Comment 10 for bug 1715271

4.12.10 contains this commit:
commit 38c36f9d1fca5b5e6d2d1be13ab82d1ce714e4b1
Author: Aaron Ma <email address hidden>
Date: Fri Aug 18 12:17:21 2017 -0700

    Input: trackpoint - add new trackpoint firmware ID

    commit ec667683c532c93fb41e100e5d61a518971060e2 upstream.

    Synaptics add new TP firmware ID: 0x2 and 0x3, for now both lower 2 bits
    are indicated as TP. Change the constant to bitwise values.

    This makes trackpoint to be recognized on Lenovo Carbon X1 Gen5 instead
    of it being identified as "PS/2 Generic Mouse".

    Signed-off-by: Aaron Ma <email address hidden>
    Signed-off-by: Dmitry Torokhov <email address hidden>
    Signed-off-by: Greg Kroah-Hartman <email address hidden>

This issue should be that stable-kernel lack this commit:
commit 293b915fd9bebf33cdc906516fb28d54649a25ac
Author: Oscar Campos <email address hidden>
Date: Tue Jul 18 17:20:36 2017 -0700

    Input: trackpoint - assume 3 buttons when buttons detection fails

    Trackpoint buttons detection fails on ThinkPad 570 and 470 series,
    this makes the middle button of the trackpoint to not being recogized.
    As I don't believe there is any trackpoint with less than 3 buttons this
    patch just assumes three buttons when the extended button information
    read fails.

    Signed-off-by: Oscar Campos <email address hidden>
    Acked-by: Peter Hutterer <email address hidden>
    Signed-off-by: Dmitry Torokhov <email address hidden>

Please try 4.13.0 kernel, not stable branch, this will make middle button work.