NBD patches for 2024-08-08

- plug CVE-2024-7409, a DoS attack exploiting nbd-server-stop
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Merge tag 'pull-nbd-2024-08-08' of https://repo.or.cz/qemu/ericb into staging

NBD patches for 2024-08-08

- plug CVE-2024-7409, a DoS attack exploiting nbd-server-stop

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# Q2qdHQf/dMydqNcPYnwEI238APyljpNvHNq6p9TYb0l5aVWisXHRlhFWM117hH7T
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# gpg: Signature made Fri 09 Aug 2024 07:49:05 AM AEST
# gpg:                using RSA key 71C2CC22B1C4602927D2F3AAA7A16B4A2527436A
# gpg: Good signature from "Eric Blake <eblake@redhat.com>" [full]
# gpg:                 aka "Eric Blake (Free Software Programmer) <ebb9@byu.net>" [full]
# gpg:                 aka "[jpeg image of size 6874]" [full]

* tag 'pull-nbd-2024-08-08' of https://repo.or.cz/qemu/ericb:
  nbd/server: CVE-2024-7409: Close stray clients at server-stop
  nbd/server: CVE-2024-7409: Drop non-negotiating clients
  nbd/server: CVE-2024-7409: Cap default max-connections to 100
  nbd/server: Plumb in new args to nbd_client_add()
  nbd: Minor style and typo fixes

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

block/monitor/block-hmp-cmds.c

@@ -402,7 +402,8 @@ void hmp_nbd_server_start(Monitor *mon, const QDict *qdict)
         goto exit;
     }
 
-    nbd_server_start(addr, NULL, NULL, 0, &local_err);
+    nbd_server_start(addr, NULL, NULL, NBD_DEFAULT_MAX_CONNECTIONS,
+                     &local_err);
     qapi_free_SocketAddress(addr);
     if (local_err != NULL) {
         goto exit;

blockdev-nbd.c

@@ -21,12 +21,18 @@
 #include "io/channel-socket.h"
 #include "io/net-listener.h"
 
+typedef struct NBDConn {
+    QIOChannelSocket *cioc;
+    QLIST_ENTRY(NBDConn) next;
+} NBDConn;
+
 typedef struct NBDServerData {
     QIONetListener *listener;
     QCryptoTLSCreds *tlscreds;
     char *tlsauthz;
     uint32_t max_connections;
     uint32_t connections;
+    QLIST_HEAD(, NBDConn) conns;
 } NBDServerData;
 
 static NBDServerData *nbd_server;
@@ -51,6 +57,14 @@ int nbd_server_max_connections(void)
 
 static void nbd_blockdev_client_closed(NBDClient *client, bool ignored)
 {
+    NBDConn *conn = nbd_client_owner(client);
+
+    assert(qemu_in_main_thread() && nbd_server);
+
+    object_unref(OBJECT(conn->cioc));
+    QLIST_REMOVE(conn, next);
+    g_free(conn);
+
     nbd_client_put(client);
     assert(nbd_server->connections > 0);
     nbd_server->connections--;
@@ -60,12 +74,20 @@ static void nbd_blockdev_client_closed(NBDClient *client, bool ignored)
 static void nbd_accept(QIONetListener *listener, QIOChannelSocket *cioc,
                        gpointer opaque)
 {
+    NBDConn *conn = g_new0(NBDConn, 1);
+
+    assert(qemu_in_main_thread() && nbd_server);
     nbd_server->connections++;
+    object_ref(OBJECT(cioc));
+    conn->cioc = cioc;
+    QLIST_INSERT_HEAD(&nbd_server->conns, conn, next);
     nbd_update_server_watch(nbd_server);
 
     qio_channel_set_name(QIO_CHANNEL(cioc), "nbd-server");
-    nbd_client_new(cioc, nbd_server->tlscreds, nbd_server->tlsauthz,
-                   nbd_blockdev_client_closed);
+    /* TODO - expose handshake timeout as QMP option */
+    nbd_client_new(cioc, NBD_DEFAULT_HANDSHAKE_MAX_SECS,
+                   nbd_server->tlscreds, nbd_server->tlsauthz,
+                   nbd_blockdev_client_closed, conn);
 }
 
 static void nbd_update_server_watch(NBDServerData *s)
@@ -79,12 +101,25 @@ static void nbd_update_server_watch(NBDServerData *s)
 
 static void nbd_server_free(NBDServerData *server)
 {
+    NBDConn *conn, *tmp;
+
     if (!server) {
         return;
     }
 
