{
  "containers": {
    "cna": {
      "affected": [
        {
          "defaultStatus": "unaffected",
          "product": "Linux",
          "programFiles": [
            "include/net/sch_generic.h",
            "net/sched/sch_codel.c",
            "net/sched/sch_fq.c",
            "net/sched/sch_fq_codel.c",
            "net/sched/sch_fq_pie.c",
            "net/sched/sch_hhf.c",
            "net/sched/sch_pie.c"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
            {
              "lessThan": "a225f44d84b8900d679c5f5a9ea46fe9c0cc7802",
              "status": "affected",
              "version": "4b549a2ef4bef9965d97cbd992ba67930cd3e0fe",
              "versionType": "git"
            },
            {
              "lessThan": "52bf272636bda69587952b35ae97690b8dc89941",
              "status": "affected",
              "version": "4b549a2ef4bef9965d97cbd992ba67930cd3e0fe",
              "versionType": "git"
            }
          ]
        },
        {
          "defaultStatus": "affected",
          "product": "Linux",
          "programFiles": [
            "include/net/sch_generic.h",
            "net/sched/sch_codel.c",
            "net/sched/sch_fq.c",
            "net/sched/sch_fq_codel.c",
            "net/sched/sch_fq_pie.c",
            "net/sched/sch_hhf.c",
            "net/sched/sch_pie.c"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
            {
              "status": "affected",
              "version": "3.5"
            },
            {
              "lessThan": "3.5",
              "status": "unaffected",
              "version": "0",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "6.16.*",
              "status": "unaffected",
              "version": "6.16.4",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "*",
              "status": "unaffected",
              "version": "6.17",
              "versionType": "original_commit_for_fix"
            }
          ]
        }
      ],
      "cpeApplicability": [
        {
          "nodes": [
            {
              "cpeMatch": [
                {
                  "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                  "versionEndExcluding": "6.16.4",
                  "versionStartIncluding": "3.5",
                  "vulnerable": true
                },
                {
                  "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                  "versionEndExcluding": "6.17",
                  "versionStartIncluding": "3.5",
                  "vulnerable": true
                }
              ],
              "negate": false,
              "operator": "OR"
            }
          ]
        }
      ],
      "descriptions": [
        {
          "lang": "en",
          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nnet/sched: Fix backlog accounting in qdisc_dequeue_internal\n\nThis issue applies for the following qdiscs: hhf, fq, fq_codel, and\nfq_pie, and occurs in their change handlers when adjusting to the new\nlimit. The problem is the following in the values passed to the\nsubsequent qdisc_tree_reduce_backlog call given a tbf parent:\n\n   When the tbf parent runs out of tokens, skbs of these qdiscs will\n   be placed in gso_skb. Their peek handlers are qdisc_peek_dequeued,\n   which accounts for both qlen and backlog. However, in the case of\n   qdisc_dequeue_internal, ONLY qlen is accounted for when pulling\n   from gso_skb. This means that these qdiscs are missing a\n   qdisc_qstats_backlog_dec when dropping packets to satisfy the\n   new limit in their change handlers.\n\n   One can observe this issue with the following (with tc patched to\n   support a limit of 0):\n\n   export TARGET=fq\n   tc qdisc del dev lo root\n   tc qdisc add dev lo root handle 1: tbf rate 8bit burst 100b latency 1ms\n   tc qdisc replace dev lo handle 3: parent 1:1 $TARGET limit 1000\n   echo \u0027\u0027; echo \u0027add child\u0027; tc -s -d qdisc show dev lo\n   ping -I lo -f -c2 -s32 -W0.001 127.0.0.1 2\u003e\u00261 \u003e/dev/null\n   echo \u0027\u0027; echo \u0027after ping\u0027; tc -s -d qdisc show dev lo\n   tc qdisc change dev lo handle 3: parent 1:1 $TARGET limit 0\n   echo \u0027\u0027; echo \u0027after limit drop\u0027; tc -s -d qdisc show dev lo\n   tc qdisc replace dev lo handle 2: parent 1:1 sfq\n   echo \u0027\u0027; echo \u0027post graft\u0027; tc -s -d qdisc show dev lo\n\n   The second to last show command shows 0 packets but a positive\n   number (74) of backlog bytes. The problem becomes clearer in the\n   last show command, where qdisc_purge_queue triggers\n   qdisc_tree_reduce_backlog with the positive backlog and causes an\n   underflow in the tbf parent\u0027s backlog (4096 Mb instead of 0).\n\nTo fix this issue, the codepath for all clients of qdisc_dequeue_internal\nhas been simplified: codel, pie, hhf, fq, fq_pie, and fq_codel.\nqdisc_dequeue_internal handles the backlog adjustments for all cases that\ndo not directly use the dequeue handler.\n\nThe old fq_codel_change limit adjustment loop accumulated the arguments to\nthe subsequent qdisc_tree_reduce_backlog call through the cstats field.\nHowever, this is confusing and error prone as fq_codel_dequeue could also\npotentially mutate this field (which qdisc_dequeue_internal calls in the\nnon gso_skb case), so we have unified the code here with other qdiscs."
