FKIE_CVE-2025-39677
Vulnerability from fkie_nvd - Published: 2025-09-05 18:15 - Updated: 2025-11-25 21:28
Severity ?
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.
References
Impacted products
| Vendor | Product | Version | |
|---|---|---|---|
| linux | linux_kernel | * | |
| linux | linux_kernel | 6.17 | |
| linux | linux_kernel | 6.17 |
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"descriptions": [
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"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."
}
],
"id": "CVE-2025-39677",
"lastModified": "2025-11-25T21:28:36.563",
"metrics": {
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"cvssData": {
"attackComplexity": "LOW",
"attackVector": "LOCAL",
"availabilityImpact": "HIGH",
"baseScore": 5.5,
"baseSeverity": "MEDIUM",
"confidentialityImpact": "NONE",
"integrityImpact": "NONE",
"privilegesRequired": "LOW",
"scope": "UNCHANGED",
"userInteraction": "NONE",
"vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H",
"version": "3.1"
},
"exploitabilityScore": 1.8,
"impactScore": 3.6,
"source": "nvd@nist.gov",
"type": "Primary"
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"published": "2025-09-05T18:15:44.043",
"references": [
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"source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"tags": [
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"url": "https://git.kernel.org/stable/c/52bf272636bda69587952b35ae97690b8dc89941"
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"url": "https://git.kernel.org/stable/c/a225f44d84b8900d679c5f5a9ea46fe9c0cc7802"
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"sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
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}
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Sightings
| Author | Source | Type | Date |
|---|
Nomenclature
- 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.
- Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
- Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
- Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
- 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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