[tf-psa-crypto] MBEDTLS_PLATFORM_GET_ENTROPY_ALT in 4.0 #212

valeriosetti · 2025-03-18T13:42:53Z

Description

This helps resolving Mbed-TLS/mbedtls#9618

CI note

Due to dependencies with the main repo PR please evaluate the CI of 10090 as validation of this PR.

PR checklist

changelog done
framework PR
mbedtls development PR [development] MBEDTLS_PLATFORM_GET_ENTROPY_ALT in 4.0 mbedtls#10090
mbedtls 3.6 PR not required: new feature
tests TODO

gilles-peskine-arm

I just did a design review. I won't be able to do a full code review.

We just realized this morning that the issue description wasn't very clear, and several things were in Janos's mind and mine but we hadn't written them down. Sorry about this. I added a note to the issue and some comments here, hopefully that should clarify things.

gilles-peskine-arm · 2025-03-19T13:13:51Z

drivers/builtin/include/mbedtls/platform.h

+ *                              in such information, so the function must handle
+ *                              this.
+ */
+int mbedtls_entropy_hardware_poll(unsigned char *output, size_t output_size,


Non-blocker: since the function is now declared in platform.h, should we call it mbedtls_platform_xxx? Same question about the configuration option. Maybe MBEDTLS_PLATFORM_GET_ENTROPY_ALT and mbedtls_platform_get_entropy()?

gilles-peskine-arm · 2025-03-19T13:15:52Z

drivers/builtin/include/mbedtls/platform.h

+ * \param[out] entropy_content  Measure of the entropy content (in bits) of the
+ *                              data written in the \p output buffer. The pointer


Please add the constraint that for the time being, the function must set entropy_content = 8 * *output_len on success. The library will be extended later to support partial entropy.

gilles-peskine-arm · 2025-03-19T13:16:39Z

drivers/builtin/include/mbedtls/platform.h

+ *                              can be \c NULL in case the caller is not interested
+ *                              in such information, so the function must handle
+ *                              this.


We don't need to support callers that don't want the value. entropy_content is guaranteed to be non-null.

gilles-peskine-arm · 2025-03-19T13:23:55Z

drivers/builtin/include/mbedtls/platform.h

+ * \brief           User defined callback function that is used from the entropy
+ *                  module to gather entropy data from some hardware device.
+ *
+ * \warning         This is not provided by Mbed TLS.


I don't think “not provided by Mbed TLS” is a useful warning. Also, it's TF-PSA-Crypto now.

On the other hand, the function's documentation should state that this function is only used when MBEDTLS_ENTROPY_HARDWARE_POLL is defined. This is otherwise not visible when you read the Doxygen-rendered documentation.

gilles-peskine-arm · 2025-03-19T13:26:32Z

ChangeLog.d/9618.txt

+     `MBEDTLS_ENTROPY_HARDWARE_POLL_MIN` can optionally optinally define the
+     minimum number of bytes that should be polled using that function before
+     declaring entropy gathering completed. If not defined, then a default
+     value of 32 bytes is used.


MBEDTLS_ENTROPY_HARDWARE_POLL_MIN should not be a configuration parameter, at least not in this form. The RNG needs a certain amount of entropy. This minimum is up to the system configuration (and in particular the required cryptographic strength for the system), not up to the hardware integration. The person writing the TRNG device driver doesn't get to choose this value.

I think we should just use the existing MBEDTLS_ENTROPY_MIN_PLATFORM here. The distinction between “platform source” and “hardware source” doesn't really make sense as it's currently described in entropy_poll.h. Actually the “platform source” is the operating system integrations that the library supports out of the box, and the “hardware source” is the one for other systems where the integrator needs to provide a driver.

gilles-peskine-arm · 2025-03-19T13:28:16Z

ChangeLog.d/9618.txt

+     removed.
+
+Features
+   * The new symbol `MBEDTLS_ENTROPY_HARDWARE_POLL` is added to allow the


The changelog entries should probably mention the function names as well as the option names, for easier searching.

gilles-peskine-arm · 2025-03-19T13:35:52Z

drivers/builtin/src/entropy_poll.h

+        void *data, unsigned char *output, size_t len, size_t *olen)
+{
+    (void) data;
+    return mbedtls_entropy_hardware_poll(output, len, olen, NULL);


The wrapper should check the entropy content and fail if it's less than full entropy, i.e. if entropy_output < 8 * *olen. Please add a unit test for that in test_suite_entropy.

