[tpmdd-devel] [PATCH RESEND 3/3] tpm-chip: Export TPM device to user space even when startup failed

[Alexander.Steffen at infineon.com]

> On Wed, Aug 30, 2017 at 12:41:51PM +0200, Peter Huewe wrote:
> >
> >
> > Am 30. August 2017 12:15:10 MESZ schrieb Jarkko Sakkinen
> <jarkko.sakkinen at linux.intel.com>:
> > >On Tue, Aug 29, 2017 at 03:17:39PM +0200, Michal Suchánek wrote:
> > >> Hello,
> > >>
> > >> On Tue, 29 Aug 2017 15:55:09 +0300
> > >> Jarkko Sakkinen <jarkko.sakkinen at linux.intel.com> wrote:
> > >>
> > >> > On Mon, Aug 28, 2017 at 05:15:58PM +0000,
> > >> > Alexander.Steffen at infineon.com wrote:
> > >> > > But is that just because nobody bothered to implement the
> > >necessary
> > >> > > logic or for some other reason?
> > >> >
> > >> > We do not want user space to access broken hardware. It's a huge
> > >risk
> > >> > for system stability and potentially could be used for evil
> > >purposes.
> > >> >
> > >> > This is not going to mainline as it is not suitable for general
> > >> > consumption. You must use a patched kernel if you want this.
> > >> >
> > >> > /Jarkko
> > >> >
> > >>
> > >> It has been pointed out that userspace applications that use direct
> > >IO
> > >> access exist for the purpose. So using a kernel driver is an
> > >> improvement over that if the interface is otherwise sane.
> > >>
> > >> What do you expect is the potential for instability or evil use?
> > >
> > >By definition the use of broken hardware can have unpredictable
> > >effects.
> > >Use a patched kernel if you want to do it.
> >
> > If the s.m.a.r.t selftest of your hard disk fails, you can still
> > access it, even though the hw selftest says it is broken.
> > Same situation.
> 
> Not sure if you can compare these directly although I get your point.
> 
> Waiting for more comments on this. At the moment I'm still dilated to
> restricted access because it gives more variables for the future.
> 
> /Jarkko

I think, this is a perfect example. In both cases we have intelligent devices, that can detect that they are broken and report it correctly (instead of just malfunction randomly). This also shows that those devices are not so broken that they completely misbehave (i.e. significantly violate their spec), they just do not provide the full functionality anymore.

Could you give me one example how you'd imagine such a device to impact stability? My patch already prevents the kernel from using broken TPMs for anything, it only provides the communication facilities for user space applications.

With regard to security/attacks, it is precisely the TPM's job to protect itself (and your secrets). You get all your security guarantees from the TPM, not the driver code, so it does not matter what the driver does, even with a broken TPM.

I understand your point of not wanting to enable functionality that you cannot disable anymore, but I fail to see why you'd ever want to disable (part of) this functionality again. Knowing more about the potential problems would allow me to prevent them from the beginning or to come up with a better safeguard than the command-based whitelisting.

Alexander
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