Bug 2346269 (CVE-2025-21703)

Summary: CVE-2025-21703 kernel: netem: Update sch->q.qlen before qdisc_tree_reduce_backlog()
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
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Priority: medium    
Version: unspecifiedCC: dfreiber, drow, jburrell, vkumar
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Hardware: All   
OS: Linux   
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Description OSIDB Bzimport 2025-02-18 15:01:15 UTC 
In the Linux kernel, the following vulnerability has been resolved:

netem: Update sch->q.qlen before qdisc_tree_reduce_backlog()

qdisc_tree_reduce_backlog() notifies parent qdisc only if child
qdisc becomes empty, therefore we need to reduce the backlog of the
child qdisc before calling it. Otherwise it would miss the opportunity
to call cops->qlen_notify(), in the case of DRR, it resulted in UAF
since DRR uses ->qlen_notify() to maintain its active list.
Comment 1 Michal Findra 2025-02-18 15:03:46 UTC 
Upstream advisory:
https://lore.kernel.org/linux-cve-announce/2025021849-CVE-2025-21703-79f1@gregkh/T