CVE-2026-80842 in Linux
Summary
by MITRE • 09/04/2026
In the Linux kernel, the following vulnerability has been resolved:
net: bridge: mcast: fix use-after-free of a master VLAN's multicast context
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 = &vlan->br_mcast_ctx with pmctx = NULL, so IGMP sent to the bridge device re-arms the context's timers after br_multicast_ctx_deinit() has already stopped them.
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.
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Analysis
by VulDB Data Team • 09/04/2026
The Linux kernel contains a critical use-after-free vulnerability within the bridge multicast subsystem, specifically affecting master VLAN configurations. This flaw arises from an improper synchronization sequence during the teardown of multicast contexts for bridged virtual LANs. The core issue lies in how br_multicast_toggle_one_vlan() manages state transitions when disabling multicast support. For standard port-VLAN interfaces, the function correctly clears the BR_VLFLAG_MCAST_ENABLED flag under the protection of br->multicast_lock before stopping the VLAN's multicast context. This ensures that lockless readers, which gate their operations on this flag via functions like br_multicast_ctx_should_use() and br_multicast_ctx_vlan_disabled(), cannot re-arm or access the context once it has been marked for removal. However, this protective handshake is bypassed when dealing with master VLANs due to a missing conditional check.
The vulnerability manifests because __vlan_del() clears the BRIDGE_VLAN_INFO_BRENTRY flag before invoking br_vlan_put_master(). Consequently, when br_multicast_toggle_one_vlan() is called for a master VLAN, it encounters the !br_vlan_is_brentry(vlan) condition and returns early without acquiring br->multicast_lock or clearing the multicast enabled flag. This leaves BR_VLFLAG_MCAST_ENABLED set to true even though the underlying context is about to be destroyed. The subsequent call to br_multicast_ctx_deinit() drains the context resources, and eventually, the VLAN structure is freed via kfree through a deferred RCU callback mechanism initiated by call_rcu(). During this window, if an incoming IGMP packet arrives at the bridge device, the transmit path in br_multicast_rcv() may still perceive the multicast context as active.
In the affected scenario for master VLANs, br_multicast_rcv() selects the vlan's br_mcast_ctx with pmctx set to NULL. Because the flag indicating disabled status was never cleared under lock protection, the kernel proceeds to re-arm the context timers by calling functions such as br_multicast_host_join and __br_multicast_add_group. This results in a write operation into memory that has already been freed or is in the process of being reclaimed. The resulting use-after-free condition can lead to kernel panics, data corruption, or potentially arbitrary code execution if an attacker can control the contents of the freed slab object. The stack trace associated with this vulnerability typically shows calls originating from br_dev_xmit through br_multicast_rcv down to timer modification functions like __mod_timer and detach_if_pending, confirming that active multicast processing is occurring on deallocated memory structures allocated by br_vlan_add.
This flaw represents a classic race condition combined with improper resource management in concurrent environments. It aligns with CWE-416, Use After Free, where the program continues to use a pointer after it has been freed, leading to undefined behavior. From an attack surface perspective, this vulnerability is exploitable remotely if IGMP traffic can be directed at the affected bridge interface, placing it within the scope of ATT&CK technique T1059, Command and Scripting Interpreter, or more broadly as part of lateral movement via network service exploitation. The lack of proper locking during state transition allows a reader thread to proceed with operations that assume valid memory structures while another context is tearing them down.
To mitigate this vulnerability, the kernel code must be updated to ensure consistent handling of master VLANs compared to port-VLANs. Specifically, the br_multicast_toggle_one_vlan function should check for BRIDGE_VLAN_INFO_BRENTRY status when enabling multicast contexts, mirroring the existing checks for global disabled states. More critically, during disabling operations, the flag BR_VLFLAG_MCAST_ENABLED must always be cleared under the protection of br->multicast_lock before initiating the deinitialization and freeing processes. This ensures that any concurrent readers attempting to access the context will see it as disabled and abort their operations safely. System administrators should apply kernel updates provided by their distribution vendors immediately, as this issue affects core networking functionality and poses a significant stability risk under high multicast traffic loads or specific VLAN configuration changes.