[RFC] efi/tpm: add efi.tpm_log as a reserved region in 820_table_firmware

[Usama Arif]

On 12/09/2024 11:51, Ard Biesheuvel wrote:
> On Thu, 12 Sept 2024 at 12:23, Usama Arif <usamaarif642 at gmail.com> wrote:
>>
>>
>>
>> On 11/09/2024 12:51, Ard Biesheuvel wrote:
>>> On Wed, 11 Sept 2024 at 12:41, Usama Arif <usamaarif642 at gmail.com> wrote:
>>>>
>>>> Looking at the TPM spec [1]
>>>>
>>>> If the ACPI TPM2 table contains the address and size of the Platform
>>>> Firmware TCG log, firmware “pins” the memory associated with the
>>>> Platform FirmwareTCG log, and reports this memory as “Reserved” memory
>>>> via the INT 15h/E820 interface.
>>>>
>>>> It looks like the firmware should pass this as reserved in e820 memory
>>>> map. However, it doesn't seem to. The firmware being tested on is:
>>>> dmidecode -s bios-version
>>>> edk2-20240214-2.el9
>>>>
>>>> When this area is not reserved, it comes up as usable in
>>>> /sys/firmware/memmap. This means that kexec, which uses that memmap
>>>> to find usable memory regions, can select the region where efi.tpm_log
>>>> is and overwrite it and relocate_kernel.
>>>>
>>>> Having a fix in firmware can be difficult to get through. As a secondary
>>>> fix, this patch marks that region as reserved in e820_table_firmware if it
>>>> is currently E820_TYPE_RAM so that kexec doesn't use it for kernel segments.
>>>>
>>>> [1] https://trustedcomputinggroup.org/wp-content/uploads/PC-ClientPlatform_Profile_for_TPM_2p0_Systems_v49_161114_public-review.pdf
>>>>
>>>> Signed-off-by: Usama Arif <usamaarif642 at gmail.com>
>>>
>>> I would expect the EFI memory map to E820 conversion implemented in
>>> the EFI stub to take care of this.
>>>
>>
>> So I have been making a prototype with EFI stub, and the unfinished version is looking like a
>> horrible hack.
>>
>> The only way to do this in libstub is to pass log_tbl all the way from efi_retrieve_tcg2_eventlog
>> to efi_stub_entry and from there to setup_e820.
>> While going through the efi memory map and converting it to e820 table in setup_e820, you have to check
>> if log_tbl falls in any of the ranges and if the range is E820_TYPE_RAM. If that condition is satisfied,
>> then you have to split that range into 3. i.e. the E820_TYPE_RAM range before tpm_log, the tpm_log
>> E820_TYPE_RESERVED range, and the E820_TYPE_RAM range after. There are no helper functions, so this
>> splitting involves playing with a lot of pointers, and it looks quite ugly. I believe doing this
>> way is more likely to introduce bugs.
>>
>> If we are having to compensate for an EFI bug, would it make sense to do it in the way done
>> in RFC and do it in kernel rather than libstub? It is simple and very likely to be bug free.
>>
> 
> I don't see how this could be an EFI bug, given that it does not deal
> with E820 tables at all.


EFI passes memory descriptors to libstub, libstub converts it to e820.

I believe the right behaviour should be that EFI creates an EFI_RESERVED_TYPE region for 
that TPM log memory. Then libstub would automatically convert that EFI_RESERVED_TYPE
to E820_TYPE_RESERVED in setup_e820 [1].

[1] https://elixir.bootlin.com/linux/v6.10.9/source/drivers/firmware/efi/libstub/x86-stub.c#L573