Also the entropy collection should fail if *olen is too small and as a result there isn't enough entropy, collected from all sources. I believe this is covered by the existing tests in test_suite_entropy, but please double-check.

The wrapper should check the entropy content and fail if it's less than full entropy, i.e. if entropy_output < 8 * *olen. Please add a unit test for that in test_suite_entropy.

While adding the check on the wrapper and testing it with the new component tf_psa_crypto_platform_get_entropy_alt I found that tests are already failing if the returned entropy content is not correct, so there seems to be no need for a new test step. As reference implementation I'm using the one proposed in the companion PR Mbed-TLS/mbedtls-framework#151.

valeriosetti · 2025-04-01T20:06:36Z

I think that CI failures in test_psasim and test_suite_with_psasim can be ignored in this PR as the same tests in the main PR are OK

valeriosetti · 2025-04-01T20:07:56Z

I just forced push the last commit following the merge of Mbed-TLS/mbedtls-framework#151. No other change was made

valeriosetti · 2025-04-04T12:35:58Z

I think that CI failures in test_psasim and test_suite_with_psasim can be ignored in this PR as the same tests in the main PR are OK

Thinking on this a bit more I think I can explain why psasim tests fail in this PR. The PR on the main repo modifies config.py (only in the main repo) so that the full config doesn't set MBEDTLS_PLATFORM_GET_ENTROPY_ALT.
The problem is that when Mbed-TLS/mbedtls#10090 is tested then it pick this PR as the tf-psa-crypto version. The opposite is not true: when this PR is tested on the CI, the main repo is checked out at development and there the full config enables MBEDTLS_PLATFORM_GET_ENTROPY_ALT.

Signed-off-by: Valerio Setti <[email protected]>

This is necessary to let entropy being able to gather entropy data from the native platform source. Signed-off-by: Valerio Setti <[email protected]>

…_entropy() Signed-off-by: Valerio Setti <[email protected]>

Signed-off-by: Valerio Setti <[email protected]>

Since this is now an internal function, output_size always comes from our library code, and we can assume that it has a “reasonable” size — tens of bytes, definitely less than ULONG_MAX which is 2^32. So the loop is no longer needed. Signed-off-by: Valerio Setti <[email protected]>

Signed-off-by: Valerio Setti <[email protected]>

…py() Signed-off-by: Valerio Setti <[email protected]>

…alt() Signed-off-by: Valerio Setti <[email protected]>

…lt() Test also the case in which the callback is returning less data than requested. In this case the entropy module will call the callback until it reaches the minimum required amount of data. Signed-off-by: Valerio Setti <[email protected]>

…alt() Ensure that all forced values on mbedtls_platform_get_entropy() are reset before quitting the test functions. This prevents weird failures on subsequent tests that might be executed. Signed-off-by: Valerio Setti <[email protected]>

valeriosetti · 2025-04-29T09:00:14Z

@gilles-peskine-arm @bjwtaylor I just updated the framework reference (since Mbed-TLS/mbedtls-framework#165 was merged) and rebased in top of development branch to solve CI issues with main repo.

…ropy() Signed-off-by: Valerio Setti <[email protected]>

valeriosetti · 2025-04-30T08:17:41Z

Remaining CI failures are:

[2025-04-29T14:35:20.193Z] test/selftest.c:229:14: error: implicit declaration of function 'mbedtls_platform_entropy_poll' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
[2025-04-29T14:35:20.193Z]     result = mbedtls_platform_entropy_poll(NULL,

which is reasonable since the main repo is tested on the development branch. We can rely on the CI of Mbed-TLS/mbedtls#10090 to validate this PR.

gilles-peskine-arm · 2025-04-30T08:33:11Z

It's problematic to merge something in the crypto repository that breaks the CI, since it means crypto will be broken on all branches that don't have this PR merged yet.

Can you please add a transition function mbedtls_platform_entropy_poll that's just good enough for how it's used in selftest.c, with a comment stating that it's going to be removed soon? And might as well make a PR that reverts that commit, to be merged in a few weeks.

…py_poll() This is temporary and going to be removed by Mbed-TLS#273 Signed-off-by: Valerio Setti <[email protected]>

Signed-off-by: Valerio Setti <[email protected]>

valeriosetti · 2025-04-30T09:58:39Z

Can you please add a transition function mbedtls_platform_entropy_poll that's just good enough for how it's used in selftest.c, with a comment stating that it's going to be removed soon? And might as well make a PR that reverts that commit, to be merged in a few weeks.

