CVE-2024-46797 in Linux
Summary
by MITRE • 09/18/2024
In the Linux kernel, the following vulnerability has been resolved:
powerpc/qspinlock: Fix deadlock in MCS queue
If an interrupt occurs in queued_spin_lock_slowpath() after we increment qnodesp->count and before node->lock is initialized, another CPU might see stale lock values in get_tail_qnode(). If the stale lock value happens to match the lock on that CPU, then we write to the "next" pointer of the wrong qnode. This causes a deadlock as the former CPU, once it becomes the head of the MCS queue, will spin indefinitely until it's "next" pointer is set by its successor in the queue.
Running stress-ng on a 16 core (16EC/16VP) shared LPAR, results in occasional lockups similar to the following:
$ stress-ng --all 128 --vm-bytes 80% --aggressive \ --maximize --oomable --verify --syslog \ --metrics --times --timeout 5m
watchdog: CPU 15 Hard LOCKUP ...... NIP [c0000000000b78f4] queued_spin_lock_slowpath+0x1184/0x1490
LR [c000000001037c5c] _raw_spin_lock+0x6c/0x90
Call Trace: 0xc000002cfffa3bf0 (unreliable) _raw_spin_lock+0x6c/0x90 raw_spin_rq_lock_nested.part.135+0x4c/0xd0 sched_ttwu_pending+0x60/0x1f0 __flush_smp_call_function_queue+0x1dc/0x670 smp_ipi_demux_relaxed+0xa4/0x100 xive_muxed_ipi_action+0x20/0x40 __handle_irq_event_percpu+0x80/0x240 handle_irq_event_percpu+0x2c/0x80 handle_percpu_irq+0x84/0xd0 generic_handle_irq+0x54/0x80 __do_irq+0xac/0x210 __do_IRQ+0x74/0xd0 0x0 do_IRQ+0x8c/0x170 hardware_interrupt_common_virt+0x29c/0x2a0 --- interrupt: 500 at queued_spin_lock_slowpath+0x4b8/0x1490 ...... NIP [c0000000000b6c28] queued_spin_lock_slowpath+0x4b8/0x1490
LR [c000000001037c5c] _raw_spin_lock+0x6c/0x90
--- interrupt: 500 0xc0000029c1a41d00 (unreliable) _raw_spin_lock+0x6c/0x90 futex_wake+0x100/0x260 do_futex+0x21c/0x2a0 sys_futex+0x98/0x270 system_call_exception+0x14c/0x2f0 system_call_vectored_common+0x15c/0x2ec
The following code flow illustrates how the deadlock occurs. For the sake of brevity, assume that both locks (A and B) are contended and we call the queued_spin_lock_slowpath() function.
CPU0 CPU1 ---- ---- spin_lock_irqsave(A) | spin_unlock_irqrestore(A) | spin_lock(B) | | | ▼ | id = qnodesp->count++; | (Note that nodes[0].lock == A) |
| | ▼ | Interrupt | (happens before "nodes[0].lock = B") |
| | ▼ | spin_lock_irqsave(A) | | | ▼ | id = qnodesp->count++ | nodes[1].lock = A |
| | ▼ | Tail of MCS queue | | spin_lock_irqsave(A) ▼ | Head of MCS queue ▼ | CPU0 is previous tail ▼ | Spin indefinitely ▼ (until "nodes[1].next != NULL") prev = get_tail_qnode(A, CPU0)
| ▼ prev == &qnodes[CPU0].nodes[0]
(as qnodes ---truncated---
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Analysis
by VulDB Data Team • 04/06/2026
The vulnerability described in CVE-2024-46797 resides within the Linux kernel's powerpc architecture implementation of queued spinlocks, specifically in the MCS queue handling mechanism. This flaw manifests as a deadlock condition that occurs during the execution of the queued_spin_lock_slowpath() function, which is part of the kernel's locking subsystem designed to manage concurrent access to shared resources across multiple processors. The issue is particularly significant in multi-core environments where the timing of interrupt handling and lock acquisition can lead to inconsistent state management within the queue structure.
The technical root cause stems from a race condition that arises when an interrupt occurs between the increment of qnodesp->count and the initialization of node->lock within the queued_spin_lock_slowpath() function. During this brief window, another CPU core may observe stale lock values through the get_tail_qnode() function, which can result in incorrect pointer manipulation when setting the "next" field of a queue node. This misconfiguration causes the former CPU that should become the head of the MCS queue to spin indefinitely, waiting for a "next" pointer that will never be set properly. The vulnerability is classified under CWE-362 as a race condition that can lead to deadlock conditions, and it directly impacts the kernel's ability to maintain proper synchronization primitives.
The operational impact of this vulnerability is severe, particularly in high-concurrency scenarios such as those involving stress-ng testing with multiple threads and virtual machines. The deadlock manifests as hard lockups where the system becomes unresponsive, as evidenced by the watchdog timeout messages and call traces showing the lockup occurring in the queued_spin_lock_slowpath function. The stress-ng command with 128 workers, VM memory allocation, and aggressive settings effectively triggers the race condition, leading to systems becoming unresponsive for extended periods. This vulnerability affects shared LPAR environments with multiple cores, making it particularly dangerous in virtualized and cloud computing scenarios where multiple processes compete for the same lock resources. The attack surface is primarily limited to powerpc architectures running Linux kernels with queued spinlock implementations, but the implications are broad given the critical nature of locking mechanisms in kernel space.
Mitigation strategies for this vulnerability involve ensuring that the Linux kernel is updated to a version that includes the fix for this race condition in the queued spinlock implementation. System administrators should prioritize applying kernel updates that address CVE-2024-46797, particularly in production environments running powerpc-based systems with high concurrency workloads. The fix typically involves reordering the sequence of operations within the queued_spin_lock_slowpath() function to ensure proper initialization of lock values before allowing other CPUs to observe them, thereby preventing the race condition that leads to the deadlock. Organizations should also implement monitoring systems to detect potential lockup conditions and establish recovery procedures that can gracefully handle such scenarios. Additionally, system architects should consider reducing the complexity of lock contention in applications running on affected systems, and implement alternative synchronization mechanisms where possible to minimize exposure to this specific vulnerability. The ATT&CK framework categorizes this as a privilege escalation technique through kernel exploitation, as the deadlock condition can potentially be leveraged to cause system instability or denial of service in critical infrastructure environments.