The Linux kernel recently patched a vulnerability involving pfifo_tail_enqueue, which could potentially be used for privilege escalation exploits. Specifically, this vulnerability impacts the packet queueing structure within the kernel, leading to a discrepancy between the sums of the children's qlen and the parent's qlen. This post will provide an in-depth analysis of the issue, the affected functions, and the implications for potential exploit scenarios.

Problematic Behavior

The vulnerability in question lies within the kernel's pfifo_tail_enqueue function. Under normal conditions, when scheduling limits are reached, the function is expected to drop a packet in the queue, decrease the queue length (qlen) by one, enqueue the new packet, and then increase the qlen once again. It then returns a NET_XMIT_CN status code, as expected.

However, if the scheduler's limit (sch->limit) is set to and the function is triggered on a scheduler without any packets, the "drop a packet" step does not execute as expected, leaving the scheduler's qlen unchanged at . As the new packet is enqueued and the qlen increases by one, it is still possible to leverage this function to exploit the changed qlen and return the NET_XMIT_CN status code.

Exploit Details

The exploit involves two separate qdiscs within the kernel: Qdisc_A and Qdisc_B. Qdisc_A must have a '->graft()' function, such as hfsc, to create a parent-child relationship between the two qdiscs. When a packet is enqueued through Qdisc_A, the following process is triggered:

pfifo_tail_enqueue() returns NET_XMIT_CN

4. hfsc_enqueue() checks for NET_XMIT_SUCCESS and instead sees NET_XMIT_CN, failing to increase the qlen of Qdisc_A

Ultimately, this leads to an imbalance between the parent (Qdisc_A) and child (Qdisc_B) qlens. This is not only an issue when using 'hfsc' as the type for Qdisc_A; other types, such as 'drr', can also result in the same situation. This design flaw ultimately violates the intended behavior and design constraints.

Original References

The code snippet below demonstrates the existing vulnerability in the kernel (link to original reference):

static int pfifo_tail_enqueue(struct sk_buff *skb, struct Qdisc *sch,
                              struct sk_buff **to_free)
{
    if (likely(skb_queue_len(&sch->q) < sch->limit)) {
        __skb_queue_tail(&sch->q, skb);
        sch->qstats.backlog += qdisc_pkt_len(skb);
        sch->bstats.packets++;
        sch->bstats.bytes += qdisc_pkt_len(skb);
        return NET_XMIT_SUCCESS;
    }

    qdisc_drop(skb, sch, to_free);
    sch->qstats.drops++;
    return NET_XMIT_CN;
}

Impact and Mitigation

Exploiting this design flaw could make it possible for a malicious attacker to achieve a user-to-kernel privilege escalation, potentially compromising a system running a vulnerable version of the Linux kernel. To protect against this vulnerability, ensure that all Linux kernel installations are updated to the latest version in which this issue has been resolved. Proper patch management and staying informed about new vulnerabilities and their associated fixes can help maintain a secure environment for all users.

Conclusion

The CVE-2025-21702 Linux kernel vulnerability highlights the importance of vigilance in discovering and addressing software flaws that may have a significant impact on system security. By understanding the root causes and potential implications of this bug, developers and system administrators can ensure that their systems are appropriately patched and safe from potential privilege escalation attacks.

Timeline

Published on: 02/18/2025 15:15:18 UTC
Last modified on: 03/24/2025 15:39:02 UTC