diff --git a/block/curl.c b/block/curl.c
index 3494c6d6626568aaa8d966ccdfe596c114b4d08c..1b9b1f6341e67e4089422257e9243baf6632ff15 100644
--- a/block/curl.c
+++ b/block/curl.c
@@ -157,6 +157,11 @@ static size_t curl_read_cb(void *ptr, size_t size, size_t nmemb, void *opaque)
     if (!s || !s->orig_buf)
         goto read_end;
 
+    if (s->buf_off >= s->buf_len) {
+        /* buffer full, read nothing */
+        return 0;
+    }
+    realsize = MIN(realsize, s->buf_len - s->buf_off);
     memcpy(s->orig_buf + s->buf_off, ptr, realsize);
     s->buf_off += realsize;