit 0c24604b68fc (tcp: implement RFC 5961 4.2)

    As reported by Vijay Subramanian, we should send a challenge ACK
    instead of a dup ack if a SYN flag is set on a packet received out of
    window.

    This permits the ratelimiting to work as intended, and to increase
    correct SNMP counters.

    Suggested-by: Vijay Subramanian <subramanian.vijay@gmail.com>
    Signed-off-by: Eric Dumazet <edumazet@google.com>
    Acked-by: Vijay Subramanian <subramanian.vijay@gmail.com>
    Cc: Kiran Kumar Kella <kkiran@broadcom.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    net/ipv4/tcp_input.c | 6 +++++-
    1 file changed, 5 insertions(+), 1 deletion(-)

    --- a/net/ipv4/tcp_input.c
    +++ b/net/ipv4/tcp_input.c
    @@ -5314,8 +5314,11 @@ static bool tcp_validate_incoming(struct
    * an acknowledgment should be sent in reply (unless the RST
    * bit is set, if so drop the segment and return)".
    */
    - if (!th->rst)
    + if (!th->rst) {
    + if (th->syn)
    + goto syn_challenge;
    tcp_send_dupack(sk, skb);
    + }
    goto discard;
    }

    @@ -5345,6 +5348,7 @@ static bool tcp_validate_incoming(struct
    * RFC 5691 4.2 : Send a challenge ack
    */
    if (th->syn) {
    +syn_challenge:
    if (syn_inerr)
    TCP_INC_STATS_BH(sock_net(sk), TCP_MIB_INERRS);
    NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPSYNCHALLENGE);



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