Re: [tpmdd-devel] [PATCH 4/4] keys, trusted: seal/unseal with TPM 2.0 chips

[Jarkko Sakkinen]

On Sun, Oct 04, 2015 at 06:57:42PM +0000, Fuchs, Andreas wrote:
> Hi Jarkko,
> 
> thanks for the clearification...
> 
> However, I'd recommend against doing so.
>
> Furthermore, if there is a resource-manager running in userspace,
> applications only get virtual handles and TPM might be empty
> actually...
> 
> If that's what you're aiming for, I'd recommend passing the pointer to
> a context-saved-blob and have the kernel load the key this way. That
> ensures no problems with resource-manager and handle-mixups.

TPM 1.x interface has the same race if you do not use the default value
for the 'keyhandle' option.

In practice a processs in TCB (or root) would do all the keyctl magic so
I do not see huge issue here. It can be orchestrated by the
OS/distribution. From my point of view you are over-engineering in wrong
place.

It would be easy to add a way to provide the sealing key as blob later
on if the simple approach chosen would not be sufficient. I'm confident
that for 99% of all real-world use cases the interface provided by the
patch set is sufficient.

> Cheers,
> Andreas

/Jarkko

> From: Jarkko Sakkinen [jarkko.sakkinen@xxxxxxxxxxxxxxx]
> Sent: Saturday, October 03, 2015 12:26
> To: Fuchs, Andreas
> Cc: tpmdd-devel@xxxxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; David Howells; gregkh@xxxxxxxxxxxxxxxxxxx; open list:KEYS-TRUSTED; open list:KEYS-TRUSTED; James Morris; David Safford; akpm@xxxxxxxxxxxxxxxxxxxx; Serge E. Hallyn
> Subject: Re: [tpmdd-devel] [PATCH 4/4] keys,    trusted: seal/unseal with TPM 2.0 chips
> 
> On Sat, Oct 03, 2015 at 10:00:59AM +0000, Fuchs, Andreas wrote:
> > Hi Jarkko,
> >
> > [snip]
> >
> > diff --git a/security/keys/trusted.h b/security/keys/trusted.h
> > index ff001a5..fc32c47 100644
> > --- a/security/keys/trusted.h
> > +++ b/security/keys/trusted.h
> > @@ -12,6 +12,13 @@
> >  #define TPM_RETURN_OFFSET              6
> >  #define TPM_DATA_OFFSET                        10
> >
> > +/* Transient object handles start from 0x80000000 in TPM 2.0, which makes it
> > + * a sane default.
> > + */
> > +
> > +#define TPM1_SRKHANDLE 0x40000000
> > +#define TPM2_SRKHANDLE 0x80000000
> > +
> >  #define LOAD32(buffer, offset) (ntohl(*(uint32_t *)&buffer[offset]))
> >  #define LOAD32N(buffer, offset)        (*(uint32_t *)&buffer[offset])
> >  #define LOAD16(buffer, offset) (ntohs(*(uint16_t *)&buffer[offset]))
> >
> > This TPM2_SRKHANDLE is unfortunately wrong.
> >
> > Transient handles are assigned and returned by the TPM following the
> > commands TPM2_CreatePrimary, TPM2_LoadObject and TPM2_ContextLoad. You
> > can only use transient handles as returned by the TPM in order to
> > refer to the corresponding object created inside the TPM via these
> > commands. They can never assumed to be constant. The fact that TPMs
> > return 0x80000000 for the first loaded Object and 0x80000001 for the
> > second is merely a coincidence... ;-)
> >
> > TPM2 also has no (single) SRK anymore. You have to create your own SRK
> > / Storage Primary Keys via TPM2_CreatePrimary and use the transient
> > handle returned from there. This however requires SH-authorization,
> > usually via Policy IMHO, so not easy to manage. So IMHO, this might be
> > something for the future but for the moment relying on a persistent
> > key would be better...
> >
> > For persistent SRKs it should become a convention to have those
> > around. Those handles start with 0x81000000 and the SRKs (or Storage
> > primary Keys) shall live within 0x81000000 to 0x8100FFFF (see
> > http://www.trustedcomputinggroup.org/resources/registry_of_reserved_tpm_20_handles_and_localities)
> >
> > I'd recommend to rely on the existence of a handle inside this range
> > with an empty auth-value. So maybe install a persistent SRK to
> > 0x81000000 via TPM2_EvictControl and then use this from within the
> > kernel for anything following.
> > P.S. You should check for the key's TPMA_OBJECT to have fixedTPM SET.
> > I don't know if there is an actual test for owner-generated SRK
> > testing. I'll ask around though...
> >
> > Note: you can query for handles in this range via
> > TPM2_GetCapability(TPM_CAP_HANDLES, 0x81000000) and then look for
> > fitting keys.
> >
> >
> > Feel free to discuss other approaches.
> 
> I'm fully aware of all what you said. My take was to use 0x800000000 as
> a default value if you don't the handle ID explicitly in 'description'
> parameter of the add_key() syscall.
> 
> > Cheers,
> > Andreas
> 
> /Jarkko
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