From 1c2fe81349a99cf0c28549426c7ad0c06d942ae3 Mon Sep 17 00:00:00 2001 From: Eric Blake Date: Wed, 7 Aug 2024 12:23:13 -0500 Subject: [PATCH 5/5] nbd/server: CVE-2024-7409: Close stray clients at server-stop MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit RH-Author: Eric Blake RH-MergeRequest: 386: nbd/server: fix CVE-2024-7409 (qemu crash on nbd-server-stop) [rhel-9.4.z] RH-Jira: RHEL-52616 RH-Acked-by: Miroslav Rezanina RH-Commit: [4/4] 54d56c0d92d92b2b6a0f81a01853881286beae0e (ebblake/qemu-kvm) A malicious client can attempt to connect to an NBD server, and then intentionally delay progress in the handshake, including if it does not know the TLS secrets. Although the previous two patches reduce this behavior by capping the default max-connections parameter and killing slow clients, they did not eliminate the possibility of a client waiting to close the socket until after the QMP nbd-server-stop command is executed, at which point qemu would SEGV when trying to dereference the NULL nbd_server global which is no longer present. This amounts to a denial of service attack. Worse, if another NBD server is started before the malicious client disconnects, I cannot rule out additional adverse effects when the old client interferes with the connection count of the new server (although the most likely is a crash due to an assertion failure when checking nbd_server->connections > 0). For environments without this patch, the CVE can be mitigated by ensuring (such as via a firewall) that only trusted clients can connect to an NBD server. Note that using frameworks like libvirt that ensure that TLS is used and that nbd-server-stop is not executed while any trusted clients are still connected will only help if there is also no possibility for an untrusted client to open a connection but then stall on the NBD handshake. Given the previous patches, it would be possible to guarantee that no clients remain connected by having nbd-server-stop sleep for longer than the default handshake deadline before finally freeing the global nbd_server object, but that could make QMP non-responsive for a long time. So intead, this patch fixes the problem by tracking all client sockets opened while the server is running, and forcefully closing any such sockets remaining without a completed handshake at the time of nbd-server-stop, then waiting until the coroutines servicing those sockets notice the state change. nbd-server-stop now has a second AIO_WAIT_WHILE_UNLOCKED (the first is indirectly through the blk_exp_close_all_type() that disconnects all clients that completed handshakes), but forced socket shutdown is enough to progress the coroutines and quickly tear down all clients before the server is freed, thus finally fixing the CVE. This patch relies heavily on the fact that nbd/server.c guarantees that it only calls nbd_blockdev_client_closed() from the main loop (see the assertion in nbd_client_put() and the hoops used in nbd_client_put_nonzero() to achieve that); if we did not have that guarantee, we would also need a mutex protecting our accesses of the list of connections to survive re-entrancy from independent iothreads. Although I did not actually try to test old builds, it looks like this problem has existed since at least commit 862172f45c (v2.12.0, 2017) - even back when that patch started using a QIONetListener to handle listening on multiple sockets, nbd_server_free() was already unaware that the nbd_blockdev_client_closed callback can be reached later by a client thread that has not completed handshakes (and therefore the client's socket never got added to the list closed in nbd_export_close_all), despite that patch intentionally tearing down the QIONetListener to prevent new clients. Reported-by: Alexander Ivanov Fixes: CVE-2024-7409 CC: qemu-stable@nongnu.org Signed-off-by: Eric Blake Message-ID: <20240807174943.771624-14-eblake@redhat.com> Reviewed-by: Daniel P. Berrangé (cherry picked from commit 3e7ef738c8462c45043a1d39f702a0990406a3b3) Jira: https://issues.redhat.com/browse/RHEL-52616 Signed-off-by: Eric Blake --- blockdev-nbd.c | 35 ++++++++++++++++++++++++++++++++++- 1 file changed, 34 insertions(+), 1 deletion(-) diff --git a/blockdev-nbd.c b/blockdev-nbd.c index 24ba5382db..f73409ae49 100644 --- a/blockdev-nbd.c +++ b/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--; @@ -60,14 +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"); /* 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, NULL); + nbd_blockdev_client_closed, conn); } static void nbd_update_server_watch(NBDServerData *s) @@ -81,12 +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)); } -- 2.39.3