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([98.142.47.158]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-205152b1446sm11241235ad.58.2024.08.29.06.32.50 for (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 29 Aug 2024 06:32:51 -0700 (PDT) From: Steve Sakoman To: openembedded-core@lists.openembedded.org Subject: [OE-core][scarthgap 06/12] qemu: fix CVE-2024-7409 Date: Thu, 29 Aug 2024 06:32:29 -0700 Message-Id: <334f70c408ce5c95f145aa4657f343b023f7e1b4.1724938187.git.steve@sakoman.com> X-Mailer: git-send-email 2.34.1 In-Reply-To: References: MIME-Version: 1.0 List-Id: X-Webhook-Received: from li982-79.members.linode.com [45.33.32.79] by aws-us-west-2-korg-lkml-1.web.codeaurora.org with HTTPS for ; Thu, 29 Aug 2024 13:32:54 -0000 X-Groupsio-URL: https://lists.openembedded.org/g/openembedded-core/message/203926 From: Archana Polampalli A flaw was found in the QEMU NBD Server. This vulnerability allows a denial of service (DoS) attack via improper synchronization during socket closure when a client keeps a socket open as the server is taken offline. Signed-off-by: Archana Polampalli Signed-off-by: Steve Sakoman --- meta/recipes-devtools/qemu/qemu.inc | 4 + .../qemu/qemu/CVE-2024-7409-0001.patch | 167 +++++++++++++++++ .../qemu/qemu/CVE-2024-7409-0002.patch | 175 ++++++++++++++++++ .../qemu/qemu/CVE-2024-7409-0003.patch | 126 +++++++++++++ .../qemu/qemu/CVE-2024-7409-0004.patch | 164 ++++++++++++++++ 5 files changed, 636 insertions(+) create mode 100644 meta/recipes-devtools/qemu/qemu/CVE-2024-7409-0001.patch create mode 100644 meta/recipes-devtools/qemu/qemu/CVE-2024-7409-0002.patch create mode 100644 meta/recipes-devtools/qemu/qemu/CVE-2024-7409-0003.patch create mode 100644 meta/recipes-devtools/qemu/qemu/CVE-2024-7409-0004.patch diff --git a/meta/recipes-devtools/qemu/qemu.inc b/meta/recipes-devtools/qemu/qemu.inc index 50d92b04bd..a1d8a309a0 100644 --- a/meta/recipes-devtools/qemu/qemu.inc +++ b/meta/recipes-devtools/qemu/qemu.inc @@ -45,6 +45,10 @@ SRC_URI = "https://download.qemu.org/${BPN}-${PV}.tar.xz \ file://CVE-2024-4467-0003.patch \ file://CVE-2024-4467-0004.patch \ file://CVE-2024-4467-0005.patch \ + file://CVE-2024-7409-0001.patch \ + file://CVE-2024-7409-0002.patch \ + file://CVE-2024-7409-0003.patch \ + file://CVE-2024-7409-0004.patch \ " UPSTREAM_CHECK_REGEX = "qemu-(?P\d+(\.\d+)+)\.tar" diff --git a/meta/recipes-devtools/qemu/qemu/CVE-2024-7409-0001.patch b/meta/recipes-devtools/qemu/qemu/CVE-2024-7409-0001.patch new file mode 100644 index 0000000000..631e93a6d2 --- /dev/null +++ b/meta/recipes-devtools/qemu/qemu/CVE-2024-7409-0001.patch @@ -0,0 +1,167 @@ +From fb1c2aaa981e0a2fa6362c9985f1296b74f055ac Mon Sep 17 00:00:00 2001 +From: Eric Blake +Date: Wed, 7 Aug 2024 08:50:01 -0500 +Subject: [PATCH] nbd/server: Plumb in new args to nbd_client_add() + +Upcoming patches to fix a CVE need to track an opaque pointer passed +in by the owner of a client object, as well as request for a time +limit on how fast negotiation must complete. Prepare for that by +changing the signature of nbd_client_new() and adding an accessor to +get at the opaque pointer, although for now the two servers +(qemu-nbd.c and blockdev-nbd.c) do not change behavior even though +they pass in a new default timeout value. + +Suggested-by: Vladimir Sementsov-Ogievskiy +Signed-off-by: Eric Blake +Message-ID: <20240807174943.771624-11-eblake@redhat.com> +Reviewed-by: Daniel P. Berrangé +[eblake: s/LIMIT/MAX_SECS/ as suggested by Dan] +Signed-off-by: Eric Blake + +CVE: CVE-2024-7409 + +Upstream-Status: Backport [https://github.com/qemu/qemu/commit/fb1c2aaa981e0a2fa6362c9985f1296b74f055ac] + +Signed-off-by: Archana Polampalli +--- + blockdev-nbd.c | 6 ++++-- + include/block/nbd.h | 11 ++++++++++- + nbd/server.c | 20 +++++++++++++++++--- + qemu-nbd.c | 4 +++- + 4 files changed, 34 insertions(+), 7 deletions(-) + +diff --git a/blockdev-nbd.c b/blockdev-nbd.c +index 213012435..267a1de90 100644 +--- a/blockdev-nbd.c ++++ b/blockdev-nbd.c +@@ -64,8 +64,10 @@ static void nbd_accept(QIONetListener *listener, QIOChannelSocket *cioc, + 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, NULL); + } + + static void nbd_update_server_watch(NBDServerData *s) +diff --git a/include/block/nbd.h b/include/block/nbd.h +index 4e7bd6342..1d4d65922 100644 +--- a/include/block/nbd.h ++++ b/include/block/nbd.h +@@ -33,6 +33,12 @@ 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 ++ + /* Handshake phase structs - this struct is passed on the wire */ + + typedef struct NBDOption { +@@ -403,9 +409,12 @@ 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); + +diff --git a/nbd/server.c b/nbd/server.c +index 091b57119..f8881936e 100644 +--- a/nbd/server.c ++++ b/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) */ + +@@ -3160,6 +3162,7 @@ static coroutine_fn void nbd_co_client_start(void *opaque) + + qemu_co_mutex_init(&client->send_lock); + ++ /* TODO - utilize client->handshake_max_secs */ + if (nbd_negotiate(client, &local_err)) { + if (local_err) { + error_report_err(local_err); +@@ -3174,14 +3177,17 @@ static coroutine_fn void nbd_co_client_start(void *opaque) + } + + /* +- * 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; +@@ -3194,13 +3200,21 @@ 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; ++} +diff --git a/qemu-nbd.c b/qemu-nbd.c +index 186e6468b..5fa399c0b 100644 +--- a/qemu-nbd.c ++++ b/qemu-nbd.c +@@ -389,7 +389,9 @@ 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) +-- +2.40.0 diff --git a/meta/recipes-devtools/qemu/qemu/CVE-2024-7409-0002.patch b/meta/recipes-devtools/qemu/qemu/CVE-2024-7409-0002.patch new file mode 100644 index 0000000000..ca8ef0b44d --- /dev/null +++ b/meta/recipes-devtools/qemu/qemu/CVE-2024-7409-0002.patch @@ -0,0 +1,175 @@ +From c8a76dbd90c2f48df89b75bef74917f90a59b623 Mon Sep 17 00:00:00 2001 +From: Eric Blake +Date: Tue, 6 Aug 2024 13:53:00 -0500 +Subject: [PATCH] nbd/server: CVE-2024-7409: Cap default max-connections to 100 + +Allowing an unlimited number of clients to any web service is a recipe +for a rudimentary denial of service attack: the client merely needs to +open lots of sockets without closing them, until qemu no longer has +any more fds available to allocate. + +For qemu-nbd, we default to allowing only 1 connection unless more are +explicitly asked for (-e or --shared); this was historically picked as +a nice default (without an explicit -t, a non-persistent qemu-nbd goes +away after a client disconnects, without needing any additional +follow-up commands), and we are not going to change that interface now +(besides, someday we want to point people towards qemu-storage-daemon +instead of qemu-nbd). + +But for qemu proper, and the newer qemu-storage-daemon, the QMP +nbd-server-start command has historically had a default of unlimited +number of connections, in part because unlike qemu-nbd it is +inherently persistent until nbd-server-stop. Allowing multiple client +sockets is particularly useful for clients that can take advantage of +MULTI_CONN (creating parallel sockets to increase throughput), +although known clients that do so (such as libnbd's nbdcopy) typically +use only 8 or 16 connections (the benefits of scaling diminish once +more sockets are competing for kernel attention). Picking a number +large enough for typical use cases, but not unlimited, makes it +slightly harder for a malicious client to perform a denial of service +merely by opening lots of connections withot progressing through the +handshake. + +This change does not eliminate CVE-2024-7409 on its own, but reduces +the chance for fd exhaustion or unlimited memory usage as an attack +surface. On the other hand, by itself, it makes it more obvious that +with a finite limit, we have the problem of an unauthenticated client +holding 100 fds opened as a way to block out a legitimate client from +being able to connect; thus, later patches will further add timeouts +to reject clients that are not