RE: [PATCH] x86/hyperv: check cpu mask after interrupt has been disabled

[Michael Kelley]

From: Wei Liu <wei.liu@xxxxxxxxxx> Sent: Tuesday, January 5, 2021 9:51 AM
> 
> We've observed crashes due to an empty cpu mask in
> hyperv_flush_tlb_others.  Obviously the cpu mask in question is changed
> between the cpumask_empty call at the beginning of the function and when
> it is actually used later.
> 
> One theory is that an interrupt comes in between and a code path ends up
> changing the mask. Move the check after interrupt has been disabled to
> see if it fixes the issue.
> 
> Signed-off-by: Wei Liu <wei.liu@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxx
> ---
>  arch/x86/hyperv/mmu.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/hyperv/mmu.c b/arch/x86/hyperv/mmu.c
> index 5208ba49c89a..2c87350c1fb0 100644
> --- a/arch/x86/hyperv/mmu.c
> +++ b/arch/x86/hyperv/mmu.c
> @@ -66,11 +66,17 @@ static void hyperv_flush_tlb_others(const struct cpumask *cpus,
>  	if (!hv_hypercall_pg)
>  		goto do_native;
> 
> -	if (cpumask_empty(cpus))
> -		return;
> -
>  	local_irq_save(flags);
> 
> +	/*
> +	 * Only check the mask _after_ interrupt has been disabled to avoid the
> +	 * mask changing under our feet.
> +	 */
> +	if (cpumask_empty(cpus)) {
> +		local_irq_restore(flags);
> +		return;
> +	}
> +
>  	flush_pcpu = (struct hv_tlb_flush **)
>  		     this_cpu_ptr(hyperv_pcpu_input_arg);
> 
> --
> 2.20.1

Reviewed-by:  Michael Kelley <mikelley@xxxxxxxxxxxxx>