From: Eric Blake <eblake@redhat.com>

Date: Thu, 8 Jun 2017 17:26:17 -0500

Subject: [PATCH] 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>

(cherry picked from commit 0c9390d978cbf61e8f16c9f580fa96b305c43568)

---

 blockdev-nbd.c      |  6 +++++-

 include/block/nbd.h |  2 +-

 nbd/server.c        | 24 +++++++++++++++---------

 qemu-nbd.c          |  4 ++--

 4 files changed, 23 insertions(+), 13 deletions(-)

diff --git a/blockdev-nbd.c b/blockdev-nbd.c

index 8a11807df3..8d7284ac56 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;

 }

diff --git a/include/block/nbd.h b/include/block/nbd.h

index 3e373f0498..b69c30d063 100644

--- a/include/block/nbd.h

+++ b/include/block/nbd.h

@@ -160,7 +160,7 @@ 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);

diff --git a/nbd/server.c b/nbd/server.c

index edfda84d43..a98bb21a0a 100644

--- a/nbd/server.c

+++ b/nbd/server.c

@@ -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;

@@ -796,7 +796,7 @@ void nbd_client_put(NBDClient *client)

     }

 }

-static void client_close(NBDClient *client)

+static void client_close(NBDClient *client, bool negotiated)

 {

     if (client->closing) {

         return;

@@ -811,8 +811,8 @@ static void client_close(NBDClient *client)

                          NULL);

     /* Also tell the client, so that they release their reference.  */

-    if (client->close) {

-        client->close(client);

+    if (client->close_fn) {

+        client->close_fn(client, negotiated);

     }

 }

@@ -993,7 +993,7 @@ void nbd_export_close(NBDExport *exp)

     nbd_export_get(exp);

     QTAILQ_FOREACH_SAFE(client, &exp->clients, next, next) {

-        client_close(client);

+        client_close(client, true);

     }

     nbd_export_set_name(exp, NULL);

     nbd_export_set_description(exp, NULL);

@@ -1355,7 +1355,7 @@ done:

 out:

     nbd_request_put(req);

-    client_close(client);

+    client_close(client, true);

     nbd_client_put(client);

 }

@@ -1381,7 +1381,7 @@ static coroutine_fn void nbd_co_client_start(void *opaque)

     qemu_co_mutex_init(&client->send_lock);

     if (nbd_negotiate(data)) {

-        client_close(client);

+        client_close(client, false);

         goto out;

     }

@@ -1391,11 +1391,17 @@ out:

     g_free(data);

 }

+/*

+ * 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);

@@ -1412,7 +1418,7 @@ void nbd_client_new(NBDExport *exp,

     object_ref(OBJECT(client->sioc));

     client->ioc = QIO_CHANNEL(sioc);

     object_ref(OBJECT(client->ioc));

-    client->close = close_fn;

+    client->close_fn = close_fn;

     data->client = client;

     data->co = qemu_coroutine_create(nbd_co_client_start, data);

diff --git a/qemu-nbd.c b/qemu-nbd.c

index b44764eb87..483dd77a77 100644

--- a/qemu-nbd.c

+++ b/qemu-nbd.c

@@ -335,10 +335,10 @@ 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)

 {

     nb_fds--;

-    if (nb_fds == 0 && !persistent && state == RUNNING) {

+    if (negotiated && nb_fds == 0 && !persistent && state == RUNNING) {

         state = TERMINATE;

     }

     nbd_update_server_watch();