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[67.216.200.102]) by smtp.gmail.com with ESMTPSA id 5a478bee46e88-36740cca3d8sm14612471eec.14.2026.10.22.02.15.04 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 22 Oct 2026 02:15:06 -0700 (PDT) From: Cheng Yongkang To: Nikolay Aleksandrov , Ido Schimmel Cc: "David S . Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , bridge@lists.linux.dev, netdev@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [BUG] net: bridge: MST bypasses blocking when VLAN filtering is off Date: Thu, 22 Oct 2026 02:15:01 -0700 Message-Id: <20261022091501.3650765-1-teel4res@gmail.com> X-Mailer: git-send-email 2.34.1 X-Mailing-List: netdev@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Hi all, MST can be enabled while VLAN filtering is disabled and no port VLAN entries exist. nbp_vlan_init() still installs a port VLAN group, so br_mst_is_enabled() returns true even though BROPT_VLAN_ENABLED is off. With that configuration: ingress skips the normal p->state check and forces forwarding br_allowed_ingress() accepts the unfiltered frame br_allowed_egress() accepts the unfiltered frame A port in BR_STATE_BLOCKING therefore still forwards data in both directions. STP/MRP loop protection is defeated. Observed on Linux v7.3, commit 08ab31164c55f20c7a2653de3186ae333470e2c1, with vlan_filtering=0, default_pvid=0, mst_enabled=1, and a port whose IFLA_BRPORT_STATE read back as BR_STATE_BLOCKING: MST on: egress=3 ingress=3 MST off: egress=0 ingress=0 MST on: egress=3 ingress=3 Each direction sent three matching broadcast frames. This is a deterministic configuration/state mismatch, not a race. The MST fast path should require both MST and VLAN filtering, while retaining the existing non-NULL port VLAN-group lifetime check. The MST configuration itself should also be rejected if it is unusable with VLAN filtering disabled. CVE-2025-40297 used the same broad predicate for a port-deletion UAF, but its fix only added the vlgrp lifetime check. The normal data-plane forwarding bypass remains in current source; I found no separate fix or CVE. The test used a diagnostic switchdev endpoint only to establish the MRP ring state. It did not replace bridge, VLAN, MST or packet-forwarding code. The PoC and full program/console logs are available on request. This was found with a multi-agent workflow; the forwarding differential was reproduced in QEMU. Thanks, Cheng Yongkang