Fit images and EFI_LOAD_FILE2_PROTOCOL

[Ard Biesheuvel]

On Sat, 3 Oct 2020 at 13:16, François Ozog <francois.ozog at linaro.org> wrote:

>
>
> Le sam. 3 oct. 2020 à 13:14, François Ozog <francois.ozog at linaro.org> a
> écrit :
>
>>
>>
>> Le sam. 3 oct. 2020 à 10:51, Heinrich Schuchardt <xypron.glpk at gmx.de> a
>> écrit :
>>
>>> 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_r
>>>
>>>
>>>
>>> like the booti command to serve an initial RAM disk.
>>>
>>>
>>>
>>> What are your thoughts?
>>
>> that looks super interesting.
>> I propose something (in the latest desk preparing oct 14th) similar
>> except the an efi application boots the FIT.
>> I view UEFI as booting a PE coff and pass a set of config tables. Today
>> we have DTB, we could just add Initrd (you command line). Bootefi would be
>> responsible to valide the containing FIT before pushing initrd (and
>> dTB?)into the table. It would be the responsibility of the efi stub to get
>> the initrd from the config table (GUID to be defined).
>>
> the memory attributes of the initrd config table should be such that it
> can be recovered for normal use. That may be tricky though.
>

The purpose of the EFI_FILE_LOAD2_PROTOCOL based initrd loading mechanism
is to allow the EFI stub (which is tightly coupled to the kernel
arch/version/etc) to allocate the memory for the initrd, and pass it into
the LoadFile2() request, using whichever policy it wants to adhere to for
alignment, offset and/or vicinity of the kernel image. It also ensures that
any measurement performed by the bootloader for attestation or
authentication can be delayed to the point where the booting kernel assumes
ownership of the initrd contents, preventing potential TOCTOU issues where
intermediate boot stages are involved (shim+grub etc)

Creating an initrd config table would mean that the bootloader decides
where to load the initrd in memory, and only passes the address and size.
This is exactly what we wanted to avoid, because now, the bootloader has to
know all these different rules that vary between kernel version,
configurations and architectures.

For uboot's implementation of FIT based EFI_FILE_LOAD2_PROTOCOL, this might
mean that the initrd is loaded into memory first, and copied to another
location (and [re-]authenticated) when LoadFile2() is invoked. I don't
think this is a problem in the general case, but we might think about ways
to avoid this if this turns out to be a problem for memory constrained
devices with huge initrds.