diff --git a/block/mirror.c b/block/mirror.c
index 05758e55007b..2a4333474e14 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -631,11 +631,49 @@ void commit_active_start(BlockDriverState *bs, BlockDriverState *base,
                          BlockDriverCompletionFunc *cb,
                          void *opaque, Error **errp)
 {
+    int64_t length, base_length;
+    int orig_base_flags;
+
+    orig_base_flags = bdrv_get_flags(base);
+
     if (bdrv_reopen(base, bs->open_flags, errp)) {
         return;
     }
+
+    length = bdrv_getlength(bs);
+    if (length < 0) {
+        error_setg(errp, "Unable to determine length of %s", bs->filename);
+        goto error_restore_flags;
+    }
+
+    base_length = bdrv_getlength(base);
+    if (base_length < 0) {
+        error_setg(errp, "Unable to determine length of %s", base->filename);
+        goto error_restore_flags;
+    }
+
+    if (length > base_length) {
+        if (bdrv_truncate(base, length) < 0) {
+            error_setg(errp, "Top image %s is larger than base image %s, and "
+                             "resize of base image failed",
+                             bs->filename, base->filename);
+            goto error_restore_flags;
+        }
+    }
+
     bdrv_ref(base);
     mirror_start_job(bs, base, speed, 0, 0,
                      on_error, on_error, cb, opaque, errp,
                      &commit_active_job_driver, false, base);
+    if (error_is_set(errp)) {
+        goto error_restore_flags;
+    }
+
+    return;
+
+error_restore_flags:
+    /* ignore error and errp for bdrv_reopen, because we want to propagate
+     * the original error */
+    bdrv_reopen(base, orig_base_flags, NULL);
+    return;
 }