linux-kernel - Re: [RFC] efi/tpm: add efi.tpm_log as a reserved region in 820_table

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Message-ID: <CAMj1kXGi+N6AukJt6EGQTao=-1Ud_=bzwPvdjEzhmzEraFU98w@mail.gmail.com>
Date: Thu, 12 Sep 2024 12:51:57 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: Usama Arif <usamaarif642@...il.com>
Cc: linux-efi@...r.kernel.org, kexec@...ts.infradead.org, 
	ebiederm@...ssion.com, bhe@...hat.com, vgoyal@...hat.com, tglx@...utronix.de, 
	dave.hansen@...ux.intel.com, x86@...nel.org, linux-kernel@...r.kernel.org, 
	leitao@...ian.org, rmikey@...a.com, gourry@...rry.net
Subject: Re: [RFC] efi/tpm: add efi.tpm_log as a reserved region in 820_table_firmware

On Thu, 12 Sept 2024 at 12:23, Usama Arif <usamaarif642@...il.com> wrote:
>
>
>
> On 11/09/2024 12:51, Ard Biesheuvel wrote:
> > On Wed, 11 Sept 2024 at 12:41, Usama Arif <usamaarif642@...il.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@...il.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.