[U-Boot] Secure update of uboot devices?

[Kim Phillips]

On Fri, 30 Dec 2011 13:37:47 +0100
Wolfgang Denk <wd at denx.de> wrote:

> Dear Andreas,
> 
> In message <CAB+EkH4j-UoUyHb=XgDbGRncX=Oq6+3+MNjWStiuojoOYUcMPw at mail.gmail.com> you wrote:
> >
> > sha1sum sum is yes enough to verify that no files have been modified on the
> > file system on the already installed Linux device.
> 
> It is also good enough to ensure that the files on any distribution
> media have not been corrupted or modified in some way.  Of course it
> dies not protect against intentional modifications.
> 
> > But my case here is if one need to update the software on the device out
> > somewhere in the world we have now made a usb stick and uboot looks for
> > special files first on the usb stick before it continues normal boot. How
> > can one ensure that the software on the usb stick is not altered on the way
> > to include some additional unwanted features?
> 
> You cannot.  Actually you would have to insure first that the U-Boot
> running on that system has not been tampered with.  If I were to
> attack such a system, I'd probably first install (or otherwise run) a
> version of U-boot that has any such security checks disabled or
> removed.

That depends on your hardware.  SoCs with Freescale SEC v4+ h/w can
enable a trusted boot mode after writing a private key to
special-purpose on-chip key memory and subsequently blowing a fuse.
The trusted boot mode ensures a continuous root of trust by booting
an initial (u-)bootloader from on-chip firmware that verifies the
authenticity of the u-boot image it loads before executing it.  The
initial bootloader is written in a similar fashion to the private
key of the chip, and similarly can never be overwritten.  Subsequent
loads, e.g., u-boot->kernel, kernel->app, are free to inherit that
same root of trust.

The Freescale BSP version of u-boot includes some of Freescale's
secure boot work [1], but since then it's been modified to use the
dedicated crypto unit to do the crypto and therefore boot much
faster. Ideally u-boot would be modified to use either s/w or h/w
crypto, but unfortunately I haven't had the time to look into it.

Kim

[1] I don't know where to find the latest that uses the h/w to do
the crypto right now, but there's some s/w crypto based code
available here:

http://git.freescale.com/git/cgit.cgi/ppc/sdk/u-boot.git/log/