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Date:   Thu, 6 May 2021 15:35:07 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     kvm@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <seanjc@google.com>, x86@kernel.org,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: KVM: x86: Prevent deadlock against tk_core.seq
Message-ID: <YJPwi0FSObIjOSd7@hirez.programming.kicks-ass.net>
References: <87h7jgm1zy.ffs@nanos.tec.linutronix.de>
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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>