Re: [PATCH] KVM: x86/xen: Fix kvm_xen_has_interrupt() sleeping in kvm_vcpu_block()

[Paolo Bonzini <pbonzini@redhat.com>]

On 23/10/21 21:47, David Woodhouse wrote:
> From: David Woodhouse <dwmw@amazon.co.uk>
> 
> In kvm_vcpu_block, the current task is set to TASK_INTERRUPTIBLE before
> making a final check whether the vCPU should be woken from HLT by any
> incoming interrupt.
> 
> This is a problem for the get_user() in __kvm_xen_has_interrupt(), which
> really shouldn't be sleeping when the task state has already been set.
> I think it's actually harmless as it would just manifest itself as a
> spurious wakeup, but it's causing a debug warning:
> 
> [  230.963649] do not call blocking ops when !TASK_RUNNING; state=1 set at [<00000000b6bcdbc9>] prepare_to_swait_exclusive+0x30/0x80
> 
> Fix the warning by turning it into an *explicit* spurious wakeup. When
> invoked with !task_is_running(current) (and we might as well add
> in_atomic() there while we're at it), just return 1 to indicate that
> an IRQ is pending, which will cause a wakeup and then something will
> call it again in a context that *can* sleep so it can fault the page
> back in.
> 
> Cc: stable@vger.kernel.org
> Fixes: 40da8ccd724f ("KVM: x86/xen: Add event channel interrupt vector upcall")
> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
> 
> ---
>   arch/x86/kvm/xen.c | 27 ++++++++++++++++++++++-----
>   1 file changed, 22 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
> index 9ea9c3dabe37..8f62baebd028 100644
> --- a/arch/x86/kvm/xen.c
> +++ b/arch/x86/kvm/xen.c
> @@ -190,6 +190,7 @@ void kvm_xen_update_runstate_guest(struct kvm_vcpu *v, int state)
>   
>   int __kvm_xen_has_interrupt(struct kvm_vcpu *v)
>   {
> +	int err;
>   	u8 rc = 0;
>   
>   	/*
> @@ -216,13 +217,29 @@ int __kvm_xen_has_interrupt(struct kvm_vcpu *v)
>   	if (likely(slots->generation == ghc->generation &&
>   		   !kvm_is_error_hva(ghc->hva) && ghc->memslot)) {
>   		/* Fast path */
> -		__get_user(rc, (u8 __user *)ghc->hva + offset);
> -	} else {
> -		/* Slow path */
> -		kvm_read_guest_offset_cached(v->kvm, ghc, &rc, offset,
> -					     sizeof(rc));
> +		pagefault_disable();
> +		err = __get_user(rc, (u8 __user *)ghc->hva + offset);
> +		pagefault_enable();
> +		if (!err)
> +			return rc;
>   	}
>   
> +	/* Slow path */
> +
> +	/*
> +	 * This function gets called from kvm_vcpu_block() after setting the
> +	 * task to TASK_INTERRUPTIBLE, to see if it needs to wake immediately
> +	 * from a HLT. So we really mustn't sleep. If the page ended up absent
> +	 * at that point, just return 1 in order to trigger an immediate wake,
> +	 * and we'll end up getting called again from a context where we *can*
> +	 * fault in the page and wait for it.
> +	 */
> +	if (in_atomic() || !task_is_running(current))
> +		return 1;
> +
> +	kvm_read_guest_offset_cached(v->kvm, ghc, &rc, offset,
> +				     sizeof(rc));
> +
>   	return rc;
>   }
>   
> 
> 

Queued, thanks.

Paolo