Re: [PATCH] KVM: VMX: fix crash cleanup when KVM wasn't used

Sean Christopherson
2020-08-22T03:40:56Z

On Thu, Aug 20, 2020 at 01:08:22PM -0700, Jim Mattson wrote:
> On Wed, Apr 1, 2020 at 1:13 AM Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
> > ---
> > arch/x86/kvm/vmx/vmx.c | 12 +++++++-----
> > 1 file changed, 7 insertions(+), 5 deletions(-)
> >
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index 3aba51d782e2..39a5dde12b79 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -2257,10 +2257,6 @@ static int hardware_enable(void)
> > !hv_get_vp_assist_page(cpu))
> > return -EFAULT;
> >
> > - INIT_LIST_HEAD(&per_cpu(loaded_vmcss_on_cpu, cpu));
> > - INIT_LIST_HEAD(&per_cpu(blocked_vcpu_on_cpu, cpu));
> > - spin_lock_init(&per_cpu(blocked_vcpu_on_cpu_lock, cpu));
> > -
> > r = kvm_cpu_vmxon(phys_addr);
> > if (r)
> > return r;
> > @@ -8006,7 +8002,7 @@ module_exit(vmx_exit);
> >
> > static int __init vmx_init(void)
> > {
> > - int r;
> > + int r, cpu;
> >
> > #if IS_ENABLED(CONFIG_HYPERV)
> > /*
> > @@ -8060,6 +8056,12 @@ static int __init vmx_init(void)
> > return r;
> > }
> >
> > + for_each_possible_cpu(cpu) {
> > + INIT_LIST_HEAD(&per_cpu(loaded_vmcss_on_cpu, cpu));
> > + INIT_LIST_HEAD(&per_cpu(blocked_vcpu_on_cpu, cpu));
> > + spin_lock_init(&per_cpu(blocked_vcpu_on_cpu_lock, cpu));
> > + }
>
> Just above this chunk, we have:
>
> r = vmx_setup_l1d_flush(vmentry_l1d_flush_param);
> if (r) {
> vmx_exit();
> ...
>
> If we take that early exit, because vmx_setup_l1d_flush() fails, we
> won't initialize loaded_vmcss_on_cpu. However, vmx_exit() calls
> kvm_exit(), which calls on_each_cpu(hardware_disable_nolock, NULL, 1).
> Hardware_disable_nolock() then calls kvm_arch_hardware_disable(),
> which calls kvm_x86_ops.hardware_disable() [the vmx.c
> hardware_disable()], which calls vmclear_local_loaded_vmcss().
>
> I believe that vmclear_local_loaded_vmcss() will then try to
> dereference a NULL pointer, since per_cpu(loaded_vmcss_on_cpu, cpu) is
> uninitialzed.

I agree the code is a mess (kvm_init() and kvm_exit() included), but I'm
pretty sure hardware_disable_nolock() is guaranteed to be a nop as it's
impossible for kvm_usage_count to be non-zero if vmx_init() hasn't
finished.

> > #ifdef CONFIG_KEXEC_CORE
> > rcu_assign_pointer(crash_vmclear_loaded_vmcss,
> > crash_vmclear_local_loaded_vmcss);
> > --
> > 2.25.1
> >