tpm: allow the user to select the compiled algorithms

Simon reports that after enabling all algorithms on the TPM some boards
fail since they don't have enough storage to accommodate the ~5KB growth.

The choice of hash algorithms is determined by the platform and the TPM
configuration. Failing to cap a PCR in a bank which the platform left
active is a security vulnerability. It might allow  unsealing of secrets
if an attacker can replay a good set of measurements into an unused bank.

If MEASURED_BOOT or EFI_TCG2_PROTOCOL is enabled our Kconfig will enable
all supported hashing algorithms. We still want to allow users to add a
TPM and not enable measured boot via EFI or bootm though and at the same
time, control the compiled algorithms for size reasons.

So let's add a function tpm2_allow_extend() which checks the TPM active
PCRs banks against the one U-Boot was compiled with. We only allow
extending PCRs if the algorithms selected during build match the TPM
configuration.

It's worth noting that this is only added for TPM2.0, since TPM1.2 is
lacking a lot of code at the moment to read the available PCR banks.
We unconditionally enable SHA1 when a TPM is selected, which is the only
hashing algorithm v1.2 supports.

Signed-off-by: Ilias Apalodimas <[email protected]>
Reviewed-by: Simon Glass <[email protected]>
Tested-by: Simon Glass <[email protected]>  # chromebook-link

1 files changed, 4 insertions, 0 deletions

diff --git a/boot/Kconfig b/boot/Kconfig
index a212f262274..11175fb7bb2 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