[PATCH 0/5] Add support for embedding public key in platform's dtb
Heinrich Schuchardt
xypron.glpk at gmx.de
Thu Apr 8 13:21:43 CEST 2021
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.
>
> 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.
Best regards
Heinrich
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