Re: [PATCH] KVM: arm64: Fix NULL pointer access issue

[Marc Zyngier <maz@kernel.org>]

On Mon, 01 Sep 2025 11:36:11 +0100,
James Clark <james.clark@linaro.org> wrote:
> 
> 
> 
> On 01/09/2025 11:01 am, Yingchao Deng wrote:
> > When linux is booted in EL1, macro "host_data_ptr()" is a wrapper that
> > resolves to "&per_cpu_ptr_nvhe_sym(kvm_host_data, cpu)",
> > is_hyp_mode_available() return false during kvm_arm_init, the per-CPU base
> > pointer __kvm_nvhe_kvm_arm_hyp_percpu_base[cpu] remains uninitialized.
> > Consequently, any access via per_cpu_ptr_nvhe_sym(kvm_host_data, cpu)
> > will result in a NULL pointer.
> > 
> > Add is_kvm_arm_initialised() condition check to ensure that kvm_arm_init
> > completes all necessary initialization steps, including init_hyp_mode.
> > 
> > Fixes: 054b88391bbe2 ("KVM: arm64: Support trace filtering for guests")
> > Signed-off-by: Yingchao Deng <yingchao.deng@oss.qualcomm.com>
> > ---
> > Add a check to prevent accessing uninitialized per-CPU data.
> > ---
> >   arch/arm64/kvm/debug.c | 7 ++++---
> >   1 file changed, 4 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c
> > index 381382c19fe4741980c79b08bbdab6a1bcd825ad..add58056297293b4eb337028773b1b018ecc9d35 100644
> > --- a/arch/arm64/kvm/debug.c
> > +++ b/arch/arm64/kvm/debug.c
> > @@ -233,7 +233,7 @@ void kvm_debug_handle_oslar(struct kvm_vcpu *vcpu, u64 val)
> >   void kvm_enable_trbe(void)
> >   {
> >   	if (has_vhe() || is_protected_kvm_enabled() ||
> > -	    WARN_ON_ONCE(preemptible()))
> > +	    WARN_ON_ONCE(preemptible()) || !is_kvm_arm_initialised())
> 
> Hi Yingchao,
> 
> There shouldn't be a warning for this, at least for the case where
> it's not initialized and never will be. If you're never going to run a
> guest these functions can all skip, the same way for !has_vhe() etc.

It's not a warning. It's a bona-fide crash:

void kvm_enable_trbe(void)
{
	if (has_vhe() || is_protected_kvm_enabled() ||
	    WARN_ON_ONCE(preemptible()))
		return;

	host_data_set_flag(TRBE_ENABLED); <--- Explodes here
}

So the write of the flag has to be skipped if KVM is available, even
if KVM is compiled in.

	M.

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