CVE-2024-36979 in Linux
Summary
by MITRE • 06/19/2024
In the Linux kernel, the following vulnerability has been resolved:
net: bridge: mst: fix vlan use-after-free
syzbot reported a suspicious rcu usage[1] in bridge's mst code. While
fixing it I noticed that nothing prevents a vlan to be freed while walking the list from the same path (br forward delay timer). Fix the rcu usage and also make sure we are not accessing freed memory by making br_mst_vlan_set_state use rcu read lock.
[1]
WARNING: suspicious RCU usage 6.9.0-rc6-syzkaller #0 Not tainted ----------------------------- net/bridge/br_private.h:1599 suspicious rcu_dereference_protected() usage! ... stack backtrace: CPU: 1 PID: 8017 Comm: syz-executor.1 Not tainted 6.9.0-rc6-syzkaller #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/27/2024 Call Trace: __dump_stack lib/dump_stack.c:88 [inline]
dump_stack_lvl+0x241/0x360 lib/dump_stack.c:114 lockdep_rcu_suspicious+0x221/0x340 kernel/locking/lockdep.c:6712 nbp_vlan_group net/bridge/br_private.h:1599 [inline]
br_mst_set_state+0x1ea/0x650 net/bridge/br_mst.c:105 br_set_state+0x28a/0x7b0 net/bridge/br_stp.c:47 br_forward_delay_timer_expired+0x176/0x440 net/bridge/br_stp_timer.c:88 call_timer_fn+0x18e/0x650 kernel/time/timer.c:1793 expire_timers kernel/time/timer.c:1844 [inline]
__run_timers kernel/time/timer.c:2418 [inline]
__run_timer_base+0x66a/0x8e0 kernel/time/timer.c:2429 run_timer_base kernel/time/timer.c:2438 [inline]
run_timer_softirq+0xb7/0x170 kernel/time/timer.c:2448 __do_softirq+0x2c6/0x980 kernel/softirq.c:554 invoke_softirq kernel/softirq.c:428 [inline]
__irq_exit_rcu+0xf2/0x1c0 kernel/softirq.c:633 irq_exit_rcu+0x9/0x30 kernel/softirq.c:645 instr_sysvec_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1043 [inline]
sysvec_apic_timer_interrupt+0xa6/0xc0 arch/x86/kernel/apic/apic.c:1043 asm_sysvec_apic_timer_interrupt+0x1a/0x20 arch/x86/include/asm/idtentry.h:702 RIP: 0010:lock_acquire+0x264/0x550 kernel/locking/lockdep.c:5758 Code: 2b 00 74 08 4c 89 f7 e8 ba d1 84 00 f6 44 24 61 02 0f 85 85 01 00 00 41 f7 c7 00 02 00 00 74 01 fb 48 c7 44 24 40 0e 36 e0 45 c7 44 25 00 00 00 00 00 43 c7 44 25 09 00 00 00 00 43 c7 44 25 RSP: 0018:ffffc90013657100 EFLAGS: 00000206 RAX: 0000000000000001 RBX: 1ffff920026cae2c RCX: 0000000000000001 RDX: dffffc0000000000 RSI: ffffffff8bcaca00 RDI: ffffffff8c1eaa60 RBP: ffffc90013657260 R08: ffffffff92efe507 R09: 1ffffffff25dfca0 R10: dffffc0000000000 R11: fffffbfff25dfca1 R12: 1ffff920026cae28 R13: dffffc0000000000 R14: ffffc90013657160 R15: 0000000000000246
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Analysis
by VulDB Data Team • 10/04/2025
The vulnerability CVE-2024-36979 addresses a critical use-after-free condition within the Linux kernel's bridge multicast spanning tree protocol implementation. This flaw exists in the mst (multicast spanning tree) code of the network bridge subsystem, specifically affecting how VLAN (Virtual Local Area Network) structures are managed during bridge forward delay timer operations. The issue was identified through syzbot, an automated bug finder that detected suspicious RCU (Read-Copy Update) usage patterns in the bridge module's multicast spanning tree implementation. The vulnerability manifests when a VLAN structure is freed while another code path is simultaneously traversing the same list structure, creating a scenario where freed memory could be accessed, leading to potential system instability or exploitation.
The technical flaw stems from improper synchronization mechanisms in the bridge multicast spanning tree code where the br_mst_vlan_set_state function fails to properly acquire RCU read locks before accessing VLAN structures. This creates a race condition where a timer callback function br_forward_delay_timer_expired executes while VLAN structures may be in the process of being freed by another concurrent path. The stack trace reveals that the vulnerability occurs during timer expiration handling in the bridge spanning tree timer subsystem, where nbp_vlan_group function is called with suspicious RCU usage. This pattern violates fundamental RCU locking principles and represents a CWE-416 use-after-free vulnerability, as defined in the Common Weakness Enumeration catalog. The specific line of code causing the issue is located in net/bridge/br_private.h at line 1599, where rcu_dereference_protected is incorrectly used without proper locking context.
The operational impact of this vulnerability extends beyond simple system crashes to potentially enable privilege escalation or denial of service attacks within networked environments. When exploited, the use-after-free condition could allow malicious actors to corrupt kernel memory structures, potentially leading to arbitrary code execution with kernel privileges. This is particularly concerning in virtualized environments or network infrastructure devices where the bridge subsystem is heavily utilized. The vulnerability affects systems running Linux kernel versions that include the affected bridge multicast spanning tree implementation, particularly those using bridge forwarding delay timer functionality. The risk is elevated in high-traffic network environments where bridge operations are frequent and concurrent access to VLAN structures is common.
Mitigation strategies for CVE-2024-36979 involve applying the kernel patch that corrects the RCU usage patterns and ensures proper locking mechanisms are implemented before accessing VLAN structures. The fix requires modifying br_mst_vlan_set_state to properly acquire rcu read locks before traversing the VLAN list, preventing access to freed memory structures. System administrators should prioritize applying the patched kernel version to all affected systems, particularly network infrastructure devices, virtual machines, and containers that utilize bridge networking functionality. Additionally, monitoring for suspicious kernel warnings related to RCU usage should be implemented as part of security operations procedures. The fix aligns with ATT&CK technique T1068 by addressing kernel-level privilege escalation vectors and follows security best practices for concurrent access control in kernel modules, as recommended by the Linux kernel security team and documented in kernel security guidelines. Organizations should also consider implementing network segmentation and access controls to limit potential attack surfaces where bridge functionality is exposed.