On 08.04.21 12:10, Sughosh Ganu wrote: > hi Heinrich, > > On Thu, 8 Apr 2021 at 14:17, Heinrich Schuchardt <[email protected] > <mailto:[email protected]>> wrote: > > On 08.04.21 08:53, Sughosh Ganu wrote: > > hi Simon, > > > > On Wed, 7 Apr 2021 at 21:44, Simon Glass <[email protected] > <mailto:[email protected]> > > <mailto:[email protected] <mailto:[email protected]>>> wrote: > > > > Hi, > > > > On Wed, 7 Apr 2021 at 23:54, Sughosh Ganu > <[email protected] <mailto:[email protected]> > > <mailto:[email protected] > <mailto:[email protected]>>> 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
