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    Subject[PATCH 4.14 58/62] tcp: fix "old stuff" D-SACK causing SACK to be treated as D-SACK
    Date
    From: Pengcheng Yang <yangpc@wangsu.com>

    [ Upstream commit c9655008e7845bcfdaac10a1ed8554ec167aea88 ]

    When we receive a D-SACK, where the sequence number satisfies:
    undo_marker <= start_seq < end_seq <= prior_snd_una
    we consider this is a valid D-SACK and tcp_is_sackblock_valid()
    returns true, then this D-SACK is discarded as "old stuff",
    but the variable first_sack_index is not marked as negative
    in tcp_sacktag_write_queue().

    If this D-SACK also carries a SACK that needs to be processed
    (for example, the previous SACK segment was lost), this SACK
    will be treated as a D-SACK in the following processing of
    tcp_sacktag_write_queue(), which will eventually lead to
    incorrect updates of undo_retrans and reordering.

    Fixes: fd6dad616d4f ("[TCP]: Earlier SACK block verification & simplify access to them")
    Signed-off-by: Pengcheng Yang <yangpc@wangsu.com>
    Signed-off-by: Eric Dumazet <edumazet@google.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    net/ipv4/tcp_input.c | 5 ++++-
    1 file changed, 4 insertions(+), 1 deletion(-)

    --- a/net/ipv4/tcp_input.c
    +++ b/net/ipv4/tcp_input.c
    @@ -1750,8 +1750,11 @@ tcp_sacktag_write_queue(struct sock *sk,
    }

    /* Ignore very old stuff early */
    - if (!after(sp[used_sacks].end_seq, prior_snd_una))
    + if (!after(sp[used_sacks].end_seq, prior_snd_una)) {
    + if (i == 0)
    + first_sack_index = -1;
    continue;
    + }

    used_sacks++;
    }

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