[PATCH v10 10/15] FWU: Add support for the FWU Multi Bank Update feature

[Sughosh Ganu]

On Tue, 27 Sept 2022 at 22:19, Jassi Brar <jassisinghbrar at gmail.com> wrote:
>
> On Tue, Sep 27, 2022 at 2:22 AM Sughosh Ganu <sughosh.ganu at linaro.org> wrote:
> >
> > On Mon, 26 Sept 2022 at 20:24, Jassi Brar <jassisinghbrar at gmail.com> wrote:
> > >
> > > On Mon, Sep 26, 2022 at 4:01 AM Sughosh Ganu <sughosh.ganu at linaro.org> wrote:
> > > > On Mon, 26 Sept 2022 at 08:25, Jassi Brar <jassisinghbrar at gmail.com> wrote:
> > >
> > > > > .....
> > > > > >
> > > > > > +static __maybe_unused efi_status_t fwu_post_update_process(bool fw_accept_os)
> > > > > > +{
> > > > > > +       int status;
> > > > > > +       u32 update_index;
> > > > > > +       efi_status_t ret;
> > > > > > +
> > > > > > +       status = fwu_plat_get_update_index(&update_index);
> > > > > > +       if (status < 0) {
> > > > > > +               log_err("Failed to get the FWU update_index value\n");
> > > > > > +               return EFI_DEVICE_ERROR;
> > > > > > +       }
> > > > > > +
> > > > > > +       /*
> > > > > > +        * All the capsules have been updated successfully,
> > > > > > +        * update the FWU metadata.
> > > > > > +        */
> > > > > > +       log_debug("Update Complete. Now updating active_index to %u\n",
> > > > > > +                 update_index);
> > > > > > +       status = fwu_update_active_index(update_index);
> > > > > >
> > > > > Do we want to check if all images in the bank are updated via capsules
> > > > > before switching the bank?
> > > >
> > > > This function does get called only when the update status for every
> > > > capsule is a success. Even if one of the capsules does not get
> > > > updated, the active index will not get updated.
> > > >
> > > .... but we don't check if the capsule for each image in the bank is
> > > provided for update.
> >
> > Yes, we have had this discussion earlier. Neither the FWU spec, nor
> > the capsule update spec in UEFI puts that restriction that all images
> > on the platform need to be updated. If a user wants to ensure such a
> > behaviour, they may choose some kind of image packaging like FIP or
> > FIT which would mean that all the images on the platform are being
> > updated. But this is not something to be ensured by the FWU update
> > code.
> >
> > >
> > > > >
> > > > > A developer will make sure all images are provided in one go, so that
> > > > > the switch is successful.
> > > > > But a malicious user may force some old vulnerable image back into use
> > > > > by updating all but that image.
> > > >
> > > > That I believe is to be handled through a combination of implementing
> > > > a rollback protection mechanism, along with capsule authentication.
> > > > These are separate to the implementation of the multi bank updates
> > > > that these patches are aiming for.
> > > >
> > > This sounds like : we don't worry about buffer-overflow
> > > vulnerabilities because the system will be secured and hardened by
> > > other mechanisms.
> >
> > Not sure how this is related. The aim of the FWU spec is for providing
> > a means for a platform to maintain multiple partitions of images and
> > to specify the metadata structure for the bookkeeping of the different
> > partitions and images on those partitions. If we need a more secure
> > and hardened system, we do have the Dependable Boot spec, which is
> > talking precisely about these things. Those are indeed separate
> > features or aspects from what the FWU spec is talking about. And this
> > patchset is implementing the FWU spec.
> >
>
> I am out of ways to explain. Best of luck.

Let me try one last time. If you still do not agree with what I say,
let us agree to disagree :)

You mentioned earlier that a malicious user may force some old
vulnerable image back into use by updating all but that image. Now a
few points to consider here. The FWU spec recommends that platforms
follow the Platform Security Boot Guide [1] which specifies the
trusted boot flow for platforms where the firmware images being booted
on the platform, including the NWd bootloader(BL33 in tf-a jargon) are
verified before getting booted. So, if the platform is booting with
the trusted boot flow, the above scenario should not occur. Secondly,
at the time of the update, the capsule can be authenticated before
getting written to the storage. This again should be able to identify
a malicious image getting written to the device. So this hypothesis of
a malicious image being present should not occur with authenticated
capsules and a trusted boot flow. And lastly, if a supposedly good
image is found to have some vulnerability because of which an image
with a fix should be installed and the vulnerable image not allowed to
boot, this is precisely what we have rollback protection for.

You also mentioned that a platform might not support these
functionalities. Well, if these are not being supported, we don't have
much of a safety net here. Moreover, one more point to think here is
that if it is possible for the user to install and boot a vulnerable
image on the platform in the first place, it will not be protected by
updating all the images -- the malicious user can still put a corrupt
image as part of the update image bundle. The protection against
booting a corrupt image or a buggy image has to come through
mechanisms like trusted boot and rollback protection.

-sughosh

[1] - https://developer.arm.com/documentation/den0072/0101/?lang=en