STORE: Make sure the loader to be registered is complete

Most of the loader function pointers are crucial, they must be defined
unconditionally.  Therefore, let's make sure OSSL_STORE_register_loader
refuses to register incomplete loaders
Reviewed-by: NBen Kaduk <kaduk@mit.edu>
(Merged from https://github.com/openssl/openssl/pull/3805)

crypto/err/openssl.txt

@@ -1990,6 +1990,7 @@ OSSL_STORE_R_BAD_PASSWORD_READ:115:bad password read
 OSSL_STORE_R_ERROR_VERIFYING_PKCS12_MAC:113:error verifying pkcs12 mac
 OSSL_STORE_R_INVALID_SCHEME:106:invalid scheme
 OSSL_STORE_R_IS_NOT_A:112:is not a
+OSSL_STORE_R_LOADER_INCOMPLETE:116:loader incomplete
 OSSL_STORE_R_NOT_A_CERTIFICATE:100:not a certificate
 OSSL_STORE_R_NOT_A_CRL:101:not a crl
 OSSL_STORE_R_NOT_A_KEY:102:not a key

crypto/store/store_err.c

@@ -85,6 +85,8 @@ static const ERR_STRING_DATA OSSL_STORE_str_reasons[] = {
     {ERR_PACK(ERR_LIB_OSSL_STORE, 0, OSSL_STORE_R_INVALID_SCHEME),
     "invalid scheme"},
     {ERR_PACK(ERR_LIB_OSSL_STORE, 0, OSSL_STORE_R_IS_NOT_A), "is not a"},
+    {ERR_PACK(ERR_LIB_OSSL_STORE, 0, OSSL_STORE_R_LOADER_INCOMPLETE),
+    "loader incomplete"},
     {ERR_PACK(ERR_LIB_OSSL_STORE, 0, OSSL_STORE_R_NOT_A_CERTIFICATE),
     "not a certificate"},
     {ERR_PACK(ERR_LIB_OSSL_STORE, 0, OSSL_STORE_R_NOT_A_CRL), "not a crl"},

crypto/store/store_register.c

@@ -153,6 +153,14 @@ int ossl_store_register_loader_int(OSSL_STORE_LOADER *loader)
         return 0;
     }
 
+    /* Check that functions we absolutely require are present */
+    if (loader->open == NULL || loader->load == NULL || loader->eof == NULL
+        || loader->error == NULL || loader->close == NULL) {
+        OSSL_STOREerr(OSSL_STORE_F_OSSL_STORE_REGISTER_LOADER_INT,
+                      OSSL_STORE_R_LOADER_INCOMPLETE);
+        return 0;
+    }
+
     if (!RUN_ONCE(&registry_init, do_registry_init)) {
         OSSL_STOREerr(OSSL_STORE_F_OSSL_STORE_REGISTER_LOADER_INT,
                       ERR_R_MALLOC_FAILURE);

include/openssl/storeerr.h

@@ -62,6 +62,7 @@ int ERR_load_OSSL_STORE_strings(void);
 # define OSSL_STORE_R_ERROR_VERIFYING_PKCS12_MAC          113
 # define OSSL_STORE_R_INVALID_SCHEME                      106
 # define OSSL_STORE_R_IS_NOT_A                            112
+# define OSSL_STORE_R_LOADER_INCOMPLETE                   116
 # define OSSL_STORE_R_NOT_A_CERTIFICATE                   100
 # define OSSL_STORE_R_NOT_A_CRL                           101
 # define OSSL_STORE_R_NOT_A_KEY                           102