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

[Etienne Carriere]

On Thu, 11 May 2023 at 10:15, Sumit Garg <sumit.garg@xxxxxxxxxx> wrote:
>
> On Thu, 11 May 2023 at 10:36, Etienne Carriere
> <etienne.carriere@xxxxxxxxxx> wrote:
> >
> > Dearl all,
> >
> > Typo in my previous post!
> >
> > On Thu, 11 May 2023 at 06:47, Etienne Carriere
> > <etienne.carriere@xxxxxxxxxx> wrote:
> > >
> > > 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.
> >
> > Typo, sorry, I meant
> > "That said, this change does **NOT** fix an existing issue in fTPM integration."
> >
> > BR,
> > Etienne
> >
> > > 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.
>
> How about if the fTPM TEE kernel driver is built as a module and
> removed at runtime by a malicious user-space client?

This is why this change can help to lower the attack surface IMHO.

Note that if a malicious user manages to load a malicious module,
there is nothing OP-TEE or fTPM can do about it. I guess it is the
same situation for all tpm drivers in the Linux kernel.

etienne

>
> -Sumit
>
> > >
> > > [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