Fix LMS and XMSS cryptocb software fallback

Author: padelsbach

wolfcrypt/src/wc_lms.c

@@ -1422,15 +1422,16 @@ int wc_LmsKey_SigsLeft(LmsKey* key)
             if (cbRet == 0) {
                 return (sigsLeft != 0) ? 1 : 0;
             }
-            /* The device owns the private state; no safe software fallback
-             * exists because key->priv_raw does not reflect HSM state. */
             if (cbRet != WC_NO_ERR_TRACE(CRYPTOCB_UNAVAILABLE)) {
+                /* Device returned an actual error; the device owns the
+                 * private state so no safe software fallback exists. */
                 WOLFSSL_MSG("PqcStatefulSigSigsLeft returned an error");
+                return 0;
             }
-            else {
-                WOLFSSL_MSG("LMS SigsLeft not supported by device");
-            }
-            return 0;
+            /* Cryptocb declined. priv_raw reflects software state from the
+             * CRYPTOCB_UNAVAILABLE fall-through in MakeKey/Reload, so the
+             * software check below is valid. */
+            WOLFSSL_MSG("LMS SigsLeft not supported by device, using software");
         }
 #endif
         ret = wc_hss_sigsleft(key->params, key->priv_raw);

wolfcrypt/src/wc_xmss.c

@@ -1521,15 +1521,16 @@ int  wc_XmssKey_SigsLeft(XmssKey* key)
         if (cbRet == 0) {
             return (sigsLeft != 0) ? 1 : 0;
         }
-        /* The device owns the private state; no safe software fallback
-         * exists because key->sk does not reflect HSM state. */
         if (cbRet != WC_NO_ERR_TRACE(CRYPTOCB_UNAVAILABLE)) {
+            /* Device returned an actual error; the device owns the
+             * private state so no safe software fallback exists. */
             WOLFSSL_MSG("PqcStatefulSigSigsLeft returned an error");
+            return 0;
         }
-        else {
-            WOLFSSL_MSG("XMSS SigsLeft not supported by device");
-        }
-        return 0;
+        /* Cryptocb declined. Fall through to the software path: state
+         * reflects the CRYPTOCB_UNAVAILABLE fall-through in MakeKey/Reload,
+         * and read_private_key can repopulate sk from NV storage. */
+        WOLFSSL_MSG("XMSS SigsLeft not supported by device, using software");
     }
 #endif