fix: restore compatibility with libp11 0.4.13

[nkraetzschmar]

This module is no longer compatible with the openssl pkcs11 engine since libp11 version 0.4.13.

Compatibility between this module and libp11 has been broken since OpenSC/libp11@da8cd1f.

Root cause

This change introduced some automatic clearing of keys when no more valid sessions exist. However, since checking if a session is valid is done by checking if C_GetSessionInfo returns CKR_OK¹ this currently always marks all sessions as invalid as C_GetSessionInfo always returns CKR_FUNCTION_NOT_SUPPORTED.

Therefore keys get cleared too early, causing undefined behaviour.

Initially this caused a segfault and since OpenSC/libp11@7f6eba0 the visible behaviour of the bug changed to instead result in errors of the form:

Public Key operation error
003C7E99FFFF0000:error:020000B3:rsa routines:rsa_ossl_private_encrypt:missing private key:../crypto/rsa/rsa_ossl.c:367:

Likely due to the new key->object_class == CKO_PRIVATE_KEY check always failing on cleared keys.

Fix

This PR restores the old behaviour by now always returning CKR_OK in C_GetSessionInfo.

Disclaimer: I lack the in-depth understanding of pkcs11 modules to properly asses wether implementing C_GetSessionInfo this way may cause other issues.

Footnotes

1. https://github.com/OpenSC/libp11/blob/6d669183c7b241ce47ecce28744837ad92814f5c/src/p11_slot.c#L156 ↩

[JackOfMostTrades]

Thank you for the detailed PR! I was reading up on this function and I think if this is going to return CKR_OK then it should probably fill in the session info. I spent some time in the manuals and I think the session info fields are probably best filled in as below. Could you update the implementation with something along these lines?

CK_RV C_GetSessionInfo(CK_SESSION_HANDLE hSession, CK_SESSION_INFO_PTR pInfo) {
    CkSession *session = (CkSession*)hSession;
    if (session == NULL) {
        return CKR_SESSION_HANDLE_INVALID;
    }
    memset(pInfo, 0, sizeof(*pInfo));
    pInfo->slotID = session->slot_id;
    pInfo->state = CKS_RW_USER_FUNCTIONS;
    pInfo->flags = CKF_RW_SESSION | CKF_SERIAL_SESSION;
    return CKR_OK;
}