Re: TPM selftest failure in 4.15

Mimi Zohar
2018-02-09T21:23:51Z

On Fri, 2018-02-09 at 08:23 -0800, James Bottomley wrote:
> On Fri, 2018-02-09 at 07:26 -0500, Mimi Zohar wrote:
> > On Fri, 2018-02-09 at 12:02 +0200, Jarkko Sakkinen wrote:
> > >
> > > On Thu, Feb 08, 2018 at 09:02:00AM -0800, James Bottomley wrote:
> > > >
> > > > There is an identified regression: the TPM driver will now
> > > > periodically
> > > > fail to attach. However, there's no point reviewing until we
> > > > agree
> > > > what the fix is. I was just waiting to verify this fixed my
> > > > problem
> > > > (which means seeing the messages it spits out proving the TPM has
> > > > remained in self test). I have now seen this and the driver
> > > > still
> > > > works, so I can submit a formal patch.
> > >
> > > For the self-test the duration falls down to 2 seconds as the specs
> > > do
> > > not contain any well-defined duration for it, or at least I haven't
> > > found it.
> > >
> > > I see three alternative ways the fix the self-test:
> > >
> > > 1. Execute self-test with fullTest = YES.
> > > 2. Have a flag TPM_CHIP_TESTING that is set when the self-test is
> > > started. Issue a warning on time-out. Check for this flag in
> > > tpm_transmit_cmd() and tpm_write() and resend self-test command
> > > if the flag is stil test before the actual command. Return
> > > -EBUSY
> > > and print a warning if self-test is still active.
> > > 3. Increase the duration to the "correct" value if we have one.
> >
> > Please take into account that the TPM must complete initialization
> > BEFORE IMA looks for the TPM, otherwise IMA goes into TPM-bypass
> > mode. This consistently happens with a full self-test (option 1).
> > Increasing the wait duration will most likely cause this to happen as
> > well (option 2).
> >
> > Based on James' patch description, which says "There are various
> > theories that resending the self-test command actually causes the
> > tests to restart and thus triggers more TPM_RC_TESTING returns until
> > the timeout is exceeded", I think IMA's detecting the TPM, by doing a
> > PCR read, will cause the self-test to restart (option 2).
>
> Actually, no, only doing another selftest seems to restart. Any other
> command will either get a normal return (if the TPM has already tested
> the subsystem) or an RC_TESTING return indicate it's still being
> tested.

Jarkko's option 2 suggested resending the self-test, thus my comment.

> It seems that the PCRs are one of the earliest things to come
> on-line, so IMA always works. This is on my current laptop where I'm
> running this patch with IMA: I no longer see the IMA bypass message and
> IMA comes up normally.

Right, as long as the TPM isn't doing a full self-test.

Mimi