CVE-2026-64012 in Linux
Summary
by MITRE • 07/19/2026
In the Linux kernel, the following vulnerability has been resolved:
net/sched: sch_sfb: Replace direct dequeue call with peek and qdisc_dequeue_peeked
When sfb has children (eg qfq qdisc) whose peek() callback is qdisc_peek_dequeued(), we could get a kernel panic. When the parent of such qdiscs (eg illustrated in patch #3 as tbf) wants to retrieve an skb from its child (sfb in this case), it will do the following: 1a. do a peek() - and when sensing there's an skb the child can offer, then - the child in this case(sfb) calls its child's (qfq) peek. qfq does the right thing and will return the gso_skb queue packet. Note: if there wasnt a gso_skb entry then qfq will store it there. 1b. invoke a dequeue() on the child (sfb). And herein lies the problem. - sfb will call the child's dequeue() which will essentially just try to grab something of qfq's queue.
[ 127.594489][ T453] KASAN: null-ptr-deref in range [0x0000000000000048-0x000000000000004f]
[ 127.594741][ T453] CPU: 2 UID: 0 PID: 453 Comm: ping Not tainted 7.1.0-rc1-00035-gac961974495b-dirty #793 PREEMPT(full)
[ 127.595059][ T453] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
[ 127.595254][ T453] RIP: 0010:qfq_dequeue+0x35c/0x1650 [sch_qfq]
[ 127.595461][ T453] Code: 00 fc ff df 80 3c 02 00 0f 85 17 0e 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 76 0c 00 00 48 b8 00 00 00 00 00 fc ff df 4c 8b
[ 127.596081][ T453] RSP: 0018:ffff88810e5af440 EFLAGS: 00010216
[ 127.596337][ T453] RAX: dffffc0000000000 RBX: 0000000000000000 RCX: dffffc0000000000
[ 127.596623][ T453] RDX: 0000000000000009 RSI: 0000001880000000 RDI: ffff888104fd82b0
[ 127.596917][ T453] RBP: ffff888104fd8000 R08: ffff888104fd8280 R09: 1ffff110211893a3
[ 127.597165][ T453] R10: 1ffff110211893a6 R11: 1ffff110211893a7 R12: 0000001880000000
[ 127.597404][ T453] R13: ffff888104fd82b8 R14: 0000000000000048 R15: 0000000040000000
[ 127.597644][ T453] FS: 00007fc380cbfc40(0000) GS:ffff88816f2a8000(0000) knlGS:0000000000000000
[ 127.597956][ T453] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 127.598160][ T453] CR2: 00005610aa9890a8 CR3: 000000010369e000 CR4: 0000000000750ef0
[ 127.598390][ T453] PKRU: 55555554
[ 127.598509][ T453] Call Trace:
[ 127.598629][ T453] <TASK>
[ 127.598718][ T453] ? mark_held_locks+0x40/0x70
[ 127.598890][ T453] ? srso_alias_return_thunk+0x5/0xfbef5
[ 127.599053][ T453] sfb_dequeue+0x88/0x4d0
[ 127.599174][ T453] ? ktime_get+0x137/0x230
[ 127.599328][ T453] ? srso_alias_return_thunk+0x5/0xfbef5
[ 127.599480][ T453] ? qdisc_peek_dequeued+0x7b/0x350 [sch_qfq]
[ 127.599670][ T453] ? srso_alias_return_thunk+0x5/0xfbef5
[ 127.599831][ T453] tbf_dequeue+0x6b1/0x1098 [sch_tbf]
[ 127.599988][ T453] __qdisc_run+0x169/0x1900
The right thing to do in #1b is to grab the skb off gso_skb queue. This patchset fixes that issue by changing #1b to use qdisc_dequeue_peeked() method instead.
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Analysis
by VulDB Data Team • 07/19/2026
This vulnerability resides within the Linux kernel's traffic control subsystem, specifically in the sfb (Stochastic Fairness Based) queuing discipline implementation. The issue manifests when sfb operates as a parent qdisc with children such as qfq (Quick Fair Queuing), creating a complex queuing hierarchy that can lead to kernel panics under certain conditions. The root cause stems from improper handling of packet dequeue operations within this nested queuing structure, where direct dequeue calls bypass proper queue state management.
The technical flaw occurs during the interaction between parent and child queuing disciplines when a parent qdisc like tbf attempts to retrieve packets from its sfb child. The process involves two distinct steps: first, a peek operation that identifies available packets, followed by a dequeue operation to actually remove them from the queue. During step 1a, sfb correctly calls its child's peek() method which properly handles gso_skb queue entries through qfq's qdisc_peek_dequeued callback. However, in step 1b, when sfb attempts to perform a direct dequeue operation on its child qfq, it fails to account for the specific state management requirements of the gso_skb queue mechanism.
The kernel panic results from a null pointer dereference occurring at the qfq_dequeue function where the code attempts to access memory address 0x0000000000000048-0x000000000000004f, indicating that the sfb implementation incorrectly assumes certain queue structures are properly initialized when they may be in an inconsistent state. This memory access violation happens because the direct dequeue call bypasses the proper peek-then-dequeue pattern that should ensure queue consistency and proper state management. The stack trace shows the call sequence leading to the crash through sfb_dequeue, qdisc_peek_dequeued, and ultimately tbf_dequeue, demonstrating how the failure propagates through the queuing hierarchy.
This vulnerability directly relates to CWE-476 Null Pointer Dereference and can be categorized under ATT&CK technique T1059.003 Command and Scripting Interpreter: Python, though more specifically as a kernel-level system stability issue. The operational impact includes complete system crashes and potential denial of service conditions, particularly affecting network functionality on systems running affected kernel versions. Systems utilizing complex queuing configurations with nested qdisc hierarchies are most vulnerable, making this especially critical for network infrastructure devices, servers handling heavy traffic loads, and any system requiring sophisticated traffic control policies.
The recommended mitigation involves applying the patch that replaces direct dequeue calls with proper peek and qdisc_dequeue_peeked method usage. This change ensures that when sfb retrieves packets from its children, it properly handles the gso_skb queue state management by first peeking at available packets and then using the appropriate dequeue_peeked mechanism that maintains queue consistency. The fix enforces proper queuing discipline behavior by ensuring that packet retrieval operations respect the underlying queue structure's integrity, particularly when dealing with gso_skb entries that require special handling. This approach aligns with established kernel networking best practices for maintaining atomicity and consistency in complex queuing scenarios while preventing the null pointer dereference condition that leads to kernel panics.