@gilles-peskine-arm I just created #273 to track this

valeriosetti · 2025-04-30T10:22:41Z

@gilles-peskine-arm although I added the stub function for mbedtls_platform_entropy_poll there are remaining failures on the CI related to the fact that the main branch actually needs some more fixes on its codebase to make this PR in (configuration header files, config.py, etc - see Mbed-TLS/mbedtls#10090 for the full list).
I'm not sure I can make the CI fully happy in this PR.,,

valeriosetti mentioned this pull request Mar 18, 2025

[framework] MBEDTLS_PLATFORM_GET_ENTROPY_ALT in 4.0 Mbed-TLS/mbedtls-framework#151

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[development] MBEDTLS_PLATFORM_GET_ENTROPY_ALT in 4.0 Mbed-TLS/mbedtls#10090

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valeriosetti changed the title ~~[tf-psa-crypto] MBEDTLS_ENTROPY_HARDWARE_ALT in 4.0~~ [tf-psa-crypto] MBEDTLS_PLATFORM_GET_ENTROPY_ALT in 4.0 Mar 24, 2025

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valeriosetti added 14 commits April 29, 2025 10:58

drivers: update documentation about entropy gathering sources

3e00cff

Signed-off-by: Valerio Setti <[email protected]>

drivers: keep a single prototype for mbedtls_platform_get_entropy()

0ba4b2b

Signed-off-by: Valerio Setti <[email protected]>

configs: add PLATFORM_C to configs using ENTROPY_C

cb66076

This is necessary to let entropy being able to gather entropy data from the native platform source. Signed-off-by: Valerio Setti <[email protected]>

drivers: platform: fix Windows implementation of mbedtls_platform_get…

d823f46

…_entropy() Signed-off-by: Valerio Setti <[email protected]>

docs: add migration guide for entropy gathering functions

f129b23

Signed-off-by: Valerio Setti <[email protected]>

docs: platform-entropy-gathering: more focus on migration aspects

9790a14

Signed-off-by: Valerio Setti <[email protected]>

crypto_config: update description of MBEDTLS_PLATFORM_GET_ENTROPY_ALT

b278f8b

Signed-off-by: Valerio Setti <[email protected]>

changelog: minor fixes

f4d4c18

Signed-off-by: Valerio Setti <[email protected]>

doc: platform-entropy-gathering: update

1cdadd0

Signed-off-by: Valerio Setti <[email protected]>

drivers: platform: update documentation of mbedtls_platform_get_entro…

ce64c95

…py() Signed-off-by: Valerio Setti <[email protected]>

tests: test_suite_entropy: improve entropy_test_platform_get_entropy_…

7a9af47

…alt() Signed-off-by: Valerio Setti <[email protected]>

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valeriosetti requested review from bjwtaylor and gilles-peskine-arm April 29, 2025 08:59

drivers: platform: fix Win implementation of mbedtls_platform_get_ent…

4b31670

…ropy() Signed-off-by: Valerio Setti <[email protected]>

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Entropy: remove empty definition of mbedtls_platform_entropy_poll() #273

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valeriosetti added 2 commits April 30, 2025 11:51

drivers: platform: add empty implementation of mbedtls_platform_entro…

26fdaa1

…py_poll() This is temporary and going to be removed by Mbed-TLS#273 Signed-off-by: Valerio Setti <[email protected]>

framework: update reference

88c818c

Signed-off-by: Valerio Setti <[email protected]>

valeriosetti force-pushed the issue9618-tfpsacrypto branch from ced4965 to 88c818c Compare April 30, 2025 09:51

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[tf-psa-crypto] MBEDTLS_PLATFORM_GET_ENTROPY_ALT in 4.0 #212

[tf-psa-crypto] MBEDTLS_PLATFORM_GET_ENTROPY_ALT in 4.0 #212

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		* \param[out] entropy_content Measure of the entropy content (in bits) of the
		* data written in the \p output buffer. The pointer

[tf-psa-crypto] MBEDTLS_PLATFORM_GET_ENTROPY_ALT in 4.0 #212

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[tf-psa-crypto] MBEDTLS_PLATFORM_GET_ENTROPY_ALT in 4.0 #212

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