        }
      ],
      "metrics": [
        {
          "cvssV3_1": {
            "baseScore": 7.8,
            "baseSeverity": "HIGH",
            "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H",
            "version": "3.1"
          },
          "scenarios": [
            {
              "lang": "en",
              "value": "AV:L - The vulnerability is reached exclusively through RTM_NEWQDISC/tc netlink messages that invoke the qdisc -\u003echange() handlers on a local netlink socket. Per kernel scoring guidance, tc/netlink qdisc configuration is a local attack vector.\nAC:L - The commit message contains a fully deterministic reproducer (tbf parent + fq child, ping to fill the queue, then change the limit and graft) with no race, no memory-layout dependence, and no non-default kernel config \u2014 fq and fq_codel are built in on virtually all distributions and fq_codel is the default qdisc.\nPR:L - tc_modify_qdisc is gated only by netlink_net_capable(skb, CAP_NET_ADMIN), which is evaluated against net-\u003euser_ns, so an unprivileged user obtains it with `unshare -Urn` and configures qdiscs on the loopback device in the new namespace. No real root in the init namespace is needed.\nUI:N - The attacker performs the entire sequence \u2014 creating the qdisc hierarchy, generating traffic into the queue, changing the limit, and grafting \u2014 with no victim action required.\nS:U - The corrupted state and its consequences are confined to the kernel\u0027s qdisc tree within the same security authority; there is no VM, IOMMU, or sandbox boundary crossed.\nC:H - The qdisc queue-accounting desync is the same primitive that has repeatedly escalated to use-after-free on qdisc class objects via wrongly-driven cops-\u003eqlen_notify() (CVE-2025-21703, CVE-2025-38000/38001), and a UAF on those objects yields attacker-controlled reads of freed kernel memory.\nI:H - The bug produces an attacker-controlled u32 underflow of qstats.backlog that propagates through qdisc_tree_reduce_backlog into every ancestor qdisc, corrupting kernel state that drives class active-list maintenance and enqueue/drop decisions; the resulting accounting desync is exploitable for heap corruption in the same manner as the prior net/sched accounting CVEs.\nA:H - The underflowed backlog permanently blackholes the device (bfifo/plug limit checks and sch_red\u0027s qavg see a ~4 GiB queue and drop everything), and the unguarded pre-fix trim loops give an outright kernel oops via qdisc_pkt_len(NULL) in codel/pie/fq_pie/fq_codel or an infinite loop in hhf while holding sch_tree_lock and RTNL."
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-08-05T12:04:29.256Z",
        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
      },
      "references": [
        {
          "url": "https://git.kernel.org/stable/c/a225f44d84b8900d679c5f5a9ea46fe9c0cc7802"
        },
        {
          "url": "https://git.kernel.org/stable/c/52bf272636bda69587952b35ae97690b8dc89941"
        }
      ],
      "title": "net/sched: Fix backlog accounting in qdisc_dequeue_internal",
      "x_generator": {
        "engine": "bippy-1.2.0"
      }
    }
  },
  "cveMetadata": {
    "assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
    "assignerShortName": "Linux",
    "cveId": "CVE-2025-39677",
    "datePublished": "2025-09-05T17:20:43.145Z",
    "dateReserved": "2025-04-16T07:20:57.112Z",
    "dateUpdated": "2026-08-05T12:04:29.256Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2"
}