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

On 14.06.24 09:01, Ilias Apalodimas wrote:

On Fri, 14 Jun 2024 at 09:59, Heinrich Schuchardt xypron.glpk@gmx.de wrote:

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On 6/14/24 08:03, Ilias Apalodimas wrote:

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Hi Simon,

On Mon, 10 Jun 2024 at 17:59, Simon Glass sjg@chromium.org wrote:

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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@chromium.org

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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

So this patch should also consider CMD_TPM_V2 and CMD_TPM_V1.

TPM v1 only needs SHA-1.

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.

Best regards

Heinrich

Regards /Ilias

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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@gmx.de

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So this would make the TPM pretty useless. Can't you remove something that doesn't break functionality?

Thanks /Ilias

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      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