CVE-2025-39749

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.