Fix EVP_PKEY_eq() to be possible to use with strictly private keys

EVP_PKEY_eq() assumed that an EVP_PKEY always has the public key
component if it has a private key component.  However, this assumption
no longer strictly holds true, at least for provider backed keys.
EVP_PKEY_eq() therefore needs to be modified to specify that the
private key should be checked too (at the discretion of what's
reasonable for the implementation doing the actual comparison).

Fixes #16267

Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/16765)

crypto/evp/p_lib.c

@@ -343,7 +343,7 @@ int EVP_PKEY_eq(const EVP_PKEY *a, const EVP_PKEY *b)
 
     if (a->keymgmt != NULL || b->keymgmt != NULL)
         return evp_pkey_cmp_any(a, b, (SELECT_PARAMETERS
-                                       | OSSL_KEYMGMT_SELECT_PUBLIC_KEY));
+                                       | OSSL_KEYMGMT_SELECT_KEYPAIR));
 
     /* All legacy keys */
     if (a->type != b->type)

doc/man3/EVP_PKEY_copy_parameters.pod

@@ -37,8 +37,8 @@ in B<from> and B<to> are both present and match this function has no effect.
 The function EVP_PKEY_parameters_eq() checks the parameters of keys
 B<a> and B<b> for equality.
 
-The function EVP_PKEY_eq() checks the public key components and parameters
-(if present) of keys B<a> and B<b> for equality.
+The function EVP_PKEY_eq() checks the keys B<a> and B<b> for equality,
+including their parameters if they are available.
 
 =head1 NOTES
 
@@ -47,14 +47,40 @@ EVP_PKEY_copy_parameters() is to handle public keys in certificates where the
 parameters are sometimes omitted from a public key if they are inherited from
 the CA that signed it.
 
-Since OpenSSL private keys contain public key components too the function
-EVP_PKEY_eq() can also be used to determine if a private key matches
-a public key.
-
 The deprecated functions EVP_PKEY_cmp() and EVP_PKEY_cmp_parameters() differ in
-their return values  compared to other _cmp() functions. They are aliases for
+their return values compared to other _cmp() functions. They are aliases for
 EVP_PKEY_eq() and EVP_PKEY_parameters_eq().
 
+The function EVP_PKEY_cmp() previously only checked the key parameters
+(if there are any) and the public key, assuming that there always was
+a public key and that private key equality could be derived from that.
+Because it's no longer assumed that the private key in an L<EVP_PKEY(3)> is
+always accompanied by a public key, the comparison can not rely on public
+key comparison alone.
+
+Instead, EVP_PKEY_eq() (and therefore also EVP_PKEY_cmp()) now compares:
+
+=over 4
+
+=item 1.
+
+the key parameters (if there are any)
+
+=item 2.
+
+the public keys or the private keys of the two B<EVP_PKEY>s, depending on
+what they both contain.
+
+=back
+
+=begin comment
+
+Exactly what is compared is ultimately at the discretion of the provider
+that holds the key, as they will compare what makes sense to them that fits
+the selector bits they are passed.
+
+=end comment
+
 =head1 RETURN VALUES
 
 The function EVP_PKEY_missing_parameters() returns 1 if the public key