Re: KVM: x86: Prevent deadlock against tk_core.seq

Peter Zijlstra
2021-05-06T13:37:09Z

On Thu, May 06, 2021 at 03:21:37PM +0200, Thomas Gleixner wrote:
> syzbot reported a possible deadlock in pvclock_gtod_notify():
>
> CPU 0 CPU 1
> write_seqcount_begin(&tk_core.seq);
> pvclock_gtod_notify() spin_lock(&pool->lock);
> queue_work(..., &pvclock_gtod_work) ktime_get()
> spin_lock(&pool->lock); do {
> seq = read_seqcount_begin(tk_core.seq)
> ...
> } while (read_seqcount_retry(&tk_core.seq, seq);
>
> While this is unlikely to happen, it's possible.
>
> Delegate queue_work() to irq_work() which postpones it until the
> tk_core.seq write held region is left and interrupts are reenabled.
>
> Fixes: 16e8d74d2da9 ("KVM: x86: notifier for clocksource changes")
> Reported-by: syzbot+6beae4000559d41d80f8@syzkaller.appspotmail.com
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>