Commit 0c9390d9 authored by Eric Blake's avatar Eric Blake Committed by Paolo Bonzini

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 ee7d7aab 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 1a6245a5 (a segfault
before we could even assert); the (masked) assertion was cleaned
up in d3780c2d (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 -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: 's avatarEric Blake <>
Message-Id: <>
Signed-off-by: 's avatarPaolo Bonzini <>
parent 457e0355
......@@ -27,6 +27,10 @@ typedef struct NBDServerData {
static NBDServerData *nbd_server;
static void nbd_blockdev_client_closed(NBDClient *client, bool ignored)
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,
return TRUE;
......@@ -162,7 +162,7 @@ void nbd_client_new(NBDExport *exp,
QIOChannelSocket *sioc,
QCryptoTLSCreds *tlscreds,
const char *tlsaclname,
void (*close)(NBDClient *));
void (*close_fn)(NBDClient *, bool));
void nbd_client_get(NBDClient *client);
void nbd_client_put(NBDClient *client);
......@@ -81,7 +81,7 @@ static QTAILQ_HEAD(, NBDExport) exports = QTAILQ_HEAD_INITIALIZER(exports);
struct NBDClient {
int refcount;
void (*close)(NBDClient *client);
void (*close_fn)(NBDClient *client, bool negotiated);
bool no_zeroes;
NBDExport *exp;
......@@ -778,7 +778,7 @@ void nbd_client_put(NBDClient *client)
static void client_close(NBDClient *client)
static void client_close(NBDClient *client, bool negotiated)
if (client->closing) {
......@@ -793,8 +793,8 @@ static void client_close(NBDClient *client)
/* Also tell the client, so that they release their reference. */
if (client->close) {
if (client->close_fn) {
client->close_fn(client, negotiated);
......@@ -975,7 +975,7 @@ void nbd_export_close(NBDExport *exp)
QTAILQ_FOREACH_SAFE(client, &exp->clients, next, next) {
client_close(client, true);
nbd_export_set_name(exp, NULL);
nbd_export_set_description(exp, NULL);
......@@ -1337,7 +1337,7 @@ done:
client_close(client, true);
......@@ -1363,7 +1363,7 @@ static coroutine_fn void nbd_co_client_start(void *opaque)
if (nbd_negotiate(data)) {
client_close(client, false);
goto out;
......@@ -1373,11 +1373,17 @@ out:
* Create a new client listener on the given export @exp, using the
* given channel @sioc. Begin servicing it in a coroutine. When the
* connection closes, call @close_fn with an indication of whether the
* client completed negotiation.
void nbd_client_new(NBDExport *exp,
QIOChannelSocket *sioc,
QCryptoTLSCreds *tlscreds,
const char *tlsaclname,
void (*close_fn)(NBDClient *))
void (*close_fn)(NBDClient *, bool))
NBDClient *client;
NBDClientNewData *data = g_new(NBDClientNewData, 1);
......@@ -1394,7 +1400,7 @@ void nbd_client_new(NBDExport *exp,
client->ioc = QIO_CHANNEL(sioc);
client->close = close_fn;
client->close_fn = close_fn;
data->client = client;
data->co = qemu_coroutine_create(nbd_co_client_start, data);
......@@ -336,10 +336,10 @@ static void nbd_export_closed(NBDExport *exp)
static void nbd_update_server_watch(void);
static void nbd_client_closed(NBDClient *client)
static void nbd_client_closed(NBDClient *client, bool negotiated)
if (nb_fds == 0 && !persistent && state == RUNNING) {
if (negotiated && nb_fds == 0 && !persistent && state == RUNNING) {
state = TERMINATE;
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