Re: One question about trusted key of keyring in Linux kernel.

[James Bottomley <jejb@linux.ibm.com>]

On Mon, 2019-12-02 at 05:55 +0000, Zhao, Shirley wrote:
> Thanks for your feedback, James.
> 
> The policy is generated by TPM command, tpm2_createpolicy, it just
> use the algorithm you mentioned, which is defined in TPM spec. 
> I re-attach my test steps as below. 
> Please help check it, is there anything wrong, especially the format
> of keyctl command. 
> 
> Firstly, the pcr policy is generated as below: 
> $ tpm2_createpolicy --policy-pcr --pcr-list sha256:7 --policy
> pcr7_bin.policy > pcr7.policy

I don't use the Intel TSS, so I can't help you with this command: you
need to ask someone who does use it it, like Phil.

> Pcr7.policy is the ascii hex of policy:
> $ cat pcr7.policy
> 321fbd28b60fcc23017d501b133bd5dbf2889814588e8a23510fe10105cb2cc9

You haven't provided enough information.  If you tell me what the pcr7
value you tied the policy to is, I can run it through the IBM TSS
policy maker and tell you if this is the correct hash.  But obviously,
since it's a hash, I can't reverse it to tell you what the policy it
mandates is.

James

> Then generate the trusted key and configure policydigest and get the
> key ID: 
> $ keyctl add trusted kmk "new 32 keyhandle=0x81000001 hash=sha256
> policydigest=`cat pcr7.policy`" @u
> 874117045
> 
> Save the trusted key. 
> $ keyctl pipe 874117045 > kmk.blob
> 
> Reboot and load the key. 
> Start a auth session to generate the policy:
> $ tpm2_startauthsession -S session.ctx
> session-handle: 0x3000000
> $ tpm2_pcrlist -L sha256:7 -o pcr7.sha256 $ tpm2_policypcr -S
> session.ctx -L sha256:7 -F pcr7.sha256 -f pcr7.policy
> policy-digest:
> 0x321FBD28B60FCC23017D501B133BD5DBF2889814588E8A23510FE10105CB2CC9
> 
> Input the policy handle to load trusted key:
> $ keyctl add trusted kmk "load `cat kmk.blob` keyhandle=0x81000001
> policyhandle=0x3000000" @u
> add_key: Operation not permitted
> 
> The error should be policy check failed, because I use TPM command to
> unseal directly with error of policy check failed. 
> $ tpm2_unseal -c 0x81000001 -L sha256:7
> ERROR on line: "81" in file: "./lib/log.h": Tss2_Sys_Unseal(0x99D) -
> tpm:session(1):a policy check failed ERROR on line: "213" in file:
> "tools/tpm2_unseal.c": Unseal failed!
> ERROR on line: "166" in file: "tools/tpm2_tool.c": Unable to run
> tpm2_unseal
> 
> - Shirley 
> 
> -----Original Message-----
> From: James Bottomley <jejb@linux.ibm.com> 
> Sent: Monday, December 2, 2019 12:17 PM
> To: Zhao, Shirley <shirley.zhao@intel.com>; Mimi Zohar <zohar@linux.i
> bm.com>; Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>; Jonathan
> Corbet <corbet@lwn.net>
> Cc: linux-integrity@vger.kernel.org; keyrings@vger.kernel.org; linux-
> doc@vger.kernel.org; linux-kernel@vger.kernel.org; 'Mauro Carvalho
> Chehab' <mchehab+samsung@kernel.org>; Zhu, Bing <bing.zhu@intel.com>;
> Chen, Luhai <luhai.chen@intel.com>
> Subject: Re: One question about trusted key of keyring in Linux
> kernel.
> 
> On Mon, 2019-12-02 at 01:44 +0000, Zhao, Shirley wrote:
> > Hi, James,
> > 
> > The value of PCR7 is not changed. I have checked it with TPM
> > command 
> > tpm_pcrlist.
> > 
> > So I think the problem is how to use the option policydigest and 
> > policyhandle? Is there any example?
> > Maybe the format in my command is not correct. 
> 
> OK, so previously you said that using the Intel TSS the policy also
> failed after a reboot:
> 
> > The error should be policy check failed, because I use TPM command
> > to 
> > unseal directly with error of policy check failed.
> > $ tpm2_unseal -c 0x81000001 -L sha256:7 ERROR on line: "81" in
> > file: 
> > "./lib/log.h": Tss2_Sys_Unseal(0x99D) - tpm:session(1):a policy
> > check 
> > failed ERROR on line: "213" in file: "tools/tpm2_unseal.c": Unseal 
> > failed!
> > ERROR on line: "166" in file: "tools/tpm2_tool.c": Unable to run 
> > tpm2_unseal
> 
> So this must mean the actual policy hash you constructed was wrong in
> some way: it didn't correspond simply to a value of pcr7 ... well
> assuming the -L sha256:7 means construct a policy of the sha256 value
> of pcr7 and use it in the unseal.
> 
> I can tell you how to construct policies using TPM2 commands, but I
> think you want to know how to do it using the Intel TSS?  In which
> case you really need to consult the experts in that TSS, like Phil
> Tricca.
> 
> For the plain TPM2 case, the policy looks like
> 
> TPM_CC_PolicyPCR || pcrs || pcrDigest
> 
> Where TPM_CC_PolicyPCR = 0000017f and for selecting pcr7 only.  pcrs
> is a complicated entity: it's a counted array of pcr selections.  For
> your policy you only need one entry, so it would be 00000001 followed
> by a single pcrSelection entry.  pcrSelection is the hash algorithm,
> the size of the selection bitmap (always 3 since every current TPM
> only has
> 24 PCRs) and a bitmap selecting the PCRs in big endian format, so for
> PCR7 using sha256 (algorithm 000b), pcrSelection = 000b 03 80 00 00. 
> And then you follow this by the hash of the PCR value you're looking
> for.  The policyhash becomes the initial policy (all zeros for the
> start of the policy chain) hashed with this.
> 
> Regards,
> 
> James
>