[PATCH v2 08/10] tpm: Add the RNG child device

Sun Mar 6 08:03:22 CET 2022

hi Simon,

On Sun, 6 Mar 2022 at 08:37, Simon Glass <sjg at chromium.org> wrote:
>
> Hi Sughosh,
>
> On Fri, 4 Mar 2022 at 06:44, Sughosh Ganu <sughosh.ganu at linaro.org> wrote:
> >
> > hi Simon,
> >
> > On Fri, 4 Mar 2022 at 08:07, Simon Glass <sjg at chromium.org> wrote:
> > >
> > > Hi Sughosh,
> > >
> > > On Thu, 3 Mar 2022 at 05:11, Sughosh Ganu <sughosh.ganu at linaro.org> wrote:
> > > >
> > > > hi Simon,
> > > >
> > > > On Thu, 3 Mar 2022 at 09:18, Simon Glass <sjg at chromium.org> wrote:
> > > > >
> > > > > Hi Sughosh,
> > > > >
> > > > > On Tue, 1 Mar 2022 at 21:53, Sughosh Ganu <sughosh.ganu at linaro.org> wrote:
> > > > > >
> > > > > > hi Simon,
> > > > > >
> > > > > > On Tue, 1 Mar 2022 at 20:29, Simon Glass <sjg at chromium.org> wrote:
> > > > > > >
> > > > > > > Hi Sughosh,
> > > > > > >
> > > > > > > On Mon, 28 Feb 2022 at 05:07, Sughosh Ganu <sughosh.ganu at linaro.org> wrote:
> > > > > > > >
> > > > > > > > The TPM device comes with the random number generator(RNG)
> > > > > > > > functionality which is built into the TPM device. Add logic to add the
> > > > > > > > RNG child device in the TPM uclass post probe callback.
> > > > > > > >
> > > > > > > > The RNG device can then be used to pass a set of random bytes to the
> > > > > > > > linux kernel, need for address space randomisation through the
> > > > > > > > EFI_RNG_PROTOCOL interface.
> > > > > > > >
> > > > > > > > Signed-off-by: Sughosh Ganu <sughosh.ganu at linaro.org>
> > > > > > > > ---
> > > > > > > >
> > > > > > > > Changes since V1: None
> > > > > > > >
> > > > > > > >  drivers/tpm/tpm-uclass.c | 58 +++++++++++++++++++++++++++++++++++++---
> > > > > > > >  1 file changed, 54 insertions(+), 4 deletions(-)
> > > > > > > >
> > > > > > > > diff --git a/drivers/tpm/tpm-uclass.c b/drivers/tpm/tpm-uclass.c
> > > > > > > > index 8619da89d8..383cc7bc48 100644
> > > > > > > > --- a/drivers/tpm/tpm-uclass.c
> > > > > > > > +++ b/drivers/tpm/tpm-uclass.c
> > > > > > > > @@ -16,6 +16,11 @@
> > > > > > > >  #include <tpm-v2.h>
> > > > > > > >  #include "tpm_internal.h"
> > > > > > > >
> > > > > > > > +#include <dm/lists.h>
> > > > > > > > +
> > > > > > > > +#define TPM_RNG1_DRV_NAME      "tpm1-rng"
> > > > > > > > +#define TPM_RNG2_DRV_NAME      "tpm2-rng"
> > > > > > > > +
> > > > > > > >  bool is_tpm1(struct udevice *dev)
> > > > > > > >  {
> > > > > > > >         return IS_ENABLED(CONFIG_TPM_V1) && tpm_get_version(dev) == TPM_V1;
> > > > > > > > @@ -147,12 +152,57 @@ int tpm_xfer(struct udevice *dev, const uint8_t *sendbuf, size_t send_size,
> > > > > > > >         return 0;
> > > > > > > >  }
> > > > > > > >
> > > > > > > > +#if IS_ENABLED(CONFIG_TPM)
> > > > > > >
> > > > > > > This should be in the Makefile so that we only build this file if TPM
> > > > > > > is enabled.
> > > > > >
> > > > > > The Makefile allows for the tpm uclass driver to be built for SPL and
> > > > > > TPL stages as well. The addition of the RNG device is to be done only
> > > > > > in the u-boot proper stage, since we enable RNG support only in u-boot
> > > > > > proper. Thanks.
> > > > >
> > > > > Well in that case, create a new SPL_TPM_RAND or similar to control
> > > > > enabling it in SPL. It should be explicit.
> > > >
> > > > I think it is easier to just protect the child addition functions
> > > > under CONFIG_TPM rather than create SPL_RNG and TPL_RNG symbols. We
> > > > don't have any requirement for generating random numbers in the SPL
> > > > and TPL stages. I feel that creating new symbols just for the sake of
> > > > not putting a check for CONFIG_TPM is a bit of an overkill, especially
> > > > since we do not have any requirement for RNG devices in the SPL/TPL
> > > > stages.
> > >
> > > What does checking for CONFIG_TPM have to do with SPL and TPL? If that
> > > option is enabled, the feature will be active in SPL and TPL too.
> >
> > Maybe I am not explaining it properly. We need the addition of the RNG
> > child device only in the u-boot proper stage, not in the SPL and TPL
> > stages. The TPM uclass driver can indeed be built for the SPL and TPL
> > stages, while the RNG uclass is needed only for u-boot proper. So, the
> > addition of the RNG child device done in the TPM uclass driver should
> > only happen in u-boot proper, and not in SPL/TPL stages. Which is the
> > reason for the CONFIG_TPM check.
>
> Let me try one more time. If this doesn't work, please chat with a colleague.
>
> The way we do that is with Kconfig options. We don't use ad-hoc
> methods to enable things in U-Boot proper but not SPL. Boards that
> have CONFIG_TPM set have it set everywhere, including in SPL. So
> checking for CONFIG_TPM is going to return true in both U-Boot proper
> and SPL. If you want different behaviour in SPL, you need
> CONFIG_SPL_TPM. But even that is indirect, as I have explained. To
> enable your driver in SPL, you need a CONFIG_SPL_...

