[PATCH v2 2/9] tpm: Avoid code bloat when not using EFI_TCG2_PROTOCOL

[Simon Glass]

Hi,

On Sat, 15 Jun 2024 at 01:03, Ilias Apalodimas
<ilias.apalodimas at linaro.org> wrote:
>
> Hi Heinrich
>
> resending the reply, I accidentally sent half of the message...
>
> On Fri, 14 Jun 2024 at 12:04, Heinrich Schuchardt <xypron.glpk at gmx.de> wrote:
> >
> > On 14.06.24 09:01, Ilias Apalodimas wrote:
> > > On Fri, 14 Jun 2024 at 09:59, Heinrich Schuchardt <xypron.glpk at gmx.de> wrote:
> > >>
> > >> On 6/14/24 08:03, Ilias Apalodimas wrote:
> > >>> Hi Simon,
> > >>>
> > >>> On Mon, 10 Jun 2024 at 17:59, Simon Glass <sjg at chromium.org> wrote:
> > >>>>
> > >>>> It does not make sense to enable all SHA algorithms unless they are
> > >>>> needed. It bloats the code and in this case, causes chromebook_link to
> > >>>> fail to build. That board does use the TPM, but not with measured boot,
> > >>>> nor EFI.
> > >>>>
> > >>>> Since EFI_TCG2_PROTOCOL already selects these options, we just need to
> > >>>> add them to MEASURED_BOOT as well.
> > >>>>
> > >>>> Note that the original commit combines refactoring and new features,
> > >>>> which makes it hard to see what is going on.
> > >>>>
> > >>>> Fixes: 97707f12fda tpm: Support boot measurements
> > >>>> Signed-off-by: Simon Glass <sjg at chromium.org>
> > >>>> ---
> > >>>>
> > >>>> Changes in v2:
> > >>>> - Put the conditions under EFI_TCG2_PROTOCOL
> > >>>> - Consider MEASURED_BOOT too
> > >>>>
> > >>>>    boot/Kconfig | 4 ++++
> > >>>>    lib/Kconfig  | 4 ----
> > >>>>    2 files changed, 4 insertions(+), 4 deletions(-)
> > >>>>
> > >>>> diff --git a/boot/Kconfig b/boot/Kconfig
> > >>>> index 6f3096c15a6..b061891e109 100644
> > >>>> --- a/boot/Kconfig
> > >>>> +++ b/boot/Kconfig
> > >>>> @@ -734,6 +734,10 @@ config LEGACY_IMAGE_FORMAT
> > >>>>    config MEASURED_BOOT
> > >>>>           bool "Measure boot images and configuration when booting without EFI"
> > >>>>           depends on HASH && TPM_V2
> > >>>> +       select SHA1
> > >>>> +       select SHA256
> > >>>> +       select SHA384
> > >>>> +       select SHA512
> > >>>>           help
> > >>>>             This option enables measurement of the boot process when booting
> > >>>>             without UEFI . Measurement involves creating cryptographic hashes
> > >>>> diff --git a/lib/Kconfig b/lib/Kconfig
> > >>>> index 189e6eb31aa..568892fce44 100644
> > >>>> --- a/lib/Kconfig
> > >>>> +++ b/lib/Kconfig
> > >>>> @@ -438,10 +438,6 @@ config TPM
> > >>>>           bool "Trusted Platform Module (TPM) Support"
> > >>>>           depends on DM
> > >>>>           imply DM_RNG
> > >>>> -       select SHA1
> > >>>> -       select SHA256
> > >>>> -       select SHA384
> > >>>> -       select SHA512
> > >>>
> > >>> I am not sure this is the right way to deal with your problem.
> > >>> The TPM main functionality is to measure and extend PCRs, so shaXXXX
> > >>> is really required. To make things even worse, you don't know the PCR
> > >>> banks that are enabled beforehand. This is a runtime config of the
> > >>> TPM.
> > >>
> > >> If neither MEASURED_BOOT nor EFI_TCG2_PROTOCOL is selected, U-Boot
> > >> cannot extend PCRs. So it seems fine to let these two select the
> > >> complete set of hashing algorithms. As Simon pointed out for
> > >> EFI_TCG2_PROTOCOL this is already done in lib/efi_loader/Kconfig.
> > >
> > > It can. The cmd we have can extend those pcrs -- e.g tpm2 pcr_extend 8
> > > 0xb0000000

That's pretty normal for U-Boot though, since we want to avoid lots of
growth for things people might want control over. We can enable or
disable the SHA for the board, if this functionality is used outside
of measured boot and tcg2, but someone is enabling the tpm command.

> >
> > So this patch should also consider CMD_TPM_V2 and CMD_TPM_V1.
> >
> > TPM v1 only needs SHA-1.
>
> I still prefer to imply all algos.

'imply' would be OK in this case as I can disable it for that board. I
don't think it is in the spirit of U-Boot though.

isn't someone checking the growth in U-Boot? Or do so few boards have
TPMs that it didn't register? The size growth was 3.2KB on
chromebook_link.

>
> >
> > In cmd/tpm-v2.c do_tpm2_pcr_extend() and do_tpm_pcr_read() assume
> > SHA256. Function tpm_pcr_extend() shows the same limitation. This bug
> > should be fixed. But as is CMD_TPM_V2 seems only to require CONFIG_SHA256.
>
> Isn't [0] fixing this?
>
> [0] https://source.denx.de/u-boot/u-boot/-/commit/89aa8463cdf3919ca4f04fc24ec8b154ff56d97e
> Thanks
> /Ilias
> >
> > Best regards
> >
> > Heinrich
> >
> > >
> > > Regards
> > > /Ilias
> > >>
> > >> Even if U-Boot does not support measured boot (EFI or non-EFI) we might
> > >> still be using the TPMs RNG.
> > >>
> > >> Reviewed-by: Heinrich Schuchardt <xypron.glpk at gmx.de>
> > >>
> > >>>
> > >>>    So this would make the TPM pretty useless. Can't you remove something
> > >>> that doesn't break functionality?
> > >>>
> > >>> Thanks
> > >>> /Ilias
> > >>>>           help
> > >>>>             This enables support for TPMs which can be used to provide security
> > >>>>             features for your board. The TPM can be connected via LPC or I2C
> > >>>> --
> > >>>> 2.34.1
> > >>>>
> > >>
> >

Regards,
Simon