KVM_CREATE_IRQCHIP has been refactored since I wrote this patch, but
the issue is still there. The IOAPIC is initialized (underneath
kvm->lock) before the call to kvm_setup_default_irq_routing (which
sets up the routing tables). If you are setting up the PIT without
kvm->lock, there is a window where ioapic_in_kernel can return true,
but default routing tables have not been configured.
On Thu, Aug 30, 2018 at 9:35 AM Radim Krčmář <rkrcmar@xxxxxxxxxx> wrote:
>
> 2018-08-29 17:54-0700, Peter Shier:
> > From: Steve Rutherford <srutherford@xxxxxxxxxx>
> >
> > Fixes a NULL pointer dereference, caused by the PIT firing an interrupt
> > before the interrupt table has been initialized.
> >
> > SET_PIT2 can race with the creation of the IRQchip. In particular,
> > if SET_PIT2 is called with a low PIT timer period (after the creation of
> > the IOAPIC, but before the instantiation of the irq routes), the PIT can
> > fire an interrupt at an uninitialized table.
>
> From what you describe, it seems that the problems is the timer
> interrupt started in
>
> kvm_vm_ioctl_set_pit2() ->
> kvm_pit_load_count() ->
> pit_load_count() ->
> create_pit_timer() ->
> hrtimer_start()
>
> that immediately fires
>
> pit_timer_fn() ->
> pit_do_work() ->
> kvm_set_irq()
>
> and the irq routes there are not set up properly.
> I don't see what we race with, though:
>
> create_pit_timer() checks ioapic_in_kernel() before creating the timer,
> so the whole KVM_CREATE_IRQCHIP must have finished and routing is
> kvm_setup_default_irq_routing() if the timer fires.
>
> The only other way to change the routing is through KVM_SET_GSI_ROUTING,
> but that doesn't have kvm->lock protection, so it shouldn't matter.
>
> Isn't the bug somewhere else?
>
> Thanks.
>
> > Signed-off-by: Steve Rutherford <srutherford@xxxxxxxxxx>
> > Signed-off-by: Peter Shier <pshier@xxxxxxxxxx>
> > Signed-off-by: Jim Mattson <jmattson@xxxxxxxxxx>
> > ---
> > arch/x86/kvm/x86.c | 14 ++++++++------
> > 1 file changed, 8 insertions(+), 6 deletions(-)
> >
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 506bd2b4b8bb7..188bc3fc907f1 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -4518,10 +4518,11 @@ long kvm_arch_vm_ioctl(struct file *filp,
> > r = -EFAULT;
> > if (copy_from_user(&u.ps, argp, sizeof u.ps))
> > goto out;
> > + mutex_lock(&kvm->lock);
> > r = -ENXIO;
> > - if (!kvm->arch.vpit)
> > - goto out;
> > - r = kvm_vm_ioctl_set_pit(kvm, &u.ps);
> > + if (kvm->arch.vpit)
> > + r = kvm_vm_ioctl_set_pit(kvm, &u.ps);
> > + mutex_unlock(&kvm->lock);
> > break;
> > }
> > case KVM_GET_PIT2: {
> > @@ -4541,10 +4542,11 @@ long kvm_arch_vm_ioctl(struct file *filp,
> > r = -EFAULT;
> > if (copy_from_user(&u.ps2, argp, sizeof(u.ps2)))
> > goto out;
> > + mutex_lock(&kvm->lock);
> > r = -ENXIO;
> > - if (!kvm->arch.vpit)
> > - goto out;
> > - r = kvm_vm_ioctl_set_pit2(kvm, &u.ps2);
> > + if (kvm->arch.vpit)
> > + r = kvm_vm_ioctl_set_pit2(kvm, &u.ps2);
> > + mutex_unlock(&kvm->lock);
> > break;
> > }
> > case KVM_REINJECT_CONTROL: {
> > --
> > 2.19.0.rc0.228.g281dcd1b4d0-goog
