CVE-2025-39677: Software Security Vulnerability Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-jcgr-c7xg-m62q",
  "modified" : "2025-11-25T21:32:03Z",
  "published" : "2025-09-05T18:31:26Z",
  "aliases" : [ "CVE-2025-39677" ],
  "details" : "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.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H"
  } ],
  "affected" : [ ],
  "references" : [ {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2025-39677"
  }, {
    "type" : "WEB",
    "url" : "https://git.kernel.org/stable/c/52bf272636bda69587952b35ae97690b8dc89941"
  }, {
    "type" : "WEB",
    "url" : "https://git.kernel.org/stable/c/a225f44d84b8900d679c5f5a9ea46fe9c0cc7802"
  } ],
  "database_specific" : {
    "cwe_ids" : [ ],
    "severity" : "MODERATE",
    "github_reviewed" : false,
    "github_reviewed_at" : null,
    "nvd_published_at" : "2025-09-05T18:15:44Z"
  }
}