[PATCH v3 0/3] selftests: tpm2: Probe for available PCR bank

[PATCH v3 0/3] selftests: tpm2: Probe for available PCR bank

[Stefan Berger]

From: Stefan Berger <stefanb@linux.ibm.com>

This series of patches fixes two issues with TPM2 selftest.
- Probes for available PCR banks
- Resets DA lock on TPM2 to avoid subsequent test failures

It also extends the test cases with support for SHA-384 and SHA-512
PCR banks.

  Stefan

v3:
 - Mention SHA-256 PCR bank as alternative in patch 1 description

v2:
 - Clarified patch 1 description 
 - Added patch 3 with support for SHA-384 and SHA-512

Stefan Berger (3):
  selftests: tpm2: Probe for available PCR bank
  selftests: tpm2: Reset the dictionary attack lock
  selftests: tpm2: Add support for SHA-384 and SHA-512

 tools/testing/selftests/tpm2/tpm2.py       | 12 ++++++-
 tools/testing/selftests/tpm2/tpm2_tests.py | 37 +++++++++++++++++-----
 2 files changed, 40 insertions(+), 9 deletions(-)

-- 
2.31.1

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

[Stefan Berger]

From: Stefan Berger <stefanb@linux.ibm.com>

Probe for an available PCR bank to accommodate devices that do not have a
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.

Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
---
 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, 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)
@@ -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)
-- 
2.31.1

[PATCH v3 2/3] selftests: tpm2: Reset the dictionary attack lock

[Stefan Berger]

From: Stefan Berger <stefanb@linux.ibm.com>

Reset the dictionary attack lock to avoid the following types of test
failures after running the test 2 times:

======================================================================
ERROR: test_unseal_with_wrong_policy (tpm2_tests.SmokeTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/root/linux-ima-namespaces/tools/testing/selftests/tpm2/tpm2_tests.py", line 105, in test_unseal_with_wrong_policy
    blob = self.client.seal(self.root_key, data, auth, policy_dig)
  File "/root/linux-ima-namespaces/tools/testing/selftests/tpm2/tpm2.py", line 620, in seal
    rsp = self.send_cmd(cmd)
  File "/root/linux-ima-namespaces/tools/testing/selftests/tpm2/tpm2.py", line 397, in send_cmd
    raise ProtocolError(cc, rc)
tpm2.ProtocolError: TPM_RC_LOCKOUT: cc=0x00000153, rc=0x00000921

Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
---
 tools/testing/selftests/tpm2/tpm2_tests.py | 1 +
 1 file changed, 1 insertion(+)

diff --git a/tools/testing/selftests/tpm2/tpm2_tests.py b/tools/testing/selftests/tpm2/tpm2_tests.py
index 6b88ff0e47b9..ae88d8866e5d 100644
--- a/tools/testing/selftests/tpm2/tpm2_tests.py
+++ b/tools/testing/selftests/tpm2/tpm2_tests.py
@@ -104,6 +104,7 @@ class SmokeTest(unittest.TestCase):
             policy_dig = self.client.get_policy_digest(handle)
         finally:
             self.client.flush_context(handle)
+            self.client.reset_da_lock()
 
         blob = self.client.seal(self.root_key, data, auth, policy_dig)
 
-- 
2.31.1

[PATCH v3 3/3] selftests: tpm2: Add support for SHA-384 and SHA-512

[Stefan Berger]

From: Stefan Berger <stefanb@linux.ibm.com>

Add support for SHA-384 and SHA-512 to the TPM2 library and extend
the test case's PCR bank probing function to also probe for SHA-384
and SHA-512 banks in case SHA-1 and SHA-256 banks are not available
for use.

Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
---
 tools/testing/selftests/tpm2/tpm2.py       | 12 +++++++++++-
 tools/testing/selftests/tpm2/tpm2_tests.py |  3 ++-
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/tpm2/tpm2.py b/tools/testing/selftests/tpm2/tpm2.py
index f34486cd7342..d0960fe0e029 100644
--- a/tools/testing/selftests/tpm2/tpm2.py
+++ b/tools/testing/selftests/tpm2/tpm2.py
@@ -37,6 +37,8 @@ TPM2_ALG_SHA1 = 0x0004
 TPM2_ALG_AES = 0x0006
 TPM2_ALG_KEYEDHASH = 0x0008
 TPM2_ALG_SHA256 = 0x000B
+TPM2_ALG_SHA384 = 0x000C
+TPM2_ALG_SHA512 = 0x000D
 TPM2_ALG_NULL = 0x0010
 TPM2_ALG_CBC = 0x0042
 TPM2_ALG_CFB = 0x0043
@@ -67,6 +69,8 @@ HR_TRANSIENT = 0x80000000
 
 SHA1_DIGEST_SIZE = 20
 SHA256_DIGEST_SIZE = 32
+SHA384_DIGEST_SIZE = 48
+SHA512_DIGEST_SIZE = 64
 
 TPM2_VER0_ERRORS = {
     0x000: "TPM_RC_SUCCESS",
@@ -186,16 +190,22 @@ RC_WARN = 0x900
 ALG_DIGEST_SIZE_MAP = {
     TPM2_ALG_SHA1: SHA1_DIGEST_SIZE,
     TPM2_ALG_SHA256: SHA256_DIGEST_SIZE,
+    TPM2_ALG_SHA384: SHA384_DIGEST_SIZE,
+    TPM2_ALG_SHA512: SHA512_DIGEST_SIZE,
 }
 
 ALG_HASH_FUNCTION_MAP = {
     TPM2_ALG_SHA1: hashlib.sha1,
-    TPM2_ALG_SHA256: hashlib.sha256
+    TPM2_ALG_SHA256: hashlib.sha256,
+    TPM2_ALG_SHA384: hashlib.sha384,
+    TPM2_ALG_SHA512: hashlib.sha512,
 }
 
 NAME_ALG_MAP = {
     "sha1": TPM2_ALG_SHA1,
     "sha256": TPM2_ALG_SHA256,
+    "sha384": TPM2_ALG_SHA384,
+    "sha512": TPM2_ALG_SHA512,
 }
 
 
diff --git a/tools/testing/selftests/tpm2/tpm2_tests.py b/tools/testing/selftests/tpm2/tpm2_tests.py
index ae88d8866e5d..3feca25038f5 100644
--- a/tools/testing/selftests/tpm2/tpm2_tests.py
+++ b/tools/testing/selftests/tpm2/tpm2_tests.py
@@ -29,7 +29,8 @@ class SmokeTest(unittest.TestCase):
 
     def determine_bank_alg(self):
         # Probe for available PCR bank
-        for bank_alg in [tpm2.TPM2_ALG_SHA1, tpm2.TPM2_ALG_SHA256]:
+        for bank_alg in [tpm2.TPM2_ALG_SHA1, tpm2.TPM2_ALG_SHA256,
+                         tpm2.TPM2_ALG_SHA384, tpm2.TPM2_ALG_SHA512]:
             try:
                 handle = self.client.start_auth_session(tpm2.TPM2_SE_TRIAL)
                 self.client.policy_pcr(handle, [17], bank_alg=bank_alg)
-- 
2.31.1

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

[Jarkko Sakkinen]

On Wed, 2021-11-24 at 19:38 -0500, Stefan Berger wrote:
> From: Stefan Berger <stefanb@linux.ibm.com>
> 
> Probe for an available PCR bank to accommodate devices that do not have a

What does "probing for an vailable PCR bank" even mean?

> 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.

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@linux.ibm.com>
> ---
>  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)

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@linux.ibm.com>
>>
>> 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@linux.ibm.com>
>> ---
>>   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)

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

[Jarkko Sakkinen]

On Fri, Nov 26, 2021 at 10:43:42PM -0500, Stefan Berger wrote:
> 
> 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@linux.ibm.com>
> > > 
> > > 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?

This would an exact description:

"Some devices do not have SHA1 bank available. Try to use SHA256 as a
fallback when this occurs."

It does not leave any open territory what the patch does. It does exactly
this, not more or less.

/Jarkko