From 23a82e87ee333c573796de5bd164cd21684fe58c Mon Sep 17 00:00:00 2001 From: Julius Werner Date: Tue, 31 Mar 2020 13:32:10 -0700 Subject: [PATCH] security/tpm: Fix compile-time elimination for SEPARATE_VERSTAGE CB:35077 pulled TPM measurement code into the bootblock, with the catch that we'll only cache PCR extensions and not actually write them to the TPM until it gets initialized in a later stage. The goal of this was to keep the heavy TPM driver code out of the size-constrained bootblock. Unfortunately, a small mistake in the tspi_tpm_is_setup() function prevents the compiler from eliminating references to the TPM driver code in the bootblock on platforms with CONFIG_VBOOT and CONFIG_SEPARATE_VERSTAGE. In those cases vboot_logic_executed() is known at compile-time to be 0, but that still makes the final expression `return 0 || tpm_is_setup;`. We know that tpm_is_setup can never be set to 1 in the bootblock, but the compiler doesn't. This patch rewrites the logic slightly to achieve the same effect in a way that the compiler can follow (because we only really need to check tpm_is_setup in the stage that actually runs the vboot code). Signed-off-by: Julius Werner Change-Id: Idc25acf1e6c02d929639e83d529cc14af80e0870 Reviewed-on: https://review.coreboot.org/c/coreboot/+/39993 Reviewed-by: Aaron Durbin Reviewed-by: Bill XIE Reviewed-by: Werner Zeh Reviewed-by: Christian Walter Tested-by: build bot (Jenkins) --- src/security/tpm/tspi/tspi.c | 14 ++++++++++++-- 1 file changed, 12 insertions(+), 2 deletions(-) diff --git a/src/security/tpm/tspi/tspi.c b/src/security/tpm/tspi/tspi.c index 4f0cc972a7..b94a0fb029 100644 --- a/src/security/tpm/tspi/tspi.c +++ b/src/security/tpm/tspi/tspi.c @@ -104,8 +104,18 @@ static uint32_t tpm_setup_epilogue(uint32_t result) static int tpm_is_setup; static inline int tspi_tpm_is_setup(void) { - if (CONFIG(VBOOT)) - return vboot_logic_executed() || tpm_is_setup; + /* + * vboot_logic_executed() only starts returning true at the end of + * verstage, but the vboot logic itself already wants to extend PCRs + * before that. So in the stage where verification actually runs, we + * need to check tpm_is_setup. Skip that check in all other stages so + * this whole function can be evaluated at compile time. + */ + if (CONFIG(VBOOT)) { + if (verification_should_run()) + return tpm_is_setup; + return vboot_logic_executed(); + } if (ENV_RAMSTAGE) return tpm_is_setup;