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The Linux IMA (Integrity Measurement Architecture) subsystem used for secure boot, file integrity, or remote attestation cannot be a loadable module for few reasons listed below: o Boot-Time Integrity: IMA’s main role is to measure and appraise files before they are used. This includes measuring critical system files during early boot (e.g., init, init scripts, login binaries). If IMA were a module, it would be loaded too late to cover those. o TPM Dependency: IMA integrates tightly with the TPM to record measurements into PCRs. The TPM must be initialized early (ideally before init_ima()), which aligns with IMA being built-in. o Security Model: IMA is part of a Trusted Computing Base (TCB). Making it a module would weaken the security model, as a potentially compromised system could delay or tamper with its initialization. IMA must be built-in to ensure it starts measuring from the earliest possible point in boot which inturn implies TPM must be initialised and ready to use before IMA. To enable integration of tpm_event_log with the IMA subsystem, the TPM drivers (tpm_crb and tpm_crb_ffa) also needs to be built-in. However with FF-A driver also being initialised at device initcall level, it can lead to an initialization order issue where: - crb_acpi_driver_init() may run before tpm_crb_ffa_driver()_init and ffa_init() - As a result, probing the TPM device via CRB over FFA is deferred - ima_init() (called as a late initcall) runs before deferred probe completes, IMA fails to find the TPM and logs the below error: | ima: No TPM chip found, activating TPM-bypass! Eventually it fails to generate boot_aggregate with PCR values. Because of the above stated dependency, the ffa driver needs to initialised before tpm_crb_ffa module to ensure IMA finds the TPM successfully when present. [ jarkko: reformatted some of the paragraphs because they were going past the 75 character boundary. ] Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com> Reviewed-by: Mimi Zohar <zohar@linux.ibm.com> Reviewed-by: Sudeep Holla <sudeep.holla@arm.com> Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org> Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org> |
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