Re: [RFC PATCH 1/4] security: ima: move ima_init into late_initcall_sync

[Mimi Zohar <zohar@linux.ibm.com>]

On Tue, 2026-04-21 at 13:50 +0100, Yeoreum Yun wrote:
> Hi Mimi,
> 
> > On Fri, 2026-04-17 at 18:57 +0100, Yeoreum Yun wrote:
> > > To generate the boot_aggregate log in the IMA subsystem with TPM PCR values,
> > > the TPM driver must be built as built-in and
> > > must be probed before the IMA subsystem is initialized.
> > > 
> > > However, when the TPM device operates over the FF-A protocol using
> > > the CRB interface, probing fails and returns -EPROBE_DEFER if
> > > the tpm_crb_ffa device — an FF-A device that provides the communication
> > > interface to the tpm_crb driver — has not yet been probed.
> > > 
> > > To ensure the TPM device operating over the FF-A protocol with
> > > the CRB interface is probed before IMA initialization,
> > > the following conditions must be met:
> > > 
> > >    1. The corresponding ffa_device must be registered,
> > >       which is done via ffa_init().
> > > 
> > >    2. The tpm_crb_driver must successfully probe this device via
> > >       tpm_crb_ffa_init().
> > > 
> > >    3. The tpm_crb driver using CRB over FF-A can then
> > >       be probed successfully. (See crb_acpi_add() and
> > >       tpm_crb_ffa_init() for reference.)
> > > 
> > > Unfortunately, ffa_init(), tpm_crb_ffa_init(), and crb_acpi_driver_init() are
> > > all registered with device_initcall, which means crb_acpi_driver_init() may
> > > be invoked before ffa_init() and tpm_crb_ffa_init() are completed.
> > > 
> > > When this occurs, probing the TPM device is deferred.
> > > However, the deferred probe can happen after the IMA subsystem
> > > has already been initialized, since IMA initialization is performed
> > > during late_initcall, and deferred_probe_initcall() is performed
> > > at the same level.
> > > 
> > > To resolve this, move ima_init() into late_inicall_sync level
> > > so that let IMA not miss TPM PCR value when generating boot_aggregate
> > > log though TPM device presents in the system.
> > > 
> > > Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
> > 
> > IMA should be initialized as early as possible. I'm really hesitant to defer
> > ima_init() to late_initcall_sync() for systems that the TPM is currently
> > initialized in time. For these systems, continue initializing IMA at
> > late_initcall(). As a compromise for those systems that the TPM isn't properly
> > initialized in time, define and instantiate the late_initcall_sync().
> > 
> > ima_init() would need to differentiate between the late_initcall and
> > late_initcall_sync.  On late_initcall(), instead of saying "No TPM chip found,
> > activating TPM-bypass!",  it should say "No TPM chip found, deferring to
> > late_initcall_sync" or something similar.
> 
> But can we really move those initialisations to be called again?
> 
> I am referring to functions such as ima_init_crypto(),
> ima_add_boot_aggregate(), and ima_measure_critical_data() in ima_init()—
> first without TPM, and then a second time once TPM becomes available.
> I don’t think that approach would work.
> 
> In other words, unless tpm_default_chip() can differentiate between a TPM
> device that is deferred and one that does not exist, we cannot distinguish
> between the “defer” case and “-EEXIST”.
> 
> It might be possible if the TPM core tracked the state when a driver returns
> -EPROBE_DEFER, but I am not sure that is the right approach.
> For deferred probe cases, the “device initialised in time” check should
> likely be done at late_initcall_sync, rather than late_initcall.
> 
> This implies that any such check performed before late_initcall_sync
> does not reflect a valid state, as it cannot distinguish between “not
> present” and “deferred”.
> 
> Therefore, I think the TPM check in IMA should be performed at
> late_initcall_sync.
> 
> 
> Am I missing something?

In ima_init() you short circuit out, when called by late_initcall(), if the TPM
hasn't been initialized.  So the rest of the ima_init() isn't called.  Roughly
something like this (needs some cleanup):

int __init ima_init(void)
{
        static int first = 1;
        int rc;

        if (ima_tpm_chip)
                return 0;

        ima_tpm_chip = tpm_default_chip();
        if (!ima_tpm_chip && first) {
                pr_info("No TPM chip found, deferring te late_initcall_sync()\n");
                first = 0;
                return 0;
        }

The rest of the code remains the same.

Mimi