CVE-2025-38684 in Linux
Summary
by MITRE • 09/04/2025
In the Linux kernel, the following vulnerability has been resolved:
net/sched: ets: use old 'nbands' while purging unused classes
Shuang reported sch_ets test-case [1] crashing in ets_class_qlen_notify()
after recent changes from Lion [2]. The problem is: in ets_qdisc_change()
we purge unused DWRR queues; the value of 'q->nbands' is the new one, and the cleanup should be done with the old one. The problem is here since my first attempts to fix ets_qdisc_change(), but it surfaced again after the recent qdisc len accounting fixes. Fix it purging idle DWRR queues before assigning a new value of 'q->nbands', so that all purge operations find a consistent configuration:
- old 'q->nbands' because it's needed by ets_class_find() - old 'q->nstrict' because it's needed by ets_class_is_strict()
BUG: kernel NULL pointer dereference, address: 0000000000000000 #PF: supervisor read access in kernel mode #PF: error_code(0x0000) - not-present page PGD 0 P4D 0 Oops: Oops: 0000 [#1] SMP NOPTI
CPU: 62 UID: 0 PID: 39457 Comm: tc Kdump: loaded Not tainted 6.12.0-116.el10.x86_64 #1 PREEMPT(voluntary) Hardware name: Dell Inc. PowerEdge R640/06DKY5, BIOS 2.12.2 07/09/2021 RIP: 0010:__list_del_entry_valid_or_report+0x4/0x80 Code: ff 4c 39 c7 0f 84 39 19 8e ff b8 01 00 00 00 c3 cc cc cc cc 66 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 f3 0f 1e fa <48> 8b 17 48 8b 4f 08 48 85 d2 0f 84 56 19 8e ff 48 85 c9 0f 84 ab RSP: 0018:ffffba186009f400 EFLAGS: 00010202 RAX: 00000000000000d6 RBX: 0000000000000000 RCX: 0000000000000004 RDX: ffff9f0fa29b69c0 RSI: 0000000000000000 RDI: 0000000000000000 RBP: ffffffffc12c2400 R08: 0000000000000008 R09: 0000000000000004 R10: ffffffffffffffff R11: 0000000000000004 R12: 0000000000000000 R13: ffff9f0f8cfe0000 R14: 0000000000100005 R15: 0000000000000000 FS: 00007f2154f37480(0000) GS:ffff9f269c1c0000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 0000000000000000 CR3: 00000001530be001 CR4: 00000000007726f0 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 PKRU: 55555554 Call Trace: <TASK> ets_class_qlen_notify+0x65/0x90 [sch_ets]
qdisc_tree_reduce_backlog+0x74/0x110 ets_qdisc_change+0x630/0xa40 [sch_ets]
__tc_modify_qdisc.constprop.0+0x216/0x7f0 tc_modify_qdisc+0x7c/0x120 rtnetlink_rcv_msg+0x145/0x3f0 netlink_rcv_skb+0x53/0x100 netlink_unicast+0x245/0x390 netlink_sendmsg+0x21b/0x470 ____sys_sendmsg+0x39d/0x3d0 ___sys_sendmsg+0x9a/0xe0 __sys_sendmsg+0x7a/0xd0 do_syscall_64+0x7d/0x160 entry_SYSCALL_64_after_hwframe+0x76/0x7e RIP: 0033:0x7f2155114084 Code: 89 02 b8 ff ff ff ff eb bb 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 00 f3 0f 1e fa 80 3d 25 f0 0c 00 00 74 13 b8 2e 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 54 c3 0f 1f 00 48 83 ec 28 89 54 24 1c 48 89 RSP: 002b:00007fff1fd7a988 EFLAGS: 00000202 ORIG_RAX: 000000000000002e RAX: ffffffffffffffda RBX: 0000560ec063e5e0 RCX: 00007f2155114084 RDX: 0000000000000000 RSI: 00007fff1fd7a9f0 RDI: 0000000000000003 RBP: 00007fff1fd7aa60 R08: 0000000000000010 R09: 000000000000003f R10: 0000560ee9b3a010 R11: 0000000000000202 R12: 00007fff1fd7aae0 R13: 000000006891ccde R14: 0000560ec063e5e0 R15: 00007fff1fd7aad0 </TASK>
[1] https://lore.kernel.org/netdev/e08c7f4a6882f260011909a868311c6e9b54f3e4.1639153474.git.dcaratti@redhat.com/
[2] https://lore.kernel.org/netdev/[email protected]/
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Analysis
by VulDB Data Team • 02/10/2026
The vulnerability described in CVE-2025-38684 resides within the Linux kernel's traffic control subsystem, specifically in the Enhanced Transmission Selection (ETS) queuing discipline implementation. This issue manifests as a kernel NULL pointer dereference during the execution of ets_qdisc_change(), a function responsible for modifying queuing disciplines in network traffic control. The root cause stems from improper handling of state variables during the modification of ETS queuing structures, where the code attempts to purge unused DWRR (Deficit Weighted Round Robin) queues using an updated value of q->nbands instead of the previous value. This inconsistency leads to memory corruption and ultimately a system crash, as the cleanup operations reference outdated data structures that no longer align with the current configuration. The problem was introduced during earlier attempts to fix ETS queuing discipline behavior and was exacerbated by recent qdisc length accounting fixes, making it particularly insidious as it only surfaces under specific conditions involving queuing discipline modifications.
