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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