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Complex policy expressions #134

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wants to merge 6 commits into from
Closed

Complex policy expressions #134

wants to merge 6 commits into from

Commits on Aug 10, 2021

  1. test-enroll.sh: Quiet shellcheck integration check

    @nicowilliams
    nicowilliams committed Aug 10, 2021
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  2. Complex policy expressions 

    This allows one to express complex policies (ones with alternations)
using a simple language.

The pattern is simply that conjunctions are {tpm2 policy*} command-lines
joined with a ';' argument, and alternations are {tpm2 policyor}
commands with arguments that are themselves policies surrounded by '('
and ')' arguments.

For example:

    $ sbin/tpm2-policy							\
	tpm2 policyor							\
	    '(' tpm2 policycommandcode TPM2_CC_Sign ')'			\
	    '(' tpm2 policycommandcode TPM2_CC_RSA_Decrypt ')' ';'	\
	tpm2 policypcr -l "sha256:11"

which allows an entity sporting such a policy to be used for signing or
decryption only, and only when PCR#11 is cleared.

The same, verbosely:

    $ sbin/tpm2-policy -v						\
	tpm2 policyor							\
	    '(' tpm2 policycommandcode TPM2_CC_Sign ')'			\
	    '(' tpm2 policycommandcode TPM2_CC_RSA_Decrypt ')' ';'	\
	tpm2 policypcr -l "sha256:11"
        Running: (AND) exec_policyOR_helper ( tpm2 policycommandcode TPM2_CC_Sign ) ( tpm2 policycommandcode TPM2_CC_RSA_Decrypt ) ;
            Running: (AND) tpm2 policycommandcode --session /tmp/tmp.jbXmSKOtzR/session-0-0 --policy /tmp/tmp.jbXmSKOtzR/policy-0-0 TPM2_CC_Sign
    cc6918b226273b08f5bd406d7f10cf160f0a7d13dfd83b7770ccbcd1aa80d811
    cc6918b226273b08f5bd406d7f10cf160f0a7d13dfd83b7770ccbcd1aa80d811
            Running: (AND) tpm2 policycommandcode --session /tmp/tmp.jbXmSKOtzR/session-0-1 --policy /tmp/tmp.jbXmSKOtzR/policy-0-1 TPM2_CC_RSA_Decrypt
    3c29869a1312094782b86df5a430caae587f5dfb16dfa3f7204151054c1340d2
    3c29869a1312094782b86df5a430caae587f5dfb16dfa3f7204151054c1340d2
        ORing: cc6918b226273b08f5bd406d7f10cf160f0a7d13dfd83b7770ccbcd1aa80d811 3c29869a1312094782b86df5a430caae587f5dfb16dfa3f7204151054c1340d2
        Running: tpm2 policyor --session /tmp/tmp.jbXmSKOtzR/session --policy /tmp/tmp.jbXmSKOtzR/policy sha256:/tmp/tmp.jbXmSKOtzR/policy-0-0,/tmp/tmp.jbXmSKOtzR/policy-0-1
    39faada14fb6d5deba4315d8bce0247813262ebf15e1aee35477d26759f1b29e
        Running: (AND) tpm2 policypcr --session /tmp/tmp.jbXmSKOtzR/session --policy /tmp/tmp.jbXmSKOtzR/policy -l sha256:11
    3b9ea8dca851fac5d077d7b6925b8cc38847619e4b9a0f026ca5cc6262ff8a1c
    3b9ea8dca851fac5d077d7b6925b8cc38847619e4b9a0f026ca5cc6262ff8a1c
    $

One can also execute a policy in a policy session using
sbin/tpm2-policy.

Internally we have `exec_policy` in functions.sh that can execute a
policy in either a trial session or in policy session, and if in a
policy session then the caller must supply the indices of alternatives
to take in the policy, otherwise the caller must not supply those.

I.e.,

    # Compute policyDigest of the policy above:
    tpm2_flushall
    tpm2 startauthsession --session session
    exec_policy session policy						\
	tpm2 policyor							\
	    '(' tpm2 policycommandcode TPM2_CC_Sign ')'			\
	    '(' tpm2 policycommandcode TPM2_CC_RSA_Decrypt ')' ';'	\
	tpm2 policypcr -l "sha256:11"

    # Execute the policy above with the second alternative:
    tpm2_flushall
    tpm2 startauthsession --session session
    exec_policy 1 session policy					\
	tpm2 policyor							\
	    '(' tpm2 policycommandcode TPM2_CC_Sign ')'			\
	    '(' tpm2 policycommandcode TPM2_CC_RSA_Decrypt ')' ';'	\
	tpm2 policypcr -l "sha256:11"

This is recursive, so it generalizes to policies of arbitrary
complexity, limited to nesting alternations up to nine (9) times, or the
shell's recursion limit if it be lower.
    @nicowilliams
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  3. Rename "EK method" to "WK method" 

    Good suggestion by Erik Larsson.
    @nicowilliams
    nicowilliams committed Aug 10, 2021
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  4. attest-enroll: Fix POLICIES not set error

    @nicowilliams
    nicowilliams committed Aug 10, 2021
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  5. Makefile: shellcheck tests/test-enroll.sh too

    @nicowilliams
    nicowilliams committed Aug 10, 2021
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  6. sbin/tpm2-policy: Support reading policy from file

    @nicowilliams
    nicowilliams committed Aug 10, 2021
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