CVE-2025-39749
Unknown Unknown - Not Provided
BaseFortify

Publication date: 2025-09-11

Last updated on: 2025-11-03

Assigner: kernel.org

Description
In the Linux kernel, the following vulnerability has been resolved: rcu: Protect ->defer_qs_iw_pending from data race On kernels built with CONFIG_IRQ_WORK=y, when rcu_read_unlock() is invoked within an interrupts-disabled region of code [1], it will invoke rcu_read_unlock_special(), which uses an irq-work handler to force the system to notice when the RCU read-side critical section actually ends. That end won't happen until interrupts are enabled at the soonest. In some kernels, such as those booted with rcutree.use_softirq=y, the irq-work handler is used unconditionally. The per-CPU rcu_data structure's ->defer_qs_iw_pending field is updated by the irq-work handler and is both read and updated by rcu_read_unlock_special(). This resulted in the following KCSAN splat: ------------------------------------------------------------------------ BUG: KCSAN: data-race in rcu_preempt_deferred_qs_handler / rcu_read_unlock_special read to 0xffff96b95f42d8d8 of 1 bytes by task 90 on cpu 8: rcu_read_unlock_special+0x175/0x260 __rcu_read_unlock+0x92/0xa0 rt_spin_unlock+0x9b/0xc0 __local_bh_enable+0x10d/0x170 __local_bh_enable_ip+0xfb/0x150 rcu_do_batch+0x595/0xc40 rcu_cpu_kthread+0x4e9/0x830 smpboot_thread_fn+0x24d/0x3b0 kthread+0x3bd/0x410 ret_from_fork+0x35/0x40 ret_from_fork_asm+0x1a/0x30 write to 0xffff96b95f42d8d8 of 1 bytes by task 88 on cpu 8: rcu_preempt_deferred_qs_handler+0x1e/0x30 irq_work_single+0xaf/0x160 run_irq_workd+0x91/0xc0 smpboot_thread_fn+0x24d/0x3b0 kthread+0x3bd/0x410 ret_from_fork+0x35/0x40 ret_from_fork_asm+0x1a/0x30 no locks held by irq_work/8/88. irq event stamp: 200272 hardirqs last enabled at (200272): [<ffffffffb0f56121>] finish_task_switch+0x131/0x320 hardirqs last disabled at (200271): [<ffffffffb25c7859>] __schedule+0x129/0xd70 softirqs last enabled at (0): [<ffffffffb0ee093f>] copy_process+0x4df/0x1cc0 softirqs last disabled at (0): [<0000000000000000>] 0x0 ------------------------------------------------------------------------ The problem is that irq-work handlers run with interrupts enabled, which means that rcu_preempt_deferred_qs_handler() could be interrupted, and that interrupt handler might contain an RCU read-side critical section, which might invoke rcu_read_unlock_special(). In the strict KCSAN mode of operation used by RCU, this constitutes a data race on the ->defer_qs_iw_pending field. This commit therefore disables interrupts across the portion of the rcu_preempt_deferred_qs_handler() that updates the ->defer_qs_iw_pending field. This suffices because this handler is not a fast path.
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Meta Information
Published
2025-09-11
Last Modified
2025-11-03
Generated
2026-05-06
AI Q&A
2025-09-11
EPSS Evaluated
2026-05-05
NVD
CVE-2025-39749
Affected Vendors & Products
Showing 2 associated CPEs
Vendor Product Version / Range
linux linux_kernel 6.1.153-1
linux linux_kernel 5.10.244-1
Helpful Resources
Exploitability
CWE
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KEV
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CWE ID Description
CWE-UNKNOWN
Attack-Flow Graph
AI Powered Q&A
Can you explain this vulnerability to me?

This vulnerability is a data race in the Linux kernel's RCU (Read-Copy-Update) subsystem. Specifically, on kernels built with CONFIG_IRQ_WORK=y, when rcu_read_unlock() is called within an interrupts-disabled region, it triggers an irq-work handler that updates a per-CPU field called defer_qs_iw_pending. Because irq-work handlers run with interrupts enabled, the handler can be interrupted by another RCU read-side critical section that also accesses this field, causing a data race. This race condition can lead to unpredictable behavior or kernel bugs. The fix disables interrupts during the critical update to prevent this race.


How can this vulnerability impact me? :

This vulnerability can cause data races in the Linux kernel's RCU subsystem, potentially leading to kernel instability, crashes, or unpredictable behavior. Such issues can affect system reliability and performance, especially on systems using affected kernel configurations. It may also complicate debugging and system maintenance due to subtle concurrency bugs.


What immediate steps should I take to mitigate this vulnerability?

The vulnerability is resolved by a kernel patch that disables interrupts across the portion of the rcu_preempt_deferred_qs_handler() that updates the ->defer_qs_iw_pending field. Immediate mitigation steps include updating your Linux kernel to a version that includes this fix. There are no specific configuration changes or commands provided to mitigate this vulnerability otherwise.


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