CVE-2019-14842

A bounds check vulnerability was found in libnbd's structured reply feature where the check was supposed to test for chunk offsets smaller than the beginning of the request but did not work because of signed/unsigned confusion. Structured reply is a feature of the newstyle NBD protocol allowing the server to send a reply in chunks. If one of these chunks contains a negative offset, then data under control of the server is written to memory before the read buffer supplied by the client. If the read buffer is located on the stack then this allows the stack, the return address from nbd_pread() to be trivially modified, allowing arbitrary code execution under the control of the server. If the buffer is located on the heap then other memory objects before the buffer can be overwritten, which again would usually lead to arbitrary code execution.