linux-kernel - Re: [PATCH v2 0/2] fix failure of integration IMA with tpm_crb

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Message-ID: <aEgwpXXftXW6JNRy@e129823.arm.com>
Date: Tue, 10 Jun 2025 14:18:29 +0100
From: Yeoreum Yun <yeoreum.yun@....com>
To: Jarkko Sakkinen <jarkko@...nel.org>
Cc: sudeep.holla@....com, peterhuewe@....de, jgg@...pe.ca,
	stuart.yoder@....com, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org, linux-integrity@...r.kernel.org
Subject: Re: [PATCH v2 0/2] fix failure of integration IMA with tpm_crb_ffa

Hi Jarkko,

> On Tue, Jun 10, 2025 at 07:03:32AM +0100, Yeoreum Yun wrote:
> > To integrate a TPM device that uses CRB over FF-A with the IMA subsystem,
> > both the tpm_crb and tpm_crb_ffa drivers must be built as built-in
> > (i.e., ARM_FFA_TRANSPORT=y, CONFIG_TCG_CRB=y, and CONFIG_TCG_CRB_FFA=y),
> > because IMA itself is built-in and the TPM device must be probed
> > before ima_init() is invoked during IMA subsystem initialization.
>
> The description of the problem and motivation to solve it should be
> first; not the actions taken.

Okay. I'll describe the problem first.

>
> >
> > To ensure this works correctly, the following initcalls must be executed in order:
> > 	1.	ffa_init()
> > 	2.	tpm_crb_ffa_driver_init()
> > 	3.	crb_acpi_driver_init()
> >
> > Unfortunately, the order of these device initcalls cannot be strictly controlled.
> > As a result:
> > 	1.	ffa_init() may be called after tpm_crb_ffa_driver_init()
> > 	2.	tpm_crb_ffa_driver_init() may be called after crb_acpi_driver_init()
> >
> > For example, the following initcall sequence may occur:
> >   0000000000000888 l  .initcall6.init>  crb_acpi_driver_init
> >   000000000000088c l  .initcall6.init>  tpm_crb_ffa_driver_init
>
> This symbol does not exist.

I don't know you said "This symbol does not exit".
When CONFIG_TCG=CRB=y, CONFIG_TCG_CRB_FFA=y and ARM_FFA_TRANSPORT=y,
above symbols exist.

crb_acpi_driver_init() generated by
  module_acpi_driver(crb_acpi_driver);
in tpm_crb.c

and tpm_crb_ffa_driver_init() generated by
  module_ffa_driver(tpm_crb_ffa_driver);
in tpm_crb_ffa.c so you can get the above symbols:

$ llvm-readelf-21 --symbols vmlinux | grep crb | grep init
171332: ffff80008203ef08    56 FUNC    LOCAL  DEFAULT    19 crb_acpi_driver_init
...
370077: ffff800080d650d8    92 FUNC    GLOBAL DEFAULT     2 tpm_crb_ffa_init

>
> >   0000000000000a9c l  .initcall6.init>  ffa_init
> >
> > In this situation, the IMA subsystem fails to integrate with the TPM device
> > because the TPM was not available at the time ima_init() was called.
> > As a result, you may see the following message in the kernel log:
> >
> >   | ima: No TPM chip found, activating TPM-bypass!
>
> TPM initializes before IMA, so there should not be a problem.

If you see my commit message it describes the situation why this happen.
when crb_acpi_driver_init() is called but before tpm_crb_ffa_init() is
called, the secure partition doesn't probe. so crb_acpi_driver_init()
would be failed wiith -EPROBE.

In this situation, init_ima() which call ima_init() can be called first.
NOTE, init_ima() is deployed in late_initcall and
the "deferred_probe device" is tried again in
deferred_probe late initcall.
However, even the deferred_probe can be call later then init_ima().

000000000000012c l       .initcall7.init>-------0000000000000000 init_ima
000000000000016c l       .initcall7.init>-------0000000000000000 deferred_probe_initcall7

That's why init_ima() is failed to init with TPM when It is deffered.

Would you let me know why you said it's not a problem?

--
Sincerely,
Yeoreum Yun