Re: [PATCH v6 1/9] spl: Kconfig: allow K3 devices to use falcon mode

[Anshul Dalal <anshuld@ti.com>]

On Wed May 28, 2025 at 9:09 PM IST, Andrew Davis wrote:
> On 5/8/25 6:37 AM, Anshul Dalal wrote:
>> On Thu May 8, 2025 at 9:32 AM IST, Vignesh Raghavendra wrote:
>>>
>>>
>>> On 5/8/2025 8:42 AM, Anshul Dalal wrote:
>>>> On Wed May 7, 2025 at 11:36 PM IST, Andrew Davis wrote:
>>>>> On 5/6/25 10:33 PM, Anshul Dalal wrote:
>>>>>> On Tue May 6, 2025 at 8:03 PM IST, Andrew Davis wrote:
>>>>>>> On 4/28/25 9:12 AM, Anshul Dalal wrote:
>>>>>>>> Falcon mode was disabled for TI_SECURE_DEVICE at commit e95b9b4437bc
>>>>>>>> ("ti_armv7_common: Disable Falcon Mode on HS devices") for older 32-bit
>>>>>>>> HS devices and can be enabled on K3 devices.
>>>>>>>>
>>>>>>>> For secure boot, the kernel with x509 headers can be packaged in a fit
>>>>>>> "can be", this is the issue. Security is not just allowing methods that
>>>>>>> are security checked, but forcing the use of such methods. Setting
>>>>>>> OS_BOOT opens up several paths that look for non-FIT images. These
>>>>>>> images do not enforce authentication like FIT does. This means one can
>>>>>>> bypass secure boot when OS_BOOT is enabled by simply placing a non-FIT
>>>>>>> boot image on the boot media.
>>>>>>>
>>>>>> As per spl_load_image_ext_os, the SPL first tries to load the file set
>>>>>> in falcon_args_file env variable but since it's not set in our case. And
>>>>>> the only way to set them is by rebuilding u-boot as uEnv.txt is not
>>>>>> supported at SPL stage.
>>>>>>
>>>>>> This means the SPL only loads CONFIG_SPL_FS_LOAD_ARGS_NAME and
>>>>>> CONFIG_SPL_FS_LOAD_KERNEL_NAME which are set as the DTB and fitImage
>>>>> What is stopping me from replacing the content of the file "fitImage"
>>>>> with a normal kernel image? When loading that image the file type
>>>>> will be detected as a normal kernel image and all FIT logic bypassed,
>>>>> including authentication, breaking our secure chain of trust.
>>>>>
>>>>> Andrew
>>>> That would require booti_setup to be executed in spl_parse_image_header,
>>>> which is not possible on the R5 SPL since the required config symbol
>>>> CMD_BOOTI is only available for ARM64 platforms.
>>>>
>>>> In the worst case we end up loading a 32-bit zImage which wouldn't
>>>> boot on the Cortex-A cores anyway and would additionally require
>>>> enabling CMD_BOOTZ (currently disabled) at build time.
>>>
>>> Is there any path where R5 SPL can be tricked to load and jump to
>>> arbitrary binary? zImage, Image, elf, bin whatever?
>>>
>>> IOW, does SPL_OS_BOOT always check for some sort of header for the image
>>> that it loads and the only type of header we have enabled here is fitImage?
>> 
>> It does check for the header and proceeds only with the desired security
>> enforced execution flow if the loaded image is FIT. For all other image
>> types, they are guarded by config symbols that are set unset in our case
>
> Are you sure?
>
> The only thing preventing this from continuing in spl_parse_image_header()
> is a check for CONFIG_SPL_PANIC_ON_RAW_IMAGE. Which is not set for us.
>
> After that we check if OS_BOOT is enabled and if so allow for kernel
> images to also pass[0]. What stops this from functioning?
>
> Andrew
>
> [0] https://github.com/u-boot/u-boot/blob/master/common/spl/spl.c#L338
>

It would not function because of the unset CONFIG_CMD_BOOTI which can
only be set on 64 bit platforms anyway[1]. Hence the following check
would fail in spl_parse_image_header:

if (CONFIG_IS_ENABLED(OS_BOOT) && IS_ENABLED(CONFIG_CMD_BOOTI))

And as I said previously in the thread[2]; worst case is we load a
32-bit zImage, support for which would have to be explicitly enabled at
build time as the respective config CMD_BOOTZ is kept unset currently.

~ Anshul

[1] https://github.com/u-boot/u-boot/blob/e04d137231f2e9e14708a32448c879125b8e308f/cmd/Kconfig#L359
[2] https://lore.kernel.org/u-boot/D9QG8DY630MX.1OV8MBZIM4R8S@ti.com/