Re: [PATCH v3 1/3] selftests: tpm2: Probe for available PCR bank

[Stefan Berger]

On 11/26/21 19:29, Jarkko Sakkinen wrote:
> On Wed, 2021-11-24 at 19:38 -0500, Stefan Berger wrote:
> > From: Stefan Berger <stefanb@xxxxxxxxxxxxx>
> >
> > Probe for an available PCR bank to accommodate devices that do not have a
> What does "probing for an vailable PCR bank" even mean?

Checking ? Testing ? What's the difference to those words?


> > SHA-1 PCR bank or whose SHA-1 bank is deactivated. Use the bank that can
> > be used for the TPM2 commands, such as the SHA-256 bank.
> This is incorrect, as the patch does not have code to query
> available hash algorithms.

I am not sure I understand. The new function determines which PCR banks 
(SHA-1 or SHA-256) the subsequent tests can use. It tries to execute the 
policy_pcr command with the given hash bank algorithm (SHA-256 or SHA-1) 
and if it returns an UnknowPcrBankError it tries the next one in the 
list. It doesn't need to query available hash algorithms. A TPM 2 may 
support SHA 256 and yet its SHA 256 bank may be deactivate.

> What the code does, and only does that, is to use SHA-256 as
> a fallback when SHA-1 is not enabled.
>
> /Jarkko
>
>
> > Signed-off-by: Stefan Berger <stefanb@xxxxxxxxxxxxx>
> > ---
> >   tools/testing/selftests/tpm2/tpm2_tests.py | 35 +++++++++++++++++-----
> >   1 file changed, 27 insertions(+), 8 deletions(-)
> >
> > diff --git a/tools/testing/selftests/tpm2/tpm2_tests.py b/tools/testing/selftests/tpm2/tpm2_tests.py
> > index 9d764306887b..6b88ff0e47b9 100644
> > --- a/tools/testing/selftests/tpm2/tpm2_tests.py
> > +++ b/tools/testing/selftests/tpm2/tpm2_tests.py
> > @@ -27,7 +27,23 @@ class SmokeTest(unittest.TestCase):
> >           result = self.client.unseal(self.root_key, blob, auth, None)
> >           self.assertEqual(data, result)
> >   
> > +    def determine_bank_alg(self):
> > +        # Probe for available PCR bank
> > +        for bank_alg in [tpm2.TPM2_ALG_SHA1, tpm2.TPM2_ALG_SHA256]:
> > +            try:
> > +                handle = self.client.start_auth_session(tpm2.TPM2_SE_TRIAL)
> > +                self.client.policy_pcr(handle, [17], bank_alg=bank_alg)
> > +                return bank_alg
> > +            except tpm2.UnknownPCRBankError:
> > +                pass
> > +            finally:
> > +                self.client.flush_context(handle)
> > +        return None
> > +
> >       def test_seal_with_policy(self):
> > +        bank_alg = self.determine_bank_alg()
> > +        self.assertIsNotNone(bank_alg)
> > +
> >           handle = self.client.start_auth_session(tpm2.TPM2_SE_TRIAL)
> >   
> >           data = ('X' * 64).encode()
> > @@ -35,7 +51,7 @@ class SmokeTest(unittest.TestCase):
> >           pcrs = [16]
> >   
> >           try:
> > -            self.client.policy_pcr(handle, pcrs)
> > +            self.client.policy_pcr(handle, pcrs, bank_alg=bank_alg)
> >               self.client.policy_password(handle)
> >   
> >               policy_dig = self.client.get_policy_digest(handle)
> > @@ -47,7 +63,7 @@ class SmokeTest(unittest.TestCase):
> >           handle = self.client.start_auth_session(tpm2.TPM2_SE_POLICY)
> >   
> >           try:
> > -            self.client.policy_pcr(handle, p"Use SHA-256 as a fallback crs)
> > +            self.client.policy_pcr(handle, pcrs, bank_alg=bank_alg)
> >               self.client.policy_password(handle)
> >   
> >               result = self.client.unseal(self.root_key, blob, auth, handle)
> > @@ -72,6 +88,9 @@ class SmokeTest(unittest.TestCase):
> >           self.assertEqual(rc, tpm2.TPM2_RC_AUTH_FAIL)
> >   
> >       def test_unseal_with_wrong_policy(self):
> > +        bank_alg = self.determine_bank_alg()
> > +        self.assertIsNotNone(bank_alg)
> > +
> >           handle = self.client.start_auth_session(tpm2.TPM2_SE_TRIAL)
> >   
> >           data = ('X' * 64).encode()
> > @@ -79,7 +98,7 @@ class SmokeTest(unittest.TestCase):
> >           pcrs = [16]
> >   
> >           try:
> > -            self.client.policy_pcr(handle, pcrs)
> > +            self.client.policy_pcr(handle, pcrs, bank_alg=bank_alg)
> >               self.client.policy_password(handle)
> >   
> >               policy_dig = self.client.get_policy_digest(handle)
> > @@ -91,13 +110,13 @@ class SmokeTest(unittest.TestCase):
> >           # Extend first a PCR that is not part of the policy and try to unseal.
> >           # This should succeed.
> >   
> > -        ds = tpm2.get_digest_size(tpm2.TPM2_ALG_SHA1)
> > -        self.client.extend_pcr(1, ('X' * ds).encode())
> > +        ds = tpm2.get_digest_size(bank_alg)
> > +        self.client.extend_pcr(1, ('X' * ds).encode(), bank_alg=bank_alg)
> >   
> >           handle = self.client.start_auth_session(tpm2.TPM2_SE_POLICY)
> >   
> >           try:
> > -            self.client.policy_pcr(handle, pcrs)
> > +            self.client.policy_pcr(handle, pcrs, bank_alg=bank_alg)
> >               self.client.policy_password(handle)
> >   
> >               result = self.client.unseal(self.root_key, blob, auth, handle)
> > @@ -109,14 +128,14 @@ class SmokeTest(unittest.TestCase):
> >   
> >           # Then, extend a PCR that is part of the policy and try to unseal.
> >           # This should fail.
> > -        self.client.extend_pcr(16, ('X' * ds).encode())
> > +        self.client.extend_pcr(16, ('X' * ds).encode(), bank_alg=bank_alg)
> >   
> >           handle = self.client.start_auth_session(tpm2.TPM2_SE_POLICY)
> >   
> >           rc = 0
> >   
> >           try:
> > -            self.client.policy_pcr(handle, pcrs)
> > +            self.client.policy_pcr(handle, pcrs, bank_alg=bank_alg)
> >               self.client.policy_password(handle)
> >   
> >               result = self.client.unseal(self.root_key, blob, auth, handle)