nbd/server: CVE-2017-15118 Stack smash on large export name

Introduced in commit f37708f6b8 (2.10).  The NBD spec says a client
can request export names up to 4096 bytes in length, even though
they should not expect success on names longer than 256.  However,
qemu hard-codes the limit of 256, and fails to filter out a client
that probes for a longer name; the result is a stack smash that can
potentially give an attacker arbitrary control over the qemu
process.

The smash can be easily demonstrated with this client:
$ qemu-io f raw nbd://localhost:10809/$(printf %3000d 1 | tr ' ' a)

If the qemu NBD server binary (whether the standalone qemu-nbd, or
the builtin server of QMP nbd-server-start) was compiled with
-fstack-protector-strong, the ability to exploit the stack smash
into arbitrary execution is a lot more difficult (but still
theoretically possible to a determined attacker, perhaps in
combination with other CVEs).  Still, crashing a running qemu (and
losing the VM) is bad enough, even if the attacker did not obtain
full execution control.

CC: qemu-stable@nongnu.org
Signed-off-by: Eric Blake <eblake@redhat.com>

nbd/server.c

@@ -386,6 +386,10 @@ static int nbd_negotiate_handle_info(NBDClient *client, uint32_t length,
         msg = "name length is incorrect";
         goto invalid;
     }
+    if (namelen >= sizeof(name)) {
+        msg = "name too long for qemu";
+        goto invalid;
+    }
     if (nbd_read(client->ioc, name, namelen, errp) < 0) {
         return -EIO;
     }