Summary
In the Linux kernel, the following vulnerability has been resolved: rcu: Protect -...
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): [] finish_task_switch+0x131/0x320
hardirqs last disabled at (200271): [] __schedule+0x129/0xd70
softirqs last enabled at (0): [] 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.
Impact
CVE-2025-39749 has a CVSS score of 7.0 (High). The vector is requires local access, low privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. No fixed version is listed yet, so configuration controls and monitoring matter more in the interim.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Frequently Asked Questions
- What is CVE-2025-39749? CVE-2025-39749 is a high-severity security vulnerability. No fixed version is listed yet.
- How severe is CVE-2025-39749? CVE-2025-39749 has a CVSS score of 7.0 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Is there a fix for CVE-2025-39749? No fixed version is listed for CVE-2025-39749 yet. Monitor the advisory for updates and apply mitigations in the interim.
- Is CVE-2025-39749 exploitable, and should I be worried? Whether CVE-2025-39749 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2025-39749 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.