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Subject: Re: KVM: x86: Cancel pvclock_gtod_work on module removal
To:     Thomas Gleixner <tglx@linutronix.de>, kvm@vger.kernel.org
Cc:     Sean Christopherson <seanjc@google.com>, x86@kernel.org,
        LKML <linux-kernel@vger.kernel.org>
References: <87czu4onry.ffs@nanos.tec.linutronix.de>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <2a18634f-b100-334e-f7b5-01c84302e27e@redhat.com>
Date:   Thu, 6 May 2021 13:35:01 +0200
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On 05/05/21 23:48, Thomas Gleixner wrote:
> Nothing prevents the following:
> 
>    pvclock_gtod_notify()
>      queue_work(system_long_wq, &pvclock_gtod_work);
>    ...
>    remove_module(kvm);
>    ...
>    work_queue_run()
>      pvclock_gtod_work()	<- UAF
> 
> Ditto for any other operation on that workqueue list head which touches
> pvclock_gtod_work after module removal.
> 
> Cancel the work in kvm_arch_exit() to prevent that.
> 
> Fixes: 16e8d74d2da9 ("KVM: x86: notifier for clocksource changes")
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> ---
> Found by inspection because of:
>    https://lkml.kernel.org/r/0000000000001d43ac05c0f5c6a0@google.com
> See also:
>    https://lkml.kernel.org/r/20210505105940.190490250@infradead.org
> 
> TL;DR: Scheduling work with tk_core.seq write held is a bad idea.
> ---
>   arch/x86/kvm/x86.c |    1 +
>   1 file changed, 1 insertion(+)
> 
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -8168,6 +8168,7 @@ void kvm_arch_exit(void)
>   	cpuhp_remove_state_nocalls(CPUHP_AP_X86_KVM_CLK_ONLINE);
>   #ifdef CONFIG_X86_64
>   	pvclock_gtod_unregister_notifier(&pvclock_gtod_notifier);
> +	cancel_work_sync(&pvclock_gtod_work);
>   #endif
>   	kvm_x86_ops.hardware_enable = NULL;
>   	kvm_mmu_module_exit();
> 

Queued, thanks (with added Cc to stable).

Paolo