CVE-2025-39744

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 Frederics suggested fix for PREEMPT_RT]

Weakness

The product reads data past the end, or before the beginning, of the intended buffer.

Affected Software

Potential Mitigations

• Assume all input is malicious. Use an “accept known good” input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.
• When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, “boat” may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as “red” or “blue.”
• Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code’s environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.
• To reduce the likelihood of introducing an out-of-bounds read, ensure that you validate and ensure correct calculations for any length argument, buffer size calculation, or offset. Be especially careful of relying on a sentinel (i.e. special character such as NUL) in untrusted inputs.

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/540.html

References

• https://git.kernel.org/stable/c/1cfa244f7198d325594e627574930b7b91df5bfe
• https://git.kernel.org/stable/c/56c5ef194f4509df63fc0f7a91ea5973ce479b1e
• https://git.kernel.org/stable/c/b41642c87716bbd09797b1e4ea7d904f06c39b7b
• https://git.kernel.org/stable/c/ddebb2a7677673cf4438a04e1a48b8ed6b0c8e9a
• https://git.kernel.org/stable/c/e7a375453cca2b8a0d2fa1b82b913f3fed7c0507