making progress. + +This is an INTENTIONAL change in behavior, and will break any client +of nbd-server-start that was not passing an explicit max-connections +parameter, yet expects more than 100 simultaneous connections. We are +not aware of any such client (as stated above, most clients aware of +MULTI_CONN get by just fine on 8 or 16 connections, and probably cope +with later connections failing by relying on the earlier connections; +libvirt has not yet been passing max-connections, but generally +creates NBD servers with the intent for a single client for the sake +of live storage migration; meanwhile, the KubeSAN project anticipates +a large cluster sharing multiple clients [up to 8 per node, and up to +100 nodes in a cluster], but it currently uses qemu-nbd with an +explicit --shared=0 rather than qemu-storage-daemon with +nbd-server-start). + +We considered using a deprecation period (declare that omitting +max-parameters is deprecated, and make it mandatory in 3 releases - +then we don't need to pick an arbitrary default); that has zero risk +of breaking any apps that accidentally depended on more than 100 +connections, and where such breakage might not be noticed under unit +testing but only under the larger loads of production usage. But it +does not close the denial-of-service hole until far into the future, +and requires all apps to change to add the parameter even if 100 was +good enough. It also has a drawback that any app (like libvirt) that +is accidentally relying on an unlimited default should seriously +consider their own CVE now, at which point they are going to change to +pass explicit max-connections sooner than waiting for 3 qemu releases. +Finally, if our changed default breaks an app, that app can always +pass in an explicit max-parameters with a larger value. + +It is also intentional that the HMP interface to nbd-server-start is +not changed to expose max-connections (any client needing to fine-tune +things should be using QMP). + +Suggested-by: Daniel P. Berrangé +Signed-off-by: Eric Blake +Message-ID: <20240807174943.771624-12-eblake@redhat.com> +Reviewed-by: Daniel P. Berrangé +[ericb: Expand commit message to summarize Dan's argument for why we +break corner-case back-compat behavior without a deprecation period] +Signed-off-by: Eric Blake + +CVE: CVE-2024-7409 + +Upstream-Status: Backport [https://github.com/qemu/qemu/commit/c8a76dbd90c2f48df89b75bef74917f90a59b623] + +Signed-off-by: Archana Polampalli +--- + block/monitor/block-hmp-cmds.c | 3 ++- + blockdev-nbd.c | 8 ++++++++ + include/block/nbd.h | 7 +++++++ + qapi/block-export.json | 4 ++-- + 4 files changed, 19 insertions(+), 3 deletions(-) + +diff --git a/block/monitor/block-hmp-cmds.c b/block/monitor/block-hmp-cmds.c +index c729cbf1e..78a697585 100644 +--- a/block/monitor/block-hmp-cmds.c ++++ b/block/monitor/block-hmp-cmds.c +@@ -415,7 +415,8 @@ 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; +diff --git a/blockdev-nbd.c b/blockdev-nbd.c +index 267a1de90..24ba5382d 100644 +--- a/blockdev-nbd.c ++++ b/blockdev-nbd.c +@@ -170,6 +170,10 @@ 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); + } +@@ -182,6 +186,10 @@ 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); + } +diff --git a/include/block/nbd.h b/include/block/nbd.h +index 1d4d65922..d4f8b21ae 100644 +--- a/include/block/nbd.h ++++ b/include/block/nbd.h +@@ -39,6 +39,13 @@ extern const BlockExportDriver blk_exp_nbd; + */ + #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 { +diff --git a/qapi/block-export.json b/qapi/block-export.json +index 7874a49ba..1d255d77e 100644 +--- a/qapi/block-export.json ++++ b/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). + # + # Returns: error if the server is already running. + # +-- +2.40.0 diff --git a/meta/recipes-devtools/qemu/qemu/CVE-2024-7409-0003.patch b/meta/recipes-devtools/qemu/qemu/CVE-2024-7409-0003.patch new file mode 100644 index 0000000000..b2b9b15c54 --- /dev/null +++ b/meta/recipes-devtools/qemu/qemu/CVE-2024-7409-0003.patch @@ -0,0 +1,126 @@ +From b9b72cb3ce15b693148bd09cef7e50110566d8a0 Mon Sep 17 00:00:00 2001 +From: Eric Blake +Date: Thu, 8 Aug 2024 16:05:08 -0500 +Subject: [PATCH] nbd/server: CVE-2024-7409: Drop non-negotiating clients + +A client that opens a socket but does not negotiate is merely hogging +qemu's resources (an open fd and a small amount of memory); and a +malicious client that can access the port where NBD is listening can +attempt a denial of service attack by intentionally opening and +abandoning lots of unfinished connections. The previous patch put a +default bound on the number of such ongoing connections, but once that +limit is hit, no more clients can connect (including legitimate ones). +The solution is to insist that clients complete handshake within a +reasonable time limit, defaulting to 10 seconds. A client that has +not successfully completed NBD_OPT_GO by then (including the case of +where the client didn't know TLS credentials to even reach the point +of NBD_OPT_GO) is wasting our time and does not deserve to stay +connected. Later patches will allow fine-tuning the limit away from +the default value (including disabling it for doing integration +testing of the handshake process itself). + +Note that this patch in isolation actually makes it more likely to see +qemu SEGV after nbd-server-stop, as any client socket still connected +when the server shuts down will now be closed after 10 seconds rather +than at the client's whims. That will be addressed in the next patch. + +For a demo of this patch in action: +$ qemu-nbd -f raw -r -t -e 10 file & +$ nbdsh --opt-mode -c ' +H = list() +for i in range(20): + print(i) + H.insert(i, nbd.NBD()) + H[i].set_opt_mode(True) + H[i].connect_uri("nbd://localhost") +' +$ kill $! + +where later connections get to start progressing once earlier ones are +forcefully dropped for taking too long, rather than hanging. + +Suggested-by: Daniel P. Berrangé +Signed-off-by: Eric Blake +Message-ID: <20240807174943.771624-13-eblake@redhat.com> +Reviewed-by: Daniel P. Berrangé +[eblake: rebase to changes earlier in series, reduce scope of timer] +Signed-off-by: Eric Blake + +CVE: CVE-2024-7409 + +Upstream-Status: Backport [https://gitlab.com/qemu-project/qemu/-/commit/b9b72cb3ce15b693148bd09cef7e50110566d8a0] + +Signed-off-by: Archana Polampalli +--- + nbd/server.c | 28 +++++++++++++++++++++++++++- + nbd/trace-events | 1 + + 2 files changed, 28 insertions(+), 1 deletion(-) + +diff --git a/nbd/server.c b/nbd/server.c +index f8881936e..6155e329a 100644 +--- a/nbd/server.c ++++ b/nbd/server.c +@@ -3155,22 +3155,48 @@ 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); + +- /* TODO - utilize client->handshake_max_secs */ ++ /* ++ * 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); + } +diff --git a/nbd/trace-events b/nbd/trace-events +index 00ae3216a..cbd0a4ab7 100644 +--- a/nbd/trace-events ++++ b/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 +-- +2.40.0 diff --git a/meta/recipes-devtools/qemu/qemu/CVE-2024-7409-0004.patch b/meta/recipes-devtools/qemu/qemu/CVE-2024-7409-0004.patch new file mode 100644 index 0000000000..9515c631ad --- /dev/null +++ b/meta/recipes-devtools/qemu/qemu/CVE-2024-7409-0004.patch @@ -0,0 +1,164 @@ +From 3e7ef738c8462c45043a1d39f702a0990406a3b3 Mon Sep 17 00:00:00 2001 +From: Eric Blake +Date: Wed, 7 Aug 2024 12:23:13 -0500 +Subject: [PATCH] nbd/server: CVE-2024-7409: Close stray clients at server-stop + +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 + +CVE: CVE-2024-7409 + +Upstream-Status: Backport [https://gitlab.com/qemu-project/qemu/-/commit/3e7ef738c8462c45043a1d39f702a0990406a3b3] + +Signed-off-by: Archana Polampalli +--- + blockdev-nbd.c | 35 ++++++++++++++++++++++++++++++++++- + 1 file changed, 34 insertions(+), 1 deletion(-) + +diff --git a/blockdev-nbd.c b/blockdev-nbd.c +index 24ba5382d..f73409ae4 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)); + } +-- +2.40.0