CVE-2026-74266 in Linux
Summary
by MITRE • 08/15/2026
In the Linux kernel, the following vulnerability has been resolved:
net/sched: sch_dualpi2: Do not call qdisc_tree_reduce_backlog during peek before restoring qlen
Whenever dualpi2 drops packets during peek, it calls qdisc_tree_reduce_backlog. An issue arises because it calls qdisc_tree_reduce_backlog before it reincrements the qlen. If qlen drops to zero, but peek returns an skb, the parent's qlen_notify callback will be executed even though dualpi2 still has 1 packet on the queue and, thus, mistakenly deactivates the parent's class which leads to a null-ptr-deref:
[ 101.427314][ T599] Oops: general protection fault, probably for non-canonical address 0xdffffc0000000009: 0000 [#1] SMP KASAN NOPTI
[ 101.427755][ T599] KASAN: null-ptr-deref in range [0x0000000000000048-0x000000000000004f]
[ 101.428048][ T599] CPU: 2 UID: 0 PID: 599 Comm: ping Not tainted 7.1.0-rc5-00284-gbce53c430ed7 #102 PREEMPT(full)
[ 101.428400][ T599] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
[ 101.428608][ T599] RIP: 0010:qfq_dequeue (net/sched/sch_qfq.c:1150) sch_qfq
[ 101.428821][ T599] Code: 00 fc ff df 80 3c 02 00 0f 85 46 0c 00 00 4c 8d 73 48 48 89 9d b8 02 00 00 48 b8 00 00 00 00 00 fc ff df 4c 89 f2 48 c1 ea 03 <80> 3c 02 00 0f 85 2d 0c 00 00 48 b8 00 00 00 00 00 fc ff df 4c 8b
All code [ 101.429348][ T599] RSP: 0018:ffff8881110df4f0 EFLAGS: 00010216
[ 101.429541][ T599] RAX: dffffc0000000000 RBX: 0000000000000000 RCX: dffffc0000000000
[ 101.429763][ T599] RDX: 0000000000000009 RSI: 00000024c0000000 RDI: ffff88811436c2b0
[ 101.429985][ T599] RBP: ffff88811436c000 R08: ffff88811436c280 R09: 1ffff11021277523
[ 101.430206][ T599] R10: 1ffff11021277526 R11: 1ffff11021277527 R12: 00000024c0000000
[ 101.430423][ T599] R13: ffff88811436c2b8 R14: 0000000000000048 R15: 0000000020000000
[ 101.430642][ T599] FS: 00007f61813e1c40(0000) GS:ffff8881691ef000(0000) knlGS:0000000000000000
[ 101.430913][ T599] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 101.431100][ T599] CR2: 00005651650850a8 CR3: 000000010ca0b000 CR4: 0000000000750ef0
[ 101.431320][ T599] PKRU: 55555554
[ 101.431433][ T599] Call Trace:
[ 101.431544][ T599] <TASK>
[ 101.431628][ T599] __qdisc_run (net/sched/sch_generic.c:322 net/sched/sch_generic.c:427 net/sched/sch_generic.c:445)
[ 101.431792][ T599] ? dev_qdisc_enqueue (./include/trace/events/qdisc.h:49 (discriminator 22) net/core/dev.c:4176 (discriminator 22))
[ 101.431941][ T599] __dev_queue_xmit (./include/net/pkt_sched.h:120 ./include/net/pkt_sched.h:117 net/core/dev.c:4292 net/core/dev.c:4831)
Fix this by only calling qdisc_tree_reduce_backlog in peek after the qlen is restored.
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Analysis
by VulDB Data Team • 08/15/2026
The vulnerability resides within the Linux kernel's networking subsystem, specifically in the dualpi2 queuing discipline implementation located at net/sched/sch_dualpi2.c. This issue represents a classic race condition and state management flaw that can lead to null pointer dereferences and system instability. The problem occurs during packet peek operations where the queuing discipline incorrectly manages the packet queue length counter before restoring it, creating a temporal inconsistency between the internal queue state and the parent queue's notification mechanisms.
The technical flaw manifests when dualpi2 drops packets during peek operations and subsequently calls qdisc_tree_reduce_backlog before reincrementing the qlen counter. This sequence creates a window where the parent queue's qlen_notify callback is invoked with an incorrect queue length value, potentially leading to premature deactivation of parent classes. The vulnerability is particularly dangerous because it operates at the kernel level where such errors can result in immediate system crashes or more subtle memory corruption issues.
When the queue length drops to zero during peek operations but a packet is still available for return, the parent class's notification mechanism incorrectly interprets this as an empty queue. This misinterpretation causes the parent's class to be deactivated, which subsequently leads to null pointer dereference when other kernel components attempt to access the now-invalidated parent structure. The stack trace shows execution flow transitioning through qdisc_run and eventually reaching qfq_dequeue where the null pointer dereference occurs, indicating that the issue propagates through multiple layers of the networking subsystem.
This vulnerability directly relates to CWE-476 which describes null pointer dereference conditions, and aligns with ATT&CK technique T1059.003 for kernel-level code execution. The issue demonstrates poor state management practices in kernel space where synchronization between different queue management components is inadequate. The fix implements a simple but critical ordering change that ensures qdisc_tree_reduce_backlog is only called after the qlen is properly restored, thereby maintaining consistency between internal queue states and external notifications.
The operational impact of this vulnerability extends beyond simple system crashes to potentially enable denial of service attacks against network services or even privilege escalation in certain scenarios. Attackers could exploit this condition by crafting specific packet flows that trigger the race condition repeatedly, causing system instability or complete network stack failures. The fix addresses the root cause by ensuring proper temporal ordering of queue state modifications and notifications, which prevents the inconsistent state that leads to the null pointer dereference condition.
Mitigation strategies should include immediate deployment of kernel updates containing the fix, along with monitoring for unusual network stack behavior that might indicate exploitation attempts. System administrators should also consider implementing network traffic shaping policies that reduce the likelihood of triggering this specific race condition during normal operations. The vulnerability highlights the importance of careful state management in kernel networking code and demonstrates how seemingly minor ordering issues can have catastrophic effects on system stability and security.