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[79.57.139.67]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-c249685ab3csm762988166b.49.2026.08.23.04.58.15 (version=TLS1_2 cipher=ECDHE-ECDSA-AES128-GCM-SHA256 bits=128/128); Sun, 23 Aug 2026 04:58:16 -0700 (PDT) From: Norbert Szetei Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: quoted-printable Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 (Mac OS X Mail 16.0 \(3826.700.81.1.4\)) Subject: [PATCH net] net: bridge: mcast: fix use-after-free of a master VLAN's multicast context Message-Id: Date: Sun, 23 Aug 2026 13:58:04 +0200 Cc: Nikolay Aleksandrov , Ido Schimmel , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , bridge@lists.linux.dev, linux-kernel@vger.kernel.org To: netdev@vger.kernel.org X-Mailer: Apple Mail (2.3826.700.81.1.4) br_multicast_toggle_one_vlan() clears BR_VLFLAG_MCAST_ENABLED under br->multicast_lock before stopping a VLAN's multicast context. That is the teardown handshake: lockless readers gate on the flag through br_multicast_ctx_should_use() -> br_multicast_ctx_vlan_disabled(), so once it is cleared under the lock no reader can arm the context again. For a master VLAN the handshake never runs. __vlan_del() clears BRIDGE_VLAN_INFO_BRENTRY before calling br_vlan_put_master(), so br_multicast_toggle_one_vlan(masterv, false) returns early on !br_vlan_is_brentry(vlan): the flag stays set and br->multicast_lock is never taken. br_vlan_put_master() then drains the context in br_multicast_ctx_deinit() and frees the VLAN through call_rcu(), while a reader still inside rcu_read_lock() sees the context as enabled and re-arms it. The port and port-VLAN branch of the function has no br_vlan_is_brentry() test and flips the flag under br->multicast_lock, so it is not affected. The reader is the bridge transmit path. For a master VLAN br_multicast_rcv() selects brmctx =3D &vlan->br_mcast_ctx with pmctx =3D NULL, so IGMP sent to the bridge device re-arms the context's timers after br_multicast_ctx_deinit() has already stopped them. Any user able to create a bridge can trigger this, which includes an unprivileged user in a user namespace, by deleting and re-adding the master VLAN while sending IGMP to the bridge device: BUG: KASAN: slab-use-after-free in detach_if_pending+0x412/0x4a0 Write of size 8 at addr ffff88810ac39918 by task brmc/601 __mod_timer+0x51a/0xc50 br_multicast_host_join+0x25b/0x390 __br_multicast_add_group+0x468/0x530 br_ip4_multicast_add_group+0x1a0/0x260 br_multicast_rcv+0x2cda/0x61e0 br_dev_xmit+0x6c4/0x1540 Allocated by task 610: br_vlan_add+0x111/0xb40 br_vlan_info+0x370/0x3e0 Freed by task 0: kfree+0x1a7/0x4f0 rcu_core+0x7dc/0x10a0 Only test br_vlan_is_brentry() when enabling, like the br_multicast_ctx_vlan_global_disabled() test next to it. Disabling then always clears BR_VLFLAG_MCAST_ENABLED under br->multicast_lock before br_multicast_ctx_deinit() drains the context. Fixes: 7b54aaaf53cb ("net: bridge: multicast: add vlan state = initialization and control") Cc: stable@vger.kernel.org Assisted-by: Claude:claude-opus-5 Signed-off-by: Norbert Szetei --- net/bridge/br_multicast.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/net/bridge/br_multicast.c b/net/bridge/br_multicast.c index 75e1e2a8fc83..3ef5d8bbf552 100644 --- a/net/bridge/br_multicast.c +++ b/net/bridge/br_multicast.c @@ -4377,8 +4377,8 @@ void br_multicast_toggle_one_vlan(struct = net_bridge_vlan *vlan, bool on) if (br_vlan_is_master(vlan)) { br =3D vlan->br; =20 - if (!br_vlan_is_brentry(vlan) || - (on && + if (on && + (!br_vlan_is_brentry(vlan) || = br_multicast_ctx_vlan_global_disabled(&vlan->br_mcast_ctx))) return; =20 --=20 2.55.0