[PATCH] image: fit: Fix not verifying data configuration

[Simon Glass]

Hi Sean,

On Thu, 13 Oct 2022 at 09:41, Sean Anderson <sean.anderson at seco.com> wrote:
>
>
>
> On 10/13/22 3:14 AM, Rasmus Villemoes wrote:
> > On 12/10/2022 18.28, Sean Anderson wrote:
> >> On 10/12/22 08:59, Simon Glass wrote:
> >>> Hi Sean,
> >>>
> >>> On Tue, 11 Oct 2022 at 17:25, Sean Anderson <sean.anderson at seco.com>
> >>> wrote:
> >>>>
> >>>> When reading data from a FIT image, we must verify the configuration we
> >>>> get it from. This is because when we have a key with required = "conf",
> >>>> the image does not need any particular signature or hash. The
> >>>> configuration is the only required verification, so we must verify it.
> >>>>
> >>>> Users of fit_get_data_node are liable to load unsigned data unless the
> >>>> user has set required = "image". Even then, they are vulnerable to
> >>>> mix-and-match attacks. This also affects other callers of
> >>>> fit_image_verify which don't first call fit_config_verify, such as
> >>>> source and imxtract. I don't think there is a backwards-compatible way
> >>>> to fix these interfaces. Fundamentally, selecting data by image when
> >>>> images are not required to be verified is unsafe.
> >>>>
> >>>> Fixes: 37feaf2f727 ("image: fit: Add some helpers for getting data")
> >>>> Signed-off-by: Sean Anderson <sean.anderson at seco.com>
> >>>> ---
> >>>>
> >>>>   boot/image-fit.c | 9 ++++++++-
> >>>>   1 file changed, 8 insertions(+), 1 deletion(-)
> >>>>
> >>>> diff --git a/boot/image-fit.c b/boot/image-fit.c
> >>>> index 9c04ff78a15..632fd405e29 100644
> >>>> --- a/boot/image-fit.c
> >>>> +++ b/boot/image-fit.c
> >>>> @@ -1948,7 +1948,14 @@ int fit_get_data_node(const void *fit, const
> >>>> char *image_uname,
> >>>>   int fit_get_data_conf_prop(const void *fit, const char *prop_name,
> >>>>                             const void **data, size_t *size)
> >>>>   {
> >>>> -       int noffset = fit_conf_get_node(fit, NULL);
> >>>> +       int ret, noffset = fit_conf_get_node(fit, NULL);
> >>>> +
> >>>> +       if (noffset < 0)
> >>>> +               return noffset;
> >>>> +
> >>>> +       ret = fit_config_verify(fit, noffset);
> >>>> +       if (ret)
> >>>> +               return ret;
> >>>>
> >>>>          noffset = fit_conf_get_prop_node(fit, noffset, prop_name);
> >>>>          return fit_get_data_tail(fit, noffset, data, size);
> >>>> --
> >>>> 2.35.1.1320.gc452695387.dirty
> >>>>
> >>>
> >>> This is supposed to work by first verifying the configuration with
> >>> fit_config_verify(). After that, images in that configuration can be
> >>> freely loaded, verified by the hash that each image has.
> >>
> >> Well, this function was made to replaces several cases where code loaded
> >> a FIT image from somewhere, and then wanted to get data from an image
> >> based on the configuration. Typically they only want to extract one
> >> image, which is the common case for e.g. loading firmware. This idea of
> >> this function is to make the common case of "find me the image data from
> >> the default config and verify it" easier. If you look at the existing
> >> code which uses this function, they do not verify the configuration
> >> first. This is mainly because the original versions of this code which I
> >> replaced with this function did not verify the configuration either.
> >>
> >> So while the above process works for an integrated verification process,
> >> like what is done by bootm, it doesn't really work for one-off loading
> >> of image data. In particular, the requirements to make this secure
> >> (using required = "image" for your key), are not default. When I was
> >> trying to determine whether the source command would be OK to use to
> >> load some configuration, I looked at it and saw that it did
> >> fit_image_verify. I thought that was fine, but if you use required =
> >> "config", then all that does is verify the hash.
> >
> > Yeah, so I've raised this problem with the "source" shell command
> > previously, but never got a satisfactory answer:
> >
> > https://lore.kernel.org/u-boot/7d711133-d513-5bcb-52f2-a9dbaa9eeded@prevas.dk/
> >
> > So does your patch now mean that it's possible to get a
> > bootscript-wrapped-in-a-FIT-image verified, possibly by adding some
> > dummy (or not so dummy?) "configurations" node? Can you give a complete
> > .its showing how I can build a verifiable boot script?
>
> No. I didn't convert source because it also checks to ensure that the
> image type is correct, which fit_get_data_node doesn't check. However,
> it still uses the image name to determine the data to source, which has
> all the problems as discussed above.
>
> I think to do this right we would need either
>
> - A version of fit_image_verify which treats required = "config" as
>   required = "image". This could be used for cases where the caller
>   doesn't verify a config (such as in cases when the user specifies an
>   image directly).

Without config verification we are subject to mix-and-match attacks.

> - Add support for specifying a config node. This would be something like
>   the addr#config syntax used by bootm. Of course, this doesn't address
>   existing users of fit_get_data_node.

Yes, let's do this one.

>
> That said, if we do determine the image based on a config, we should
> definitely verify it.

Yes

Regards,
Simon