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CVE-2025-38350 Detail

Description

In the Linux kernel, the following vulnerability has been resolved: net/sched: Always pass notifications when child class becomes empty Certain classful qdiscs may invoke their classes' dequeue handler on an enqueue operation. This may unexpectedly empty the child qdisc and thus make an in-flight class passive via qlen_notify(). Most qdiscs do not expect such behaviour at this point in time and may re-activate the class eventually anyways which will lead to a use-after-free. The referenced fix commit attempted to fix this behavior for the HFSC case by moving the backlog accounting around, though this turned out to be incomplete since the parent's parent may run into the issue too. The following reproducer demonstrates this use-after-free: tc qdisc add dev lo root handle 1: drr tc filter add dev lo parent 1: basic classid 1:1 tc class add dev lo parent 1: classid 1:1 drr tc qdisc add dev lo parent 1:1 handle 2: hfsc def 1 tc class add dev lo parent 2: classid 2:1 hfsc rt m1 8 d 1 m2 0 tc qdisc add dev lo parent 2:1 handle 3: netem tc qdisc add dev lo parent 3:1 handle 4: blackhole echo 1 | socat -u STDIN UDP4-DATAGRAM:127.0.0.1:8888 tc class delete dev lo classid 1:1 echo 1 | socat -u STDIN UDP4-DATAGRAM:127.0.0.1:8888 Since backlog accounting issues leading to a use-after-frees on stale class pointers is a recurring pattern at this point, this patch takes a different approach. Instead of trying to fix the accounting, the patch ensures that qdisc_tree_reduce_backlog always calls qlen_notify when the child qdisc is empty. This solves the problem because deletion of qdiscs always involves a call to qdisc_reset() and / or qdisc_purge_queue() which ultimately resets its qlen to 0 thus causing the following qdisc_tree_reduce_backlog() to report to the parent. Note that this may call qlen_notify on passive classes multiple times. This is not a problem after the recent patch series that made all the classful qdiscs qlen_notify() handlers idempotent.


Metrics

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CVSS 4.0 Severity and Vector Strings:

NIST CVSS score
NIST: NVD
N/A
NVD assessment not yet provided.

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URL Source(s) Tag(s)
https://git.kernel.org/stable/c/103406b38c600fec1fe375a77b27d87e314aea09 kernel.org
https://git.kernel.org/stable/c/3b290923ad2b23596208c1e29520badef4356a43 kernel.org
https://git.kernel.org/stable/c/7874c9c132e906a52a187d045995b115973c93fb kernel.org
https://git.kernel.org/stable/c/a44acdd9e84a211989ff4b9b92bf3545d8456ad5 kernel.org
https://git.kernel.org/stable/c/a553afd91f55ff39b1e8a1c4989a29394c9e0472 kernel.org
https://git.kernel.org/stable/c/e269f29e9395527bc00c213c6b15da04ebb35070 kernel.org
https://git.kernel.org/stable/c/e9921b57dca05ac5f4fa1fa8e993d4f0ee52e2b7 kernel.org
https://git.kernel.org/stable/c/f680a4643c6f71e758d8fe0431a958e9a6a4f59d kernel.org

Weakness Enumeration

CWE-ID CWE Name Source

Change History

1 change records found show changes

Quick Info

CVE Dictionary Entry:
CVE-2025-38350
NVD Published Date:
07/19/2025
NVD Last Modified:
07/22/2025
Source:
kernel.org