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

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Date:   Mon, 24 Aug 2020 15:45:26 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: VMX: fix crash cleanup when KVM wasn't used

On Mon, Aug 24, 2020 at 11:57 AM Jim Mattson <jmattson@...gle.com> wrote:
>
> On Fri, Aug 21, 2020 at 8:40 PM Sean Christopherson
> <sean.j.christopherson@...el.com> wrote:
> >
> > 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@...hat.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.
>
> Unless I'm missing something, there's no check for a non-zero
> kvm_usage_count on this path. There is such a check in
> hardware_disable_all_nolock(), but not in hardware_disable_nolock().

However, cpus_hardware_enabled shouldn't have any bits set, so
everything's fine. Nothing to see here, after all.