Continue preauth after client-side failures

If the module for the selected preauth mechanism fails when processing
a KDC_ERR_MORE_PREAUTH_DATA_REQUIRED error, or fails a tryagain
operation, try again with a different preauth mech using the cached
method data.

If optimistic preauth fails on the client side, send an
unauthenticated request, allowing the mechanisms we tried
optimistically to be tried again.

ticket: 8537

src/lib/krb5/krb/get_in_tkt.c

@@ -1325,6 +1325,7 @@ init_creds_step_request(krb5_context context,
 {
     krb5_error_code code;
     krb5_preauthtype pa_type;
+    struct errinfo save = EMPTY_ERRINFO;
 
     if (ctx->loopcount >= MAX_IN_TKT_LOOPS) {
         code = KRB5_GET_IN_TKT_LOOP;
@@ -1359,38 +1360,51 @@ init_creds_step_request(krb5_context context,
     if (ctx->optimistic_padata != NULL) {
         /* Our first attempt, using an optimistic padata list. */
         TRACE_INIT_CREDS_PREAUTH_OPTIMISTIC(context);
-        code = k5_preauth(context, ctx, ctx->optimistic_padata, FALSE,
+        code = k5_preauth(context, ctx, ctx->optimistic_padata, TRUE,
                           &ctx->request->padata, &ctx->selected_preauth_type);
         krb5_free_pa_data(context, ctx->optimistic_padata);
         ctx->optimistic_padata = NULL;
-        if (code != 0)
-            goto cleanup;
+        if (code) {
+            /* Make an unauthenticated request, and possibly try again using
+             * the same mechanisms as we tried optimistically. */
+            k5_reset_preauth_types_tried(ctx);
+            krb5_clear_error_message(context);
+            code = 0;
+        }
     } if (ctx->more_padata != NULL) {
         /* Continuing after KDC_ERR_MORE_PREAUTH_DATA_REQUIRED. */
         TRACE_INIT_CREDS_PREAUTH_MORE(context, ctx->selected_preauth_type);
         code = k5_preauth(context, ctx, ctx->more_padata, TRUE,
                           &ctx->request->padata, &pa_type);
-        if (code != 0)
-            goto cleanup;
     } else if (ctx->err_reply != NULL &&
-               ctx->err_reply->error == KDC_ERR_PREAUTH_REQUIRED) {
-        /* Continuing after KDC_ERR_PREAUTH_REQUIRED, using method data. */
-        TRACE_INIT_CREDS_PREAUTH(context);
-        code = k5_preauth(context, ctx, ctx->method_padata, TRUE,
-                          &ctx->request->padata, &ctx->selected_preauth_type);
-        if (code != 0)
-            goto cleanup;
-    } else if (ctx->err_reply != NULL) {
-        /* Retry after an error other than PREAUTH_REQUIRED, using error padata
-         * to figure out what to change. */
+               ctx->err_reply->error != KDC_ERR_PREAUTH_REQUIRED) {
+        /* Retrying after an error (possibly mechanism-specific), using error
+         * padata to figure out what to change. */
         TRACE_INIT_CREDS_PREAUTH_TRYAGAIN(context, ctx->err_reply->error,
                                           ctx->selected_preauth_type);
         code = k5_preauth_tryagain(context, ctx, ctx->selected_preauth_type,
                                    ctx->err_reply, ctx->err_padata,
                                    &ctx->request->padata);
-        if (code != 0) {
-            /* couldn't come up with anything better */
+        if (code) {
+            krb5_clear_error_message(context);
             code = ctx->err_reply->error + ERROR_TABLE_BASE_krb5;
+        }
+    }
+    if (code) {
+        /* See if we can try a different preauth mech before giving up. */
+        k5_save_ctx_error(context, code, &save);
+        ctx->selected_preauth_type = KRB5_PADATA_NONE;
+    }
+
+    if (ctx->request->padata == NULL && ctx->method_padata != NULL) {
+        /* Retrying after KDC_ERR_PREAUTH_REQUIRED, or trying again with a
+         * different mechanism after a client-side failure. */
+        TRACE_INIT_CREDS_PREAUTH(context);
+        code = k5_preauth(context, ctx, ctx->method_padata, TRUE,
+                          &ctx->request->padata, &ctx->selected_preauth_type);
+        if (code) {
+            if (save.code != 0)
+                code = k5_restore_ctx_error(context, &save);
             goto cleanup;
         }
     }
@@ -1431,6 +1445,7 @@ init_creds_step_request(krb5_context context,
 cleanup:
     krb5_free_pa_data(context, ctx->request->padata);
     ctx->request->padata = NULL;
+    k5_clear_error(&save);
     return code;
 }