CVE-2025-39677 (GCVE-0-2025-39677)

Vulnerability from cvelistv5 – Published: 2025-09-05 17:20 – Updated: 2026-05-11 21:34
VLAI?
Title
net/sched: Fix backlog accounting in qdisc_dequeue_internal
Summary
In the Linux kernel, the following vulnerability has been resolved: net/sched: Fix backlog accounting in qdisc_dequeue_internal This issue applies for the following qdiscs: hhf, fq, fq_codel, and fq_pie, and occurs in their change handlers when adjusting to the new limit. The problem is the following in the values passed to the subsequent qdisc_tree_reduce_backlog call given a tbf parent: When the tbf parent runs out of tokens, skbs of these qdiscs will be placed in gso_skb. Their peek handlers are qdisc_peek_dequeued, which accounts for both qlen and backlog. However, in the case of qdisc_dequeue_internal, ONLY qlen is accounted for when pulling from gso_skb. This means that these qdiscs are missing a qdisc_qstats_backlog_dec when dropping packets to satisfy the new limit in their change handlers. One can observe this issue with the following (with tc patched to support a limit of 0): export TARGET=fq tc qdisc del dev lo root tc qdisc add dev lo root handle 1: tbf rate 8bit burst 100b latency 1ms tc qdisc replace dev lo handle 3: parent 1:1 $TARGET limit 1000 echo ''; echo 'add child'; tc -s -d qdisc show dev lo ping -I lo -f -c2 -s32 -W0.001 127.0.0.1 2>&1 >/dev/null echo ''; echo 'after ping'; tc -s -d qdisc show dev lo tc qdisc change dev lo handle 3: parent 1:1 $TARGET limit 0 echo ''; echo 'after limit drop'; tc -s -d qdisc show dev lo tc qdisc replace dev lo handle 2: parent 1:1 sfq echo ''; echo 'post graft'; tc -s -d qdisc show dev lo The second to last show command shows 0 packets but a positive number (74) of backlog bytes. The problem becomes clearer in the last show command, where qdisc_purge_queue triggers qdisc_tree_reduce_backlog with the positive backlog and causes an underflow in the tbf parent's backlog (4096 Mb instead of 0). To fix this issue, the codepath for all clients of qdisc_dequeue_internal has been simplified: codel, pie, hhf, fq, fq_pie, and fq_codel. qdisc_dequeue_internal handles the backlog adjustments for all cases that do not directly use the dequeue handler. The old fq_codel_change limit adjustment loop accumulated the arguments to the subsequent qdisc_tree_reduce_backlog call through the cstats field. However, this is confusing and error prone as fq_codel_dequeue could also potentially mutate this field (which qdisc_dequeue_internal calls in the non gso_skb case), so we have unified the code here with other qdiscs.
Severity ?
No CVSS data available.
Assigner
References
Impacted products
Vendor Product Version
Linux Linux Affected: 4b549a2ef4bef9965d97cbd992ba67930cd3e0fe , < a225f44d84b8900d679c5f5a9ea46fe9c0cc7802 (git)
Affected: 4b549a2ef4bef9965d97cbd992ba67930cd3e0fe , < 52bf272636bda69587952b35ae97690b8dc89941 (git)
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Linux Linux Affected: 3.5
Unaffected: 0 , < 3.5 (semver)
Unaffected: 6.16.4 , ≤ 6.16.* (semver)
Unaffected: 6.17 , ≤ * (original_commit_for_fix)
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{
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      "affected": [
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          "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"
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            "net/sched/sch_fq_pie.c",
            "net/sched/sch_hhf.c",
            "net/sched/sch_pie.c"
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          "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."
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-05-11T21:34:07.205Z",
        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
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          "url": "https://git.kernel.org/stable/c/52bf272636bda69587952b35ae97690b8dc89941"
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      "title": "net/sched: Fix backlog accounting in qdisc_dequeue_internal",
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    "assignerShortName": "Linux",
    "cveId": "CVE-2025-39677",
    "datePublished": "2025-09-05T17:20:43.145Z",
    "dateReserved": "2025-04-16T07:20:57.112Z",
    "dateUpdated": "2026-05-11T21:34:07.205Z",
    "state": "PUBLISHED"
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      "date": "2026-05-24",
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  }
}


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Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.

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  • Seen: The vulnerability was mentioned, discussed, or observed by the user.
  • Confirmed: The vulnerability has been validated from an analyst's perspective.
  • Published Proof of Concept: A public proof of concept is available for this vulnerability.
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  • Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
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  • Not confirmed: The user expressed doubt about the validity of the vulnerability.
  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.



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