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

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 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: default avatarEric Blake <eblake@redhat.com>
Message-Id: <20170608222617.20376-1-eblake@redhat.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent 457e0355

blockdev-nbd.c

+5 −1
Original line number Diff line number Diff line
@@ -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;

}

include/block/nbd.h

+1 −1
Original line number Diff line number Diff line
@@ -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);

nbd/server.c

+15 −9
Original line number Diff line number Diff line
@@ -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) {

        return;
@@ -793,8 +793,8 @@ 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);

    }

}
@@ -975,7 +975,7 @@ 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);
@@ -1337,7 +1337,7 @@ done: 


out:

    nbd_request_put(req);

    client_close(client);

    client_close(client, true);

    nbd_client_put(client);

}
@@ -1363,7 +1363,7 @@ 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;

    }
@@ -1373,11 +1373,17 @@ 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);
@@ -1394,7 +1400,7 @@ 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);

qemu-nbd.c

+2 −2
Original line number Diff line number Diff line
@@ -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)

{

    nb_fds--;

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

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

        state = TERMINATE;

    }

    nbd_update_server_watch();