Fit images and EFI_LOAD_FILE2_PROTOCOL

[François Ozog]

The driving idea is that there is an existing bootflow, non UEFI that
allows vmlinuz, initrd and DTB to be protected in a single FIT. The
trustworthiness of the solution is higher that regular distro on pure UEFI
systems but does not allow initrd changes as you install stuff. We need to
keep in mind the use cases: most of the cases are for production devices
where updates are not "calculated" in place but rather deployed with
various means.

I'd like to define two EBBR boot flows:
1) typical distro (with its weakness)
2) typical embedded (as of today, addressing security and mix/match
protection)

The 1) is described in slide 4 of the deck
The 2) is described in slide 5.

The UEFI interface is still OS independent but comes with an additional
opportunity: the ESP File System is "merged" with contents of a FIT (kind
of overlayfs). This way the content of the FIT can be checked and EFIStub
with EFI-LOAD_FILE2 the initrd. The bootXXXX will still indirectly point to
the FIT contained .EFI application, the firmware DTB may be overwritten by
a FIT DTB.

Is this a better scenario to establish 2)?

Cheers


FF

On Sat, 3 Oct 2020 at 15:12, Ard Biesheuvel <ardb at kernel.org> wrote:

>
>
> 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.
>
>
>

-- 
François-Frédéric Ozog | *Director Linaro Edge & Fog Computing Group*
T: +33.67221.6485
francois.ozog at linaro.org | Skype: ffozog