[PATCH] KVM: arm64: Disable preemption in kvm_arch_hardware_enable()

[Marc Zyngier]

On Mon, 10 Jul 2023 19:04:08 +0100,
Sean Christopherson <seanjc at google.com> wrote:
> 
> On Mon, Jul 03, 2023, Marc Zyngier wrote:
> > Since 0bf50497f03b ("KVM: Drop kvm_count_lock and instead protect
> > kvm_usage_count with kvm_lock"), hotplugging back a CPU whilst
> > a guest is running results in a number of ugly splats as most
> > of this code expects to run with preemption disabled, which isn't
> > the case anymore.
> > 
> > While the context is preemptable, it isn't migratable, which should
> > be enough. But we have plenty of preemptible() checks all over
> > the place, and our per-CPU accessors also disable preemption.
> > 
> > Since this affects released versions, let's do the easy fix first,
> > disabling preemption in kvm_arch_hardware_enable(). We can always
> > revisit this with a more invasive fix in the future.
> > 
> > Fixes: 0bf50497f03b ("KVM: Drop kvm_count_lock and instead protect kvm_usage_count with kvm_lock")
> > Reported-by: Kristina Martsenko <kristina.martsenko at arm.com>
> > Tested-by: Kristina Martsenko <kristina.martsenko at arm.com>
> > Signed-off-by: Marc Zyngier <maz at kernel.org>
> > Link: https://lore.kernel.org/r/aeab7562-2d39-e78e-93b1-4711f8cc3fa5@arm.com
> > Cc: stable at vger.kernek.org # v6.3, v6.4
> > ---
> >  arch/arm64/kvm/arm.c | 13 ++++++++++++-
> >  1 file changed, 12 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> > index aaeae1145359..a28c4ffe4932 100644
> > --- a/arch/arm64/kvm/arm.c
> > +++ b/arch/arm64/kvm/arm.c
> > @@ -1894,8 +1894,17 @@ static void _kvm_arch_hardware_enable(void *discard)
> >  
> >  int kvm_arch_hardware_enable(void)
> >  {
> > -	int was_enabled = __this_cpu_read(kvm_arm_hardware_enabled);
> > +	int was_enabled;
> >  
> > +	/*
> > +	 * Most calls to this function are made with migration
> > +	 * disabled, but not with preemption disabled. The former is
> > +	 * enough to ensure correctness, but most of the helpers
> > +	 * expect the later and will throw a tantrum otherwise.
> > +	 */
> > +	preempt_disable();
> > +
> > +	was_enabled = __this_cpu_read(kvm_arm_hardware_enabled);
> 
> IMO, this_cpu_has_cap() is at fault.  E.g. why not do this?
> 
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 7d7128c65161..b862477de2ce 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -3193,7 +3193,9 @@ static void __init setup_boot_cpu_capabilities(void)
>  
>  bool this_cpu_has_cap(unsigned int n)
>  {
> -       if (!WARN_ON(preemptible()) && n < ARM64_NCAPS) {
> +       __this_cpu_preempt_check("has_cap");
> +
> +       if (n < ARM64_NCAPS) {
>                 const struct arm64_cpu_capabilities *cap = cpu_hwcaps_ptrs[n];
>  
>                 if (cap)
> 

Because this check is not on at all times (it relies on
DEBUG_PREEMPT), and we really want it to be there.

	M.

-- 
Without deviation from the norm, progress is not possible.