[PATCH] Provide more details of exactly how configuration signatures are calculated

[Martin Bonner]

OK.  I've just realized there is an important warning missing from
this (if you verify a configuration signature outside u-boot, you
_must_ verify the hashes for the kernel, fdt, and ramdisk images match
too).  What is the protocol for handling that?  Submit a new patch
with title "[Patch v2] ..."?

I will hold off submitting that in case there are other comments that
need to be addressed.

Martin

On Wed, 20 Jul 2022 at 08:23, Martin Bonner <martingreybeard at gmail.com> wrote:
>
> There are a couple of uncertainties still remaining:
> - The "hashed-nodes" property is created by mkimage in image order.
> What happens if somebody manipulates a signed image to change the
> order?  Does it make any difference?  Do u-boot and fit_check_sign
> handle it in the same way?  (My preference would be that both fail the
> signature on the grounds that there is no good reason for the nodes to
> be out of order, and it suggests an attacker is trying to be clever.)
> - What happens if the image contains a DTB_NOP tag?  Is it included or
> not?  (Again, I would fail the signature if it does.)
>
> Martin
>
> On Wed, 20 Jul 2022 at 08:14, Martin Bonner <martingreybeard at gmail.com> wrote:
> >
> > I have just spent a week reverse-engineering this, so I thought I'd
> > contribute the work back to the community
> >
> > diff --git a/doc/uImage.FIT/signature.txt b/doc/uImage.FIT/signature.txt
> > index 61a72db3c7..61cdd55e40 100644
> > --- a/doc/uImage.FIT/signature.txt
> > +++ b/doc/uImage.FIT/signature.txt
> > @@ -382,6 +382,25 @@ verified later even if the FIT has been signed
> > with other keys in the
> >  meantime.
> >
> >
> > +Details
> > +-------
> > +The signature node contains a property ('hashed-nodes') which lists all the
> > +nodes that the signature was made over.  The image is walked in order and each
> > +tag processed as follows:
> > +- DTB_BEGIN_NODE: The tag and the following name are added to the signature if
> > +  the node or its parent are present in 'hashed-nodes'
> > +- DTB_END_NODE: The tag is added to the signature if the node or its parent are
> > +  present in 'hashed-nodes'
> > +- DTB_PROPERTY: The tag, the length word, the offset in the string table, and
> > +  the data are all included if the node is present in 'hashed-nodes' and the
> > +  property name is not 'data'.
> > +- DTB_END: The tag is always included.
> > +
> > +In addition, the signature contains a property 'hashed-strings' which contains
> > +the offset and length in the string table of the strings that are to be added
> > +to the signature (this is always done at the end).
> > +
> > +
> >  Verification
> >  ------------
> >  FITs are verified when loaded. After the configuration is selected a list
> >
> > --
> > Martin