nbd: Fix regression on resiliency to port scan

Back in qemu 2.5, qemu-nbd was immune to port probes (a transient
server would not quit, regardless of how many probe connections
came and went, until a connection actually negotiated).  But we
broke that in commit ee7d7aa when removing the return value to
nbd_client_new(), although that patch also introduced a bug causing
an assertion failure on a client that fails negotiation.  We then
made it worse during refactoring in commit 1a6245a (a segfault
before we could even assert); the (masked) assertion was cleaned
up in d3780c2 (still in 2.6), and just recently we finally fixed
the segfault ("nbd: Fully intialize client in case of failed
negotiation").  But that still means that ever since we added
TLS support to qemu-nbd, we have been vulnerable to an ill-timed
port-scan being able to cause a denial of service by taking down
qemu-nbd before a real client has a chance to connect.

Since negotiation is now handled asynchronously via coroutines,
we no longer have a synchronous point of return by re-adding a
return value to nbd_client_new().  So this patch instead wires
things up to pass the negotiation status through the close_fn
callback function.

Simple test across two terminals:
$ qemu-nbd -f raw -p 30001 file
$ nmap 127.0.0.1 -p 30001 && \
  qemu-io -c 'r 0 512' -f raw nbd://localhost:30001

Note that this patch does not change what constitutes successful
negotiation (thus, a client must enter transmission phase before
that client can be considered as a reason to terminate the server
when the connection ends).  Perhaps we may want to tweak things
in a later patch to also treat a client that uses NBD_OPT_ABORT
as being a 'successful' negotiation (the client correctly talked
the NBD protocol, and informed us it was not going to use our
export after all), but that's a discussion for another day.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1451614

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20170608222617.20376-1-eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 5 insertions, 1 deletions

diff --git a/blockdev-nbd.c b/blockdev-nbd.c
index dd0860f4a6..28f551a7b0 100644
--- a/blockdev-nbd.c
+++ b/blockdev-nbd.c
@@ -27,6 +27,10 @@ typedef struct NBDServerData {
 
 static NBDServerData *nbd_server;
 
+static void nbd_blockdev_client_closed(NBDClient *client, bool ignored)
+{
+    nbd_client_put(client);
+}
 
 static gboolean nbd_accept(QIOChannel *ioc, GIOCondition condition,
                            gpointer opaque)
@@ -46,7 +50,7 @@ static gboolean nbd_accept(QIOChannel *ioc, GIOCondition condition,
     qio_channel_set_name(QIO_CHANNEL(cioc), "nbd-server");
     nbd_client_new(NULL, cioc,
                    nbd_server->tlscreds, NULL,
-                   nbd_client_put);
+                   nbd_blockdev_client_closed);
     object_unref(OBJECT(cioc));
     return TRUE;
 }