Re: [PATCH] char: tpm: ftpm_tee: use kernel login identifier

[Etienne Carriere]

Hello Jarkko,

On Thu, 11 May 2023 at 00:12, Jarkko Sakkinen <jarkko@xxxxxxxxxx> wrote:
>
> On Fri May 5, 2023 at 9:43 PM EEST, Etienne Carriere wrote:
> > Changes fTPM TEE driver to open the TEE session with REE kernel login
> > identifier rather than public login. This is needed in case fTPM service
> > it denied to user land application and restricted to kernel operating
> > system services only.
> >
> > Signed-off-by: Etienne Carriere <etienne.carriere@xxxxxxxxxx>
>
>
> Can you bring up a little context here?
>
> What is REE login?
> Does it break backwards compatibility to switch?
> What kind of scenario we are talking about? What does it mean in plain
> English when fTPM service is denied.
> What is fTPM service?

By fTPM service I meant the services exposed by fTPM through its
OP-TEE interface, that are the commands a client can invoke in fTPM,
see [1].

Regarding backward compatibility, this change is backward compatible
as far as the OP-TEE entity this driver communicates with is of
revision 3.9.0 or above.
I understand this case should be addressed in some way.

In current implementation, fTPM can be invoked by Linux kernel drivers
(through Linux kernel tee API as tpm_ftpm_tee currently does) as well
as by userland application (through TEE client library API [2]).
This change makes tpm_ftpm_tee to invoke fTPM interface using a client
identifier stating it is the Linux kernel that invokes it, not a
userland application. fTPM implementation does not check the client
identity when a client opens a session toward it. Therefore using a
public identifier (TEE_IOCTL_LOGIN_PUBLIC) or the OS privilege
identifier (TEE_IOCTL_LOGIN_REE_KERNEL) does not matter, as far as
OP-TEE supports these IDs. The former is native to OP-TEE initial UAPI
[3], the latter was introduced in OP-TEE 3.9.0 [4] and Linux kernel
v5.8 [5].

That said, this change does fix an existing issue in fTPM integration.
The fTPM entity currently only accepts a single session opened towards
it. This is enforced as fTPM sets property TA_FLAG_SINGLE_INSTANCE and
does not set property TA_FLAG_MULTI_SESSION [6].
Linux kernel tpm_ftpm_tee driver currently opens a session to fTPM at
probe time and releases it at remove time so once the driver is
successfully probed, no userland application can use TEE userland
client API to open another session and communicate with fTPM.

[1] https://github.com/microsoft/ms-tpm-20-ref/blob/main/Samples/ARM32-FirmwareTPM/optee_ta/fTPM/fTPM.c#L456
[2] https://github.com/OP-TEE/optee_client
[3] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=967c9cca2cc50569efc65945325c173cecba83bd
[4] https://github.com/OP-TEE/optee_os/commit/78f462f646e7c037bea13aa6282c81f255922a4f
[5] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=104edb94cc4b3101bab33161cd861de13e85610b
[6] https://github.com/microsoft/ms-tpm-20-ref/blob/main/Samples/ARM32-FirmwareTPM/optee_ta/fTPM/user_ta_header_defines.h#L47

Regards,
Etienne

>
> > ---
> >  drivers/char/tpm/tpm_ftpm_tee.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/char/tpm/tpm_ftpm_tee.c b/drivers/char/tpm/tpm_ftpm_tee.c
> > index 528f35b14fb6..6d32e260af43 100644
> > --- a/drivers/char/tpm/tpm_ftpm_tee.c
> > +++ b/drivers/char/tpm/tpm_ftpm_tee.c
> > @@ -241,7 +241,7 @@ static int ftpm_tee_probe(struct device *dev)
> >       /* Open a session with fTPM TA */
> >       memset(&sess_arg, 0, sizeof(sess_arg));
> >       export_uuid(sess_arg.uuid, &ftpm_ta_uuid);
> > -     sess_arg.clnt_login = TEE_IOCTL_LOGIN_PUBLIC;
> > +     sess_arg.clnt_login = TEE_IOCTL_LOGIN_REE_KERNEL;
> >       sess_arg.num_params = 0;
> >
> >       rc = tee_client_open_session(pvt_data->ctx, &sess_arg, NULL);
> > --
> > 2.25.1
>
> BR, Jarkko