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CVE-2025-39749

Description

In the Linux kernel, the following vulnerability has been resolved:rcu: Protect ->defer_qs_iw_pending from data raceOn kernels built with CONFIG_IRQ_WORK=y, when rcu_read_unlock() isinvoked within an interrupts-disabled region of code [1], it will invokercu_read_unlock_special(), which uses an irq-work handler to force thesystem 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, theirq-work handler is used unconditionally.The per-CPU rcu_data structure's ->defer_qs_iw_pending field isupdated by the irq-work handler and is both read and updated byrcu_read_unlock_special(). This resulted in the following KCSAN splat:------------------------------------------------------------------------BUG: KCSAN: data-race in rcu_preempt_deferred_qs_handler / rcu_read_unlock_specialread 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/0x30write 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/0x30no locks held by irq_work/8/88.irq event stamp: 200272hardirqs last enabled at (200272): [] finish_task_switch+0x131/0x320hardirqs last disabled at (200271): [] __schedule+0x129/0xd70softirqs last enabled at (0): [] copy_process+0x4df/0x1cc0softirqs last disabled at (0): [<0000000000000000>] 0x0------------------------------------------------------------------------The problem is that irq-work handlers run with interrupts enabled, whichmeans that rcu_preempt_deferred_qs_handler() could be interrupted,and that interrupt handler might contain an RCU read-side criticalsection, which might invoke rcu_read_unlock_special(). In the strictKCSAN mode of operation used by RCU, this constitutes a data race onthe ->defer_qs_iw_pending field.This commit therefore disables interrupts across the portion of thercu_preempt_deferred_qs_handler() that updates the ->defer_qs_iw_pendingfield. This suffices because this handler is not a fast path.

POC

Reference

No PoCs from references.

Github

- https://github.com/w4zu/Debian_security