Okay. I now get your point. So this is because these config symbols
are getting included in all the stages, so I cannot use the symbol
name to identify the spl/tpl/u-boot stages. I believe earlier, the
identification used to be done based on whether a symbol is CONFIG_xxx
as against CONFIG_SPL_xxx. Now it is being done with
CONFIG_{SPL,TPL}_BUILD symbols for differentiating between stages. I
will use these instead to have the code enabled only for the u-boot
proper stage. Thing is, I don't want to define these functions for the
spl and tpl stages since they are adding to the size with no benefits.
I do understand that currently no platform is enabling tpm drivers in
the spl/tpl stage, but the driver can be enabled in those stages.

>
> Also, you should use devicetree to provide the random number
> generator, just a device_bind(). Add the node as a subnode of the TPM.
> Otherwise it cannot work with OF_PLATDATA.

As per the documentation in of-plat.rst, the OF_PLATDATA method is to
be used only for the SPL/TPL stages where there might be a size
constraint. And the tpm rng drivers are to be enabled only in the
u-boot proper stage. So I think we would not need to put a subnode in
the devicetree. Basically, the presence of the rng functionality in
the TPM device is not optional as per my understanding, so the rng
child node can just be added in the tpm uclass driver.

>
> >
> > >
> > > Also I see another problem, on further examination. You cannot start
> > > up the TPM in the pre_probe() function. That needs to be done under
> > > board control. E.g. for coral it happens in the TPL (or soon VPL)
> > > phase so cannot be done again in U-Boot proper.
> >
> > I tested running the RNG command after the TPM device has already been
> > probed and tpm_startup has been called. Even if I call the tpm_startup
> > again, I do not see any issues. Does the TPM spec prohibit calling the
> > initialisation function multiple times. I believe that the TPM device
> > should be able to handle this scenario right?
>
> I hangs on coral, for example.
>
> >
> > >
> > > So perhaps we need to remember the state of the TPM (using SPL handoff
> > > perhaps). Also you probably need to move the startup stuff to the RNG
> > > itself.
> >
> > I can move the call to tpm_startup to the RNG driver's probe function.
> > But I thought it is better that the parent device(TPM) is initialised
> > before calling the probe of the child device.
>
> We use lazy init in U-Boot, so it should be possible to probe the TPM
> without automatically starting it. Otherwise you are going to break
> some of the tpm commands.

Yes I understand. With the patch, the tpm device is being started only
when a child device is being probed. But I can remove the startup of
the tpm device from the child_pre_probe function. It will have to be
done explicitly before the rng device is to be used.

Btw, can you please comment on patch 3 of this series[1].

-sughosh

[1] - https://lists.denx.de/pipermail/u-boot/2022-March/476830.html

>
> >
> > >
> > > Perhaps we could add a new function to handle this, which can be
> > > called from your rand driver.
> > >
> > > int tpm_ensure_started(struct udevice *dev, enum tpm_startup_type mode)
> >
> > Okay, I can add this, but the question is, does calling the
> > tpm_startup function cause issues? If not, maybe this is not needed?
>
> It does on coral. I don't know whether other TPMs are tolerant of it,
> but in any case it is not a good idea.
>
> Regards,
> Simon