CVE-2025-39744 rcu: Fix rcu_read_unlock() deadloop due to IRQ work
A flaw in the Linux kernel's RCU subsystem can cause a system hang or deadlock during IRQ exit, leading to denial of service.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2025-39744?
CVE-2025-39744 is a high-severity denial of service vulnerability in Linux kernel, published on 11 Sep 2025. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
rcu: Fix rcu_read_unlock() deadloop due to IRQ work
During rcu_read_unlock_special(), if this happens during irq_exit(), we can lockup if an IPI is issued. This is because the IPI itself triggers the irq_exit() path causing a recursive lock up.
This is precisely what Xiongfeng found when invoking a BPF program on the trace_tick_stop() tracepoint As shown in the trace below. Fix by managing the irq_work state correctly.
irq_exit() __irq_exit_rcu() /* in_hardirq() returns false after this */ preempt_count_sub(HARDIRQ_OFFSET) tick_irq_exit() tick_nohz_irq_exit() tick_nohz_stop_sched_tick() trace_tick_stop() /* a bpf prog is hooked on this trace point */ __bpf_trace_tick_stop() bpf_trace_run2() rcu_read_unlock_special() /* will send a IPI to itself */ irq_work_queue_on(&rdp->defer_qs_iw, rdp->cpu);
A simple reproducer can also be obtained by doing the following in tick_irq_exit(). It will hang on boot without the patch:
static inline void tick_irq_exit(void) { + rcu_read_lock(); + WRITE_ONCE(current->rcu_read_unlock_special.b.need_qs, true); + rcu_read_unlock(); +
[neeraj: Apply Frederic's suggested fix for PREEMPT_RT]
Weakness: CWE-125
Which versions are affected by CVE-2025-39744?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2025-39744?
Apply the vendor's patch.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2025-39744 being exploited?
There is no known exploitation of CVE-2025-39744 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as AZL-67253, AZL-70807, BELL-CVE-2025-39744, DEBIAN-CVE-2025-39744, GHSA-vqg7-j8rg-9mw8, ROOT-OS-DEBIAN-11-CVE-2025-39744 and 4 more.