fix(psk): Prevent internal error with uninitialized PSK

[Ignacio-Trottaa]

fix: Handle uninitialized PSK in choose_psk

When s2n_offered_psk_list_choose_psk was called with an s2n_offered_psk
struct that was allocated but not initialized, it would result in a
NULL pointer dereference and an internal error.

This change adds a check to ensure the psk's identity data is not
NULL before proceeding, returning a usage error (S2N_ERR_NULL) instead.
A unit test is also added to verify this behavior.

Fixes: #5085

Goal

Prevent an internal error when s2n_offered_psk_list_choose_psk is called with an uninitialized PSK.

Why

Calling the function with a PSK that has a NULL identity data pointer causes a crash. The function should fail gracefully with a usage error instead of an internal one, improving the library's robustness. This addresses issue #5085.

How

A POSIX_ENSURE_REF check is added at the beginning of the function to validate that psk->identity.data is not NULL. If it is, the function now returns S2N_ERR_NULL.

Callouts

None. The fix is straightforward.

Testing

A new unit test, s2n_offered_psk_list_choose_psk_with_uninitialized_psk, was added. This test replicates the failing scenario and asserts that the function now correctly returns a failure with the S2N_ERR_NULL error code. All existing tests pass.

Related

Fixes #5085

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[CarolYeh910]

Hi @Ignacio-Trottaa, thanks for your contribution! The testing section mentioned you added a new unit test, but I didn't see it in the file diff. Do you plan to add the test to this PR? Your fix looks correct though!

[Ignacio-Trottaa]

Hi @CarolYeh910, thanks for your feedback! Sorry about the unit test; I mentioned I uploaded it, but... I forgot to upload it along with the correction.

[maddeleine]

Hey are you still working on this? We don't want to merge this without a unit test. Thanks!