Unexpected success from EVP_PKEY_fromdata

The function EVP_PKEY_fromdata will return success even in instances where it has failed to import a key successfully.

In this email to openssl-users the OP reports an instance where EVP_PKEY_fromdata indicates success when importing a public key:
https://mta.openssl.org/pipermail/openssl-users/2021-October/014479.html

The same report goes on to describe a subsequent crash which is fixed by #16911.

The problem with the EVP_PKEY_fromdata call is that the OP attempted to import a public key using OSSL_PKEY_PARAM_EC_PUB_X and OSSL_PKEY_PARAM_EC_PUB_Y which are only supported as "getters", i.e. not for import. Since the OP explicitly indicates EVP_PKEY_PUBLIC_KEY in the EVP_PKEY_fromdata call I would expect it to fail since a public key was not imported. However this is not the case. Instead it simply imports the parameters, ignores the unknown public key params and returns success, having created a parameters only key.

This seems quite unexpected.

However fixing this is quite tricky. In order to indicate an attempt to import a private key you might be expected to use EVP_PKEY_KEYPAIR as the selection parameter (there is no EVP_PKEY_PRIVATE_KEY - although if it did exist it would be the same as OSSL_KEYMGMT_SELECT_PRIVATE_KEY). However EVP_PKEY_KEYPAIR actually indicates both the public and private key are present - but in some key types (including EC) the import is deliberately tolerant of a private key without a public key. So it doesn't seem to be as simple as saying "if EVP_PKEY_PUBLIC_KEY is specified and no public key is present then fail" since in the EVP_PKEY_KEYPAIR case, EVP_PKEY_PUBLIC_KEY is incorporated but it is valid to have no public key.

Start by tracing EVP_PKEY_fromdata and the handling of OSSL_PKEY_PARAM_EC_PUB_X and OSSL_PKEY_PARAM_EC_PUB_Y, using the linked openssl-users report as the reproduction context. Compare EVP_PKEY_PUBLIC_KEY with EVP_PKEY_KEYPAIR, including EC imports that omit a public key. Done means unsuccessful public-key imports no longer report success while valid tolerant imports remain supported.