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c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1723154153; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=Zp1LX7WiE/Tm338u5H0qM12wvmECy6jktr7yJcf8lSE=; b=CxkEGkl0I8g8fHU11u1rLzbaVj1rsBEdL1D2JRmAAFMdPROBmF8BSYdxPLxbNYHP1hBVPD wRe30O5otAV4BU8X1HCJHSu3QFuurHMfveMhd7ebuq1qNKiUkIGArVSfS9EWhu9DWj/muE sYe/lktuNdq3niFLHdIuITA0ymBdKiQ= X-MC-Unique: LupBeObbO7aZih9im9sAkA-1 From: Eric Blake To: qemu-devel@nongnu.org Cc: =?UTF-8?q?Daniel=20P=2E=20Berrang=C3=A9?= , Kevin Wolf , Hanna Reitz , Vladimir Sementsov-Ogievskiy , Markus Armbruster , qemu-block@nongnu.org (open list:Block layer core) Subject: [PULL 3/5] nbd/server: CVE-2024-7409: Cap default max-connections to 100 Date: Thu, 8 Aug 2024 16:53:41 -0500 Message-ID: <20240808215529.1065336-10-eblake@redhat.com> In-Reply-To: <20240808215529.1065336-7-eblake@redhat.com> References: <20240808215529.1065336-7-eblake@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.0 on 10.30.177.17 Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Received-SPF: pass client-ip=170.10.133.124; envelope-from=eblake@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -21 X-Spam_score: -2.2 X-Spam_bar: -- X-Spam_report: (-2.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.141, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: qemu-devel-bounces+importer=patchew.org@nongnu.org X-ZohoMail-DKIM: pass (identity @redhat.com) X-ZM-MESSAGEID: 1723154214709116600 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=3D0 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. Berrang=C3=A9 Signed-off-by: Eric Blake Message-ID: <20240807174943.771624-12-eblake@redhat.com> Reviewed-by: Daniel P. Berrang=C3=A9 [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 --- qapi/block-export.json | 4 ++-- include/block/nbd.h | 7 +++++++ block/monitor/block-hmp-cmds.c | 3 ++- blockdev-nbd.c | 8 ++++++++ 4 files changed, 19 insertions(+), 3 deletions(-) diff --git a/qapi/block-export.json b/qapi/block-export.json index 665d5fd0262..ce33fe378df 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). # # Errors: # - if the server is already running diff --git a/include/block/nbd.h b/include/block/nbd.h index 1d4d65922d1..d4f8b21aecc 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/block/monitor/block-hmp-cmds.c b/block/monitor/block-hmp-cmds.c index d954bec6f1e..bdf2eb50b68 100644 --- a/block/monitor/block-hmp-cmds.c +++ b/block/monitor/block-hmp-cmds.c @@ -402,7 +402,8 @@ void hmp_nbd_server_start(Monitor *mon, const QDict *qd= ict) 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 !=3D NULL) { goto exit; diff --git a/blockdev-nbd.c b/blockdev-nbd.c index 267a1de903f..24ba5382db0 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 =3D 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 =3D socket_address_flatten(addr); + if (!has_max_connections) { + max_connections =3D NBD_DEFAULT_MAX_CONNECTIONS; + } + nbd_server_start(addr_flat, tls_creds, tls_authz, max_connections, err= p); qapi_free_SocketAddress(addr_flat); } --=20 2.46.0