% Notes about DMA and the docking station (X60/T60)

    Use case:
    ---------
    Usually when people do full disk encryption, it's not really full disk,
    instead they still have a /boot in clear.

    So an evil maid attack can still be done, in two passes:
    1) Clone the hdd, Infect the initramfs or the kernel.
    2) Wait for the user to enter its password, recover the password,
    luksOpen the hdd image.

    I wanted a real full-disk encryption so I've put grub in flash and I
    have the following: The HDD has a LUKS rootfs(containing /boot) on an
    lvm partition, so no partition is in clear.

    So when the computer boots it executes coreboot, then grub as a payload.
    Grub then opens the LUKS partition and loads the kernel and initramfs
    from there.

    To prevent hardware level tempering(like reflashing), I used nail
    polish with a lot of gilder, that acts like a seal. Then a high
    resolution picture of it is taken, to be able to tell the difference.

    The problem:
    ------------
    But then comes the docking port issue: Some LPC pins are exported
    there, such as the CLKRUN and LDRQ#.

    LDRQ# is "Encoded DMA/Bus Master Request": "Only needed by
    peripherals that need DMA or bus mastering. Requires an
    individual signal per peripheral. Peripherals may not share
    an LDRQ# signal."

    So now DMA access is possible trough the dock connector.
    So I want to be able to turn that off.

    If I got it right, the X60 has 2 superio, one is in the dock, and the
    other one is in the laptop, so we have:
                                ________________
     _________________         |                |
    |                 |        | Dock connector:|
    |Dock: NSC pc87982|<--LPC--->D_LPC_DREQ0    |
    |_________________|        |_______^________|
                                       |
                                       |
                                       |
                                       |
                    ___________________|____
                   |                   v    |
                   | SuperIO:        DLDRQ# |
                   | NSC pc87382     LDRQ#  |
                   |___________________^____|
                                       |
                                       |
                                       |
                                       |
                    ___________________|___
                   |                   v   |
                   | Southbridge:    LDRQ0 |
                   | ICH7                  |
                   |_______________________|

    The code:
    ---------
    Now if I look at the existing code, there is some superio drivers, like
    pc87382 in src/superio/nsc, the code is very small. 
    The only interesting part is the pnp_info pnp_dev_info struct.

    Now if I look inside src/mainboard/lenovo/x60 there is some more
    complete dock driver:

    Inside dock.c I see some dock_connect and dock_disconnect functions.

    Such functions are called during the initialisation (romstage.c) and
    from the X60 SMI handler (smihandler.c).

    Questions:
    ----------
    1) Would the following be sufficent to prevent DMA access from the
    outside:
    > int dock_connect(void)
    > {
    >          int timeout = 1000;
    > +        int val;
    > +        
    > +        if (get_option(&val, "dock") != CB_SUCCESS)
    > +                val = 1;
    > +        if (val == 0)
    > +                return 0;
    >          [...]
    > }
    >
    > void dock_disconnect(void) {
    > +        if (dock_present())
    > +                return;
    >          [...]
    > }
    2) Would an nvram option be ok for that? Should a Kconfig option be
    added too?

    > config DOCK_AUTODETECT
    >         bool "Autodetect"
    >         help
    >           The dock is autodetected. If unsure select this option.
    >
    > config DOCK_DISABLED
    >         bool "Disabled"
    >         help
    >           The dock is always disabled.
    >
    > config DOCK_NVRAM_ENABLE
    >         bool "Nvram"
    >         help
    >           The dock autodetection is tried only if it is also enabled
    > trough nvram.

Copyright © 2014, 2015 Leah Rowe <info@minifree.org>

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