+    /*
+     * Forcefully close the listener socket, and any clients that have
+     * not yet disconnected on their own.
+     */
     qio_net_listener_disconnect(server->listener);
     object_unref(OBJECT(server->listener));
+    QLIST_FOREACH_SAFE(conn, &server->conns, next, tmp) {
+        qio_channel_shutdown(QIO_CHANNEL(conn->cioc), QIO_CHANNEL_SHUTDOWN_BOTH,
+                             NULL);
+    }
+
+    AIO_WAIT_WHILE_UNLOCKED(NULL, server->connections > 0);
+
     if (server->tlscreds) {
         object_unref(OBJECT(server->tlscreds));
     }
@@ -168,6 +203,10 @@ void nbd_server_start(SocketAddress *addr, const char *tls_creds,
 
 void nbd_server_start_options(NbdServerOptions *arg, Error **errp)
 {
+    if (!arg->has_max_connections) {
+        arg->max_connections = NBD_DEFAULT_MAX_CONNECTIONS;
+    }
+
     nbd_server_start(arg->addr, arg->tls_creds, arg->tls_authz,
                      arg->max_connections, errp);
 }
@@ -180,6 +219,10 @@ void qmp_nbd_server_start(SocketAddressLegacy *addr,
 {
     SocketAddress *addr_flat = socket_address_flatten(addr);
 
+    if (!has_max_connections) {
+        max_connections = NBD_DEFAULT_MAX_CONNECTIONS;
+    }
+
     nbd_server_start(addr_flat, tls_creds, tls_authz, max_connections, errp);
     qapi_free_SocketAddress(addr_flat);
 }

include/block/nbd.h

@@ -33,6 +33,19 @@ typedef struct NBDMetaContexts NBDMetaContexts;
 
 extern const BlockExportDriver blk_exp_nbd;
 
+/*
+ * NBD_DEFAULT_HANDSHAKE_MAX_SECS: Number of seconds in which client must
+ * succeed at NBD_OPT_GO before being forcefully dropped as too slow.
+ */
+#define NBD_DEFAULT_HANDSHAKE_MAX_SECS 10
+
+/*
+ * NBD_DEFAULT_MAX_CONNECTIONS: Number of client sockets to allow at
+ * once; must be large enough to allow a MULTI_CONN-aware client like
+ * nbdcopy to create its typical number of 8-16 sockets.
+ */
+#define NBD_DEFAULT_MAX_CONNECTIONS 100
+
 /* Handshake phase structs - this struct is passed on the wire */
 
 typedef struct NBDOption {
@@ -403,9 +416,12 @@ AioContext *nbd_export_aio_context(NBDExport *exp);
 NBDExport *nbd_export_find(const char *name);
 
 void nbd_client_new(QIOChannelSocket *sioc,
+                    uint32_t handshake_max_secs,
                     QCryptoTLSCreds *tlscreds,
                     const char *tlsauthz,
-                    void (*close_fn)(NBDClient *, bool));
+                    void (*close_fn)(NBDClient *, bool),
+                    void *owner);
+void *nbd_client_owner(NBDClient *client);
 void nbd_client_get(NBDClient *client);
 void nbd_client_put(NBDClient *client);
 

nbd/server.c

@@ -124,12 +124,14 @@ struct NBDMetaContexts {
 struct NBDClient {
     int refcount; /* atomic */
     void (*close_fn)(NBDClient *client, bool negotiated);
+    void *owner;
 
     QemuMutex lock;
 
     NBDExport *exp;
     QCryptoTLSCreds *tlscreds;
     char *tlsauthz;
+    uint32_t handshake_max_secs;
     QIOChannelSocket *sioc; /* The underlying data channel */
     QIOChannel *ioc; /* The current I/O channel which may differ (eg TLS) */
 
@@ -1972,7 +1974,7 @@ static void nbd_export_request_shutdown(BlockExport *blk_exp)
 
     blk_exp_ref(&exp->common);
     /*
-     * TODO: Should we expand QMP NbdServerRemoveNode enum to allow a
+     * TODO: Should we expand QMP BlockExportRemoveMode enum to allow a
      * close mode that stops advertising the export to new clients but
      * still permits existing clients to run to completion? Because of
      * that possibility, nbd_export_close() can be called more than
@@ -3184,35 +3186,65 @@ static void nbd_client_receive_next_request(NBDClient *client)
     }
 }
 
+static void nbd_handshake_timer_cb(void *opaque)
+{
+    QIOChannel *ioc = opaque;
+
+    trace_nbd_handshake_timer_cb();
+    qio_channel_shutdown(ioc, QIO_CHANNEL_SHUTDOWN_BOTH, NULL);
+}
+
 static coroutine_fn void nbd_co_client_start(void *opaque)
 {
     NBDClient *client = opaque;
     Error *local_err = NULL;
+    QEMUTimer *handshake_timer = NULL;
 
     qemu_co_mutex_init(&client->send_lock);
 
