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

[Fuchs, Andreas]

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.

Cheers,
Andreas
________________________________________
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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