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CVE-2025-39677

Estado: ModificadaMedia (5.5)—

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:

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.

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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.

Detalles técnicos trazas, registros y código del informe original
   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).

CVSS

Probabilidad de explotación (EPSS)

EPSS (Exploit Prediction Scoring System, de FIRST) estima la probabilidad de que una vulnerabilidad sea explotada en 30 días. Complementa a CVSS (impacto) y a CISA KEV (explotación confirmada).

Tecnologías afectadas (1)

Linux — Linux Kernel

CWE

Referencias

JSON original (NVD)

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  "id": "CVE-2025-39677",
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          "userInteraction": "NONE",
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            "net/sched/sch_fq_pie.c",
            "net/sched/sch_hhf.c",
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  "published": "2025-09-05T18:15:44.043",
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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 ''; echo 'add child'; tc -s -d qdisc show dev lo\n   ping -I lo -f -c2 -s32 -W0.001 127.0.0.1 2>&1 >/dev/null\n   echo ''; echo 'after ping'; tc -s -d qdisc show dev lo\n   tc qdisc change dev lo handle 3: parent 1:1 $TARGET limit 0\n   echo ''; echo 'after limit drop'; tc -s -d qdisc show dev lo\n   tc qdisc replace dev lo handle 2: parent 1:1 sfq\n   echo ''; echo 'post graft'; 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's 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."
    }
  ],
  "lastModified": "2026-07-30T06:23:39.233",
  "configurations": [
    {
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}