Bug 2502372 (CVE-2026-63983)

Summary: CVE-2026-63983 kernel: net/sched: fix packet loop on netem when duplicate is on
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: rhel-process-autobot, watson-tool-maintainers
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in the Linux kernel's network emulator (netem) component. This vulnerability occurs when netem duplicates a packet and re-enqueues it, leading to a recursive packet duplication loop if multiple netem instances are configured. This uncontrolled recursion can exhaust system resources, such as memory or stack, resulting in a Denial of Service (DoS) for the affected system.
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Description OSIDB Bzimport 2026-07-19 16:04:33 UTC 
In the Linux kernel, the following vulnerability has been resolved:

net/sched: fix packet loop on netem when duplicate is on

When netem duplicates a packet it re-enqueues the copy at the root qdisc.
If another netem sits in the tree the copy can be duplicated
again, recursing until the stack or memory is exhausted.

The original duplication guard temporarily zeroed q->duplicate around
the re-enqueue, but that does not cover all cases because it is
per-qdisc state shared across all concurrent enqueue paths
and is not safe without additional locking.

Use the skb tc_depth field introduced in an earlier patch:
 - increment it on the duplicate before re-enqueue
 - skip duplication for any skb whose tc_depth is already non-zero.

This marks the packet itself rather than mutating qdisc state,
therefore it is safe regardless of tree topology or concurrency.
Comment 1 Keith Grant 2026-07-21 23:34:16 UTC 
Upstream advisory:
https://lore.kernel.org/linux-cve-announce/2026071902-CVE-2026-63983-9047@gregkh/T