On Tue, 6 Oct 2020 at 06:35, Heinrich Schuchardt xypron.glpk@gmx.de wrote:
Am 6. Oktober 2020 00:37:58 MESZ schrieb Grant Likely grant.likely@arm.com:
On 03/10/2020 09:51, Heinrich Schuchardt wrote:
Hello Ilias, hello Christian,
with commit ec80b4735a59 ("efi_loader: Implement FileLoad2 for
initramfs
loading") Ilias provided the possibility to specify a device path (CONFIG_EFI_INITRD_FILESPEC) from which an initial RAM disk can be served via the EFI_FILE_LOAD2_PROTOCOL.
Ard extended the Linux EFI stub to allow loading the initial RAM disk via the EFI_FILE_LOAD2_PROTOCOL with the utmost priority.
With commit ecc7fdaa9ef1 ("bootm: Add a bootm command for type IH_OS_EFI") Cristian enabled signed FIT images that contain a device tree and a UEFI binary (enabled by CONFIG_BOOTM_EFI=y).
In the DTE calls we have discussed that it is unfortunate that we do
not
have a method to validate initial RAM images in the UEFI context.
To me it would look like a good path forward to combine the two
ideas:
- Let the signed FIT image (of type IH_OS_EFI) contain a RAM disk
- Pass location and size to the UEFI subsystem and serve them via the EFI_FILE_LOAD2_PROTOCOL.
We could also extend the bootefi command to be callable as
bootefi $kernel_addr_r $ramdisk_addr_r:$filesize $fdt_addr_rlike the booti command to serve an initial RAM disk.
What are your thoughts?
Hi Heinrich,
I've got concerns about this approach. Even though it uses the UEFI infrastructure, images deployed in this way are U-Boot specific and won't ever be applicable on EDK2 or other UEFI implementations.
However there is another way to approach it which I think Francois touched on. If instead a UEFI stub was added to the FIT image, in the same way that the kernel has a UEFI stub, then the logic of decoding the FIT and choosing the correct DTB & initrd can be part of the image and it becomes applicable to any UEFI implementation. It would also address
Ard's concern of loading the FIT into memory, and then copying due to the EFI_FILE_LOAD2 path. The FIT stub would already know the image is in RAM, that is is reserved correctly, and just pass the correct addresses
to the kernel as part of the normal boot flow.
Signing would also be taken care of because the whole FIT can be signed, and that signature would be checked when it gets loaded.
Thoughts?
The gain of a fit image in U-Boot used for calling the Linux kernel via the EFI stub vs calling the legacy entry point comes down to providing the EFI_RNG_PROTOCOL to be used for KASLR.
For initrd a stub UEFI binary will work. But if you want to provide a kernel specific dtb with the same stub binary it will require a new service for device-tree fixups.
Both approaches are on Francois' DTE slidedeck.
When thinking of security what really is unclear to me is how we can safely provide the decryption key for the root partition. Without such a means secure boot stops being secure once Linux starts the init process. I would assume that only the secure world can provide the key.
Secure boot only deals with authentication, which does not require any secret keys.
Encrypted file systems are a completely separate matter. Typically, this is based on TPM-based local attestation, where the decryption key has been sealed into the TPM, and is only unsealed when all the boot components check out (based on their 'measurements' [aka hashes] that are cumulatively recorded in TPM hardware registers called PCRs)
In general, keeping secrets on a device is much more difficult than authenticating executable images, and typically involves some user provided secret, or h/w support. UEFI secure boot only reasons about authentication.