Re: [PATCH v2] irq_work: Fix use-after-free in irq_work_single on PREEMPT_RT

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2026-03-30 15:32:29 [+0800], Jiayuan Chen wrote:
> On PREEMPT_RT, non-HARD irq_work runs in per-CPU kthreads via
> run_irq_workd(), so irq_work_sync() uses rcuwait to wait for
> BUSY==0.
> 
> After irq_work_single() clears BUSY via atomic_cmpxchg(), it still
> dereferences @work for irq_work_is_hard() and rcuwait_wake_up().
> An irq_work_sync() caller on another CPU that enters after BUSY is
> cleared can observe BUSY==0 immediately, return, and free the work
> before those accesses complete — causing a use-after-free.
> 
> Fix this by wrapping run_irq_workd() in guard(rcu)() so that the
> entire irq_work_single() execution is within an RCU read-side
> critical section. Then add synchronize_rcu() in irq_work_sync()
> after rcuwait_wait_event() to ensure the caller waits for the RCU
> grace period before returning, preventing premature frees.
> 
> Fixes: 810979682ccc ("irq_work: Allow irq_work_sync() to sleep if irq_work() no IRQ support.")
> Suggested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Suggested-by: Steven Rostedt <rostedt@goodmis.org>
> Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>

Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

Sebastian