[PATCH 0/5] Add support for embedding public key in platform's dtb

[Sughosh Ganu]

On Thu, 8 Apr 2021 at 16:51, Heinrich Schuchardt <xypron.glpk at gmx.de> wrote:

> On 08.04.21 12:10, Sughosh Ganu wrote:
> > hi Heinrich,
> >
> > On Thu, 8 Apr 2021 at 14:17, Heinrich Schuchardt <xypron.glpk at gmx.de
> > <mailto:xypron.glpk at gmx.de>> wrote:
> >
> >     On 08.04.21 08:53, Sughosh Ganu wrote:
> >     > hi Simon,
> >     >
> >     > On Wed, 7 Apr 2021 at 21:44, Simon Glass <sjg at chromium.org
> >     <mailto:sjg at chromium.org>
> >     > <mailto:sjg at chromium.org <mailto:sjg at chromium.org>>> wrote:
> >     >
> >     >     Hi,
> >     >
> >     >     On Wed, 7 Apr 2021 at 23:54, Sughosh Ganu
> >     <sughosh.ganu at linaro.org <mailto:sughosh.ganu at linaro.org>
> >     >     <mailto:sughosh.ganu at linaro.org
> >     <mailto:sughosh.ganu at linaro.org>>> wrote:
> >     >     >
> >     >     > Patch 1 fixes an issue of selection of IMAGE_SIGN_INFO
> >     config option
> >     >     > when capsule authentication is enabled.
> >     >     >
> >     >     > Patch 2 add two config symbols, EFI_PKEY_DTB_EMBED and
> >     EFI_PKEY_FILE
> >     >     > which are used for enabling embedding of the public key in
> >     the dtb,
> >     >     > and specifying the esl file name.
> >     >     >
> >     >     > Patch 3 moves efi_capsule_auth_enabled as a weak function,
> >     which can
> >     >     > be used as a default mechanism for checking if capsule
> >     authentication
> >     >     > has been enabled.
> >     >     >
> >     >     > Patch 4 adds a default weak function for retrieving the
> >     public key
> >     >     > from the platform's dtb.
> >     >     >
> >     >     > Patch 5 adds the functionality to embed the esl file into the
> >     >     > platform's dtb during the platform build.
> >     >     >
> >     >     > I have tested this functionality on the STM32MP157C DK2
> board.
> >     >     >
> >     >     > [1] -
> >     >     https://lists.denx.de/pipermail/u-boot/2021-March/442867.html
> >     <https://lists.denx.de/pipermail/u-boot/2021-March/442867.html>
> >     >     <https://lists.denx.de/pipermail/u-boot/2021-March/442867.html
> >     <https://lists.denx.de/pipermail/u-boot/2021-March/442867.html>>
> >     >     >
> >     >     > Sughosh Ganu (5):
> >     >     >   efi_loader: Kconfig: Select IMAGE_SIGN_INFO when capsule
> >     >     >     authentication is enabled
> >     >     >   efi_loader: Kconfig: Add symbols for embedding the public
> key
> >     >     into the
> >     >     >     platform's dtb
> >     >     >   efi_capsule: Add a weak function to check whether capsule
> >     >     >     authentication is enabled
> >     >     >   efi_capsule: Add a weak function to get the public key
> >     needed for
> >     >     >     capsule authentication
> >     >     >   Makefile: Add provision for embedding public key in
> >     platform's dtb
> >     >     >
> >     >     >  Makefile                              | 10 ++++++
> >     >     >  board/emulation/common/qemu_capsule.c |  6 ----
> >     >     >  lib/efi_loader/Kconfig                | 16 ++++++++++
> >     >     >  lib/efi_loader/efi_capsule.c          | 44
> >     >     ++++++++++++++++++++++++---
> >     >     >  4 files changed, 66 insertions(+), 10 deletions(-)
> >     >     >
> >     >     > --
> >     >     > 2.17.1
> >     >     >
> >     >
> >     >     We need to rethink the use of weak functions for this sort of
> >     thing,
> >     >     or we will end up with an unnavigable mess at some point. If
> >     we need
> >     >     to adjust the flow of boot, let's adjust the flow rather than
> >     adding
> >     >     hooks everywhere.
> >     >
> >     >
> >     > There are two weak functions being added. One is for retrieving the
> >     > public key to be used for the capsule authentication, and the
> other is
> >     > for checking for whether capsule authentication has been enabled.
> The
> >     > reason why a weak function is needed is because platforms can have
> >     other
> >     > mechanisms for retrieval of the public key or for testing if
> capsule
> >     > authentication has been enabled.
> >     >
> >     > If we consider the case of public key retrieval, the majority of
> >     > platforms would be built with the device tree concatenated with the
> >     > u-boot binary. The weak function would cater to all of those
> platforms
> >     > -- having a weak function would mean that we are not required to
> >     repeat
> >     > the same functionality for every platform that uses the same
> mechanism
> >     > for extracting the public key. However, there would be platforms
> where
> >     > the public key is obtained through a different mechanism which is
> >     > platform specific. Those platforms would have to define their own
> >     > function to get the public key. Same for checking whether capsule
> >     > authentication feature has been enabled or not.
> >     >
> >     > -sughosh
> >
> >     Hello Sughosh,
> >
> >     Could you, please, explain why there could be a need to use public
> keys
> >     for capsule verification that are not compiled into U-Boot. I cannot
> see
> >     how this would increase security.
> >
> >
> > With the changes that have been made in the Makefile(patch 5/5), the
> > public key is now embedded into the platform's dtb, and subsequently
> > this dtb is concatenated with the u-boot binary to create a single
> > u-boot.bin image. This image can then be verified during the trusted
> > boot flow to check against any kind of tampering. This takes care of
> > your concern of not having the public key separately on the disk, which
> > makes it open to tampering, with the public key now embedded as part of
> > the u-boot image. You had suggested embedding the public key as part of
> > the u-boot image. I have embedded it within the platform's dtb which is
> > part of the u-boot image. This becomes a generic solution which is
> > platform and architecture agnostic. I believe concatenating the
> > platform's dtb with the u-boot binary is the standard flow for
> > production images.
>
> Embedding the key in the device-tree is fine. I am just trying to
> understand why you need the extensibility via a weak function.
>

This is to provide flexibility for any platform that might have a different
mechanism of passing/retrieving the public key. Some platforms might have
their dtb passed from an earlier stage firmware, like tf-a. Or there could
be a read-only device like a fuse which houses the keys to be used. Having
a weak default would allow such platforms to implement a platform specific
function to retrieve the public key. So if there is no technical
disadvantage of having a weak default, I think keeping this flexibility for
platforms would be good.


>
> >
> >     I cannot imagine any scenario where you would want to allow switching
> >     off capsule authentication if it has been built into U-Boot.
> >
> >
> > This is only an additional knob for any user who might want to perform a
> > capsule update without authentication -- with this additional knob, the
> > user can use the same image for updating a capsule which does not have
> > an authentication header. The user would not be required to recompile
> > the image to turn off CONFIG_EFI_CAPSULE_AUTHENTICATE config option. But
> > if you don't see this necessary, i can remove this additional check. In
> > that case, the capsule will be authenticated when
> > CONFIG_EFI_CAPSULE_AUTHENTICATE is enabled.
>
> I would prefer to reduce the number of "knobs" that you have to check
> when rolling out secure firmware.
>

Okay. I will remove this extra check in the next version. Whether the
platform authenticates the capsule or not would then depend solely on the
config option.

-sughosh