openssl-util: Query engine/provider pin via ask-password

In mkosi, we want to support signing via a hardware token. We already
support this in systemd-repart and systemd-measure. However, if the
hardware token is protected by a pin, the pin is asked as many as 20
times when building an image as the pin is not cached and thus requested
again for every operation.

Let's introduce a custom openssl ui when we use engines and providers
and plug systemd-ask-password into the process. With systemd-ask-password,
the pin can be cached in the kernel keyring, allowing us to reuse it without
querying the user again every time to enter the pin.

We use the private key URI as the keyring identifier so that the cached pin
can be shared across multiple tools.

Note that if the private key is pin protected, openssl will prompt both when
loading the private key using the pkcs11 engine and when actually signing the
roothash. To make sure our custom UI is used when signing the roothash, we have
to also configure it with ENGINE_ctrl() which takes a non-owning pointer to
the UI_METHOD object and its userdata object which we have to keep alive so we
introduce a new AskPasswordUserInterface struct which we use to keep both objects
alive together with the EVP_PKEY object.

Because the AskPasswordRequest struct stores non-owning pointers to its fields,
we change repart to store the private key URI as a global variable again instead
of the EVP_PKEY object so that we can use the private key argument as the keyring
field of the AskPasswordRequest instance without running into lifetime issues.

src/shared/openssl-util.h

@@ -1,6 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
+#include "ask-password-api.h"
 #include "iovec-util.h"
 #include "macro.h"
 #include "sha256.h"
@@ -13,6 +14,8 @@ typedef enum KeySourceType {
         _OPENSSL_KEY_SOURCE_INVALID = -EINVAL,
 } KeySourceType;
 
+typedef struct OpenSSLAskPasswordUI OpenSSLAskPasswordUI;
+
 int parse_openssl_key_source_argument(const char *argument, char **private_key_source, KeySourceType *private_key_source_type);
 
 #define X509_FINGERPRINT_SIZE SHA256_DIGEST_SIZE
@@ -26,6 +29,7 @@ int parse_openssl_key_source_argument(const char *argument, char **private_key_s
 #  include <openssl/opensslv.h>
 #  include <openssl/pkcs7.h>
 #  include <openssl/ssl.h>
+#  include <openssl/ui.h>
 #  include <openssl/x509v3.h>
 #  ifndef OPENSSL_VERSION_MAJOR
 /* OPENSSL_VERSION_MAJOR macro was added in OpenSSL 3. Thus, if it doesn't exist,  we must be before OpenSSL 3. */
@@ -130,12 +134,13 @@ int pubkey_fingerprint(EVP_PKEY *pk, const EVP_MD *md, void **ret, size_t *ret_s
 
 int digest_and_sign(const EVP_MD *md, EVP_PKEY *privkey, const void *data, size_t size, void **ret, size_t *ret_size);
 
-int openssl_load_key_from_token(KeySourceType private_key_source_type, const char *private_key_source, const char *private_key, EVP_PKEY **ret);
+int openssl_load_key_from_token(KeySourceType private_key_source_type, const char *private_key_source, const char *private_key, OpenSSLAskPasswordUI *ui, EVP_PKEY **ret_private_key);
 
 #else
 
 typedef struct X509 X509;
 typedef struct EVP_PKEY EVP_PKEY;
+typedef struct UI_METHOD UI_METHOD;
 
 static inline void *X509_free(X509 *p) {
         assert(p == NULL);
@@ -147,11 +152,17 @@ static inline void *EVP_PKEY_free(EVP_PKEY *p) {
         return NULL;
 }
 
+static inline void* UI_destroy_method(UI_METHOD *p) {
+        assert(p == NULL);
+        return NULL;
+}
+
 static inline int openssl_load_key_from_token(
                 KeySourceType private_key_source_type,
                 const char *private_key_source,
                 const char *private_key,
-                EVP_PKEY **ret) {
+                OpenSSLAskPasswordUI *ui,
+                EVP_PKEY **ret_private_key) {
 
         return -EOPNOTSUPP;
 }
@@ -160,9 +171,37 @@ static inline int openssl_load_key_from_token(
 
 DEFINE_TRIVIAL_CLEANUP_FUNC_FULL(X509*, X509_free, NULL);
 DEFINE_TRIVIAL_CLEANUP_FUNC_FULL(EVP_PKEY*, EVP_PKEY_free, NULL);
+DEFINE_TRIVIAL_CLEANUP_FUNC_FULL(UI_METHOD*, UI_destroy_method, NULL);
+
+struct OpenSSLAskPasswordUI {
+        AskPasswordRequest request;
+        UI_METHOD *method;
+};
+
+int openssl_ask_password_ui_new(OpenSSLAskPasswordUI **ret);
+
+static inline OpenSSLAskPasswordUI* openssl_ask_password_ui_free(OpenSSLAskPasswordUI *ui) {
+        if (!ui)
+                return NULL;
+
+        UI_destroy_method(ui->method);
+        return mfree(ui);
+}
+
+DEFINE_TRIVIAL_CLEANUP_FUNC_FULL(OpenSSLAskPasswordUI*, openssl_ask_password_ui_free, NULL);
 
 int x509_fingerprint(X509 *cert, uint8_t buffer[static X509_FINGERPRINT_SIZE]);
 
+int openssl_load_x509_certificate(const char *path, X509 **ret);
+
+int openssl_load_private_key(
+                KeySourceType private_key_source_type,
+                const char *private_key_source,
+                const char *private_key,
+                const AskPasswordRequest *request,
+                EVP_PKEY **ret_private_key,
+                OpenSSLAskPasswordUI **ret_user_interface);
+
 #if PREFER_OPENSSL
 /* The openssl definition */
 typedef const EVP_MD* hash_md_t;