KVM: x86: Cancel pvclock_gtod_work on module removal

KVM: x86: Cancel pvclock_gtod_work on module removal

[Thomas Gleixner]

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();

Re: KVM: x86: Cancel pvclock_gtod_work on module removal

[Paolo Bonzini]

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