+    /*
+     * Create a timer to bound the time spent in negotiation. If the
+     * timer expires, it is likely nbd_negotiate will fail because the
+     * socket was shutdown.
+     */
+    if (client->handshake_max_secs > 0) {
+        handshake_timer = aio_timer_new(qemu_get_aio_context(),
+                                        QEMU_CLOCK_REALTIME,
+                                        SCALE_NS,
+                                        nbd_handshake_timer_cb,
+                                        client->sioc);
+        timer_mod(handshake_timer,
+                  qemu_clock_get_ns(QEMU_CLOCK_REALTIME) +
+                  client->handshake_max_secs * NANOSECONDS_PER_SECOND);
+    }
+
     if (nbd_negotiate(client, &local_err)) {
         if (local_err) {
             error_report_err(local_err);
         }
+        timer_free(handshake_timer);
         client_close(client, false);
         return;
     }
 
+    timer_free(handshake_timer);
     WITH_QEMU_LOCK_GUARD(&client->lock) {
         nbd_client_receive_next_request(client);
     }
 }
 
 /*
- * Create a new client listener using the given channel @sioc.
+ * Create a new client listener using the given channel @sioc and @owner.
  * Begin servicing it in a coroutine.  When the connection closes, call
- * @close_fn with an indication of whether the client completed negotiation.
+ * @close_fn with an indication of whether the client completed negotiation
+ * within @handshake_max_secs seconds (0 for unbounded).
  */
 void nbd_client_new(QIOChannelSocket *sioc,
+                    uint32_t handshake_max_secs,
                     QCryptoTLSCreds *tlscreds,
                     const char *tlsauthz,
-                    void (*close_fn)(NBDClient *, bool))
+                    void (*close_fn)(NBDClient *, bool),
+                    void *owner)
 {
     NBDClient *client;
     Coroutine *co;
@@ -3225,13 +3257,21 @@ void nbd_client_new(QIOChannelSocket *sioc,
         object_ref(OBJECT(client->tlscreds));
     }
     client->tlsauthz = g_strdup(tlsauthz);
+    client->handshake_max_secs = handshake_max_secs;
     client->sioc = sioc;
     qio_channel_set_delay(QIO_CHANNEL(sioc), false);
     object_ref(OBJECT(client->sioc));
     client->ioc = QIO_CHANNEL(sioc);
     object_ref(OBJECT(client->ioc));
     client->close_fn = close_fn;
+    client->owner = owner;
 
     co = qemu_coroutine_create(nbd_co_client_start, client);
     qemu_coroutine_enter(co);
 }
+
+void *
+nbd_client_owner(NBDClient *client)
+{
+    return client->owner;
+}

nbd/trace-events

@@ -76,6 +76,7 @@ nbd_co_receive_request_payload_received(uint64_t cookie, uint64_t len) "Payload
 nbd_co_receive_ext_payload_compliance(uint64_t from, uint64_t len) "client sent non-compliant write without payload flag: from=0x%" PRIx64 ", len=0x%" PRIx64
 nbd_co_receive_align_compliance(const char *op, uint64_t from, uint64_t len, uint32_t align) "client sent non-compliant unaligned %s request: from=0x%" PRIx64 ", len=0x%" PRIx64 ", align=0x%" PRIx32
 nbd_trip(void) "Reading request"
+nbd_handshake_timer_cb(void) "client took too long to negotiate"
 
 # client-connection.c
 nbd_connect_thread_sleep(uint64_t timeout) "timeout %" PRIu64

qapi/block-export.json

@@ -28,7 +28,7 @@
 # @max-connections: The maximum number of connections to allow at the
 #     same time, 0 for unlimited.  Setting this to 1 also stops the
 #     server from advertising multiple client support (since 5.2;
-#     default: 0)
+#     default: 100)
 #
 # Since: 4.2
 ##
@@ -63,7 +63,7 @@
 # @max-connections: The maximum number of connections to allow at the
 #     same time, 0 for unlimited.  Setting this to 1 also stops the
 #     server from advertising multiple client support (since 5.2;
-#     default: 0).
+#     default: 100).
 #
 # Errors:
 #     - if the server is already running

qemu-nbd.c

@@ -390,7 +390,9 @@ static void nbd_accept(QIONetListener *listener, QIOChannelSocket *cioc,
 
     nb_fds++;
     nbd_update_server_watch();
-    nbd_client_new(cioc, tlscreds, tlsauthz, nbd_client_closed);
+    /* TODO - expose handshake timeout as command line option */
+    nbd_client_new(cioc, NBD_DEFAULT_HANDSHAKE_MAX_SECS,
+                   tlscreds, tlsauthz, nbd_client_closed, NULL);
 }
 
 static void nbd_update_server_watch(void)
@@ -588,7 +590,8 @@ int main(int argc, char **argv)
     pthread_t client_thread;
     const char *fmt = NULL;
     Error *local_err = NULL;
-    BlockdevDetectZeroesOptions detect_zeroes = BLOCKDEV_DETECT_ZEROES_OPTIONS_OFF;
+    BlockdevDetectZeroesOptions detect_zeroes =
+        BLOCKDEV_DETECT_ZEROES_OPTIONS_OFF;
     QDict *options = NULL;
     const char *export_name = NULL; /* defaults to "" later for server mode */
     const char *export_description = NULL;