nbd-client: Stricter enforcing of structured reply spec

Ensure that the server is not sending unexpected chunk lengths
for either the NONE or the OFFSET_DATA chunk, nor unexpected
hole length for OFFSET_HOLE.  This will flag any server as
broken that responds to a zero-length read with an OFFSET_DATA
(what our server currently does, but that's about to be fixed)
or with OFFSET_HOLE, even though we previously fixed our client
to never be able to send such a request over the wire.
Signed-off-by: NEric Blake <eblake@redhat.com>
Message-Id: <20171108215703.9295-7-eblake@redhat.com>
Reviewed-by: NVladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

block/nbd-client.c

@@ -216,7 +216,7 @@ static int nbd_parse_offset_hole_payload(NBDStructuredReplyChunk *chunk,
     offset = payload_advance64(&payload);
     hole_size = payload_advance32(&payload);
 
-    if (offset < orig_offset || hole_size > qiov->size ||
+    if (!hole_size || offset < orig_offset || hole_size > qiov->size ||
         offset > orig_offset + qiov->size - hole_size) {
         error_setg(errp, "Protocol error: server sent chunk exceeding requested"
                          " region");
@@ -281,7 +281,8 @@ static int nbd_co_receive_offset_data_payload(NBDClientSession *s,
 
     assert(nbd_reply_is_structured(&s->reply));
 
-    if (chunk->length < sizeof(offset)) {
+    /* The NBD spec requires at least one byte of payload */
+    if (chunk->length <= sizeof(offset)) {
         error_setg(errp, "Protocol error: invalid payload for "
                          "NBD_REPLY_TYPE_OFFSET_DATA");
         return -EINVAL;
@@ -293,6 +294,7 @@ static int nbd_co_receive_offset_data_payload(NBDClientSession *s,
     be64_to_cpus(&offset);
 
     data_size = chunk->length - sizeof(offset);
+    assert(data_size);
     if (offset < orig_offset || data_size > qiov->size ||
         offset > orig_offset + qiov->size - data_size) {
         error_setg(errp, "Protocol error: server sent chunk exceeding requested"
@@ -411,6 +413,11 @@ static coroutine_fn int nbd_co_do_receive_one_chunk(
                        " NBD_REPLY_FLAG_DONE flag set");
             return -EINVAL;
         }
+        if (chunk->length) {
+            error_setg(errp, "Protocol error: NBD_REPLY_TYPE_NONE chunk with"
+                       " nonzero length");
+            return -EINVAL;
+        }
         return 0;
     }