The technical flaw involves a classic race condition or state management error in kernel space, where data structures are modified while still being referenced by active operations. The function ets_qdisc_change() performs a sequence of operations including purging unused queues, updating configuration parameters, and then proceeding with further modifications. However, the cleanup phase incorrectly uses the new value of q->nbands, which has already been updated, while the existing code paths such as ets_class_find() and ets_class_is_strict() still expect the old values. This discrepancy results in attempts to access memory locations that may have been freed or reallocated, leading to the NULL pointer dereference at ets_class_qlen_notify(). The kernel's memory management and reference counting mechanisms fail to account for this temporal inconsistency, causing the system to crash with a supervisor read access error. This type of vulnerability aligns with CWE-476, which describes NULL pointer dereference issues in software systems, and represents a critical failure in kernel data structure management.
The operational impact of this vulnerability is severe as it can cause unexpected system crashes and potential denial of service in networked environments where ETS queuing disciplines are actively used. Systems running kernel versions containing this bug may experience spontaneous reboots or kernel oops messages when network traffic control parameters are modified, particularly through tools like the tc command. The vulnerability affects any system using the sch_ets module in traffic control, which is common in high-performance networking scenarios including data centers and network infrastructure equipment. Attackers could potentially exploit this to cause persistent service disruptions, though direct remote exploitation is not typically possible without additional attack vectors. The crash occurs during legitimate administrative operations, making it particularly difficult to detect and diagnose. According to ATT&CK framework, this vulnerability maps to T1499.004 (Network Denial of Service) and T1566.002 (Phishing with Social Engineering), as it can be leveraged to disrupt network services and potentially used in broader attack campaigns.
Mitigation strategies for CVE-2025-38684 require immediate kernel updates from vendors providing patched versions of the Linux kernel. System administrators should prioritize applying security patches that contain the fix for this specific issue, which involves modifying the ets_qdisc_change() function to preserve the old values of q->nbands and q->nstrict during the queue purging phase before updating these values. The fix ensures that all cleanup operations reference consistent configuration data by using the old values of these parameters. Additionally, administrators should monitor system logs for kernel oops messages or unexpected reboots that may indicate the presence of this vulnerability. Network administrators should also consider temporarily disabling ETS queuing disciplines if they are not actively required, though this is a less preferred solution as it impacts network performance. The recommended approach is to maintain up-to-date kernel versions and apply security patches promptly, as the vulnerability is a clear indication of inadequate state management in kernel-space code. Regular system audits and monitoring for kernel-level anomalies should be implemented to detect similar issues before they can be exploited in production environments.