[PATCH 1/2] KVM: x86: Initialize nr_lvt_entries to a proper default value

[PATCH 1/2] KVM: x86: Initialize nr_lvt_entries to a proper default value

[Jue Wang]

Set the default value of nr_lvt_entries to KVM_APIC_MAX_NR_LVT_ENTRIES-1
to address the cases when KVM_X86_SETUP_MCE is not called.

Fixes: 4b903561ec49 ("KVM: x86: Add Corrected Machine Check Interrupt (CMCI) emulation to lapic.")
Signed-off-by: Jue Wang <juew@google.com>
---
 arch/x86/kvm/lapic.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 8537b66cc646..257366b8e3ae 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -2524,6 +2524,7 @@ int kvm_create_lapic(struct kvm_vcpu *vcpu, int timer_advance_ns)
 
 	vcpu->arch.apic = apic;
 
+	apic->nr_lvt_entries = KVM_APIC_MAX_NR_LVT_ENTRIES - 1;
 	apic->regs = (void *)get_zeroed_page(GFP_KERNEL_ACCOUNT);
 	if (!apic->regs) {
 		printk(KERN_ERR "malloc apic regs error for vcpu %x\n",
-- 
2.37.0.rc0.161.g10f37bed90-goog

[PATCH 2/2] KVM: x86: Fix access to vcpu->arch.apic when the irqchip is not in kernel

[Jue Wang]

Fix an access to vcpu->arch.apic when KVM_X86_SETUP_MCE is called
without KVM_CREATE_IRQCHIP called or KVM_CAP_SPLIT_IRQCHIP is
enabled.

Fixes: 4b903561ec49 ("KVM: x86: Add Corrected Machine Check Interrupt (CMCI) emulation to lapic.")
Signed-off-by: Jue Wang <juew@google.com>
---
 arch/x86/kvm/x86.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 4322a1365f74..d81020dd0fea 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4820,8 +4820,9 @@ static int kvm_vcpu_ioctl_x86_setup_mce(struct kvm_vcpu *vcpu,
 		if (mcg_cap & MCG_CMCI_P)
 			vcpu->arch.mci_ctl2_banks[bank] = 0;
 	}
-	vcpu->arch.apic->nr_lvt_entries =
-		KVM_APIC_MAX_NR_LVT_ENTRIES - !(mcg_cap & MCG_CMCI_P);
+	if (vcpu->arch.apic)
+		vcpu->arch.apic->nr_lvt_entries =
+			KVM_APIC_MAX_NR_LVT_ENTRIES - !(mcg_cap & MCG_CMCI_P);
 
 	static_call(kvm_x86_setup_mce)(vcpu);
 out:
-- 
2.37.0.rc0.161.g10f37bed90-goog

Re: [PATCH 2/2] KVM: x86: Fix access to vcpu->arch.apic when the irqchip is not in kernel

[Siddh Raman Pant]

On Fri, 01 Jul 2022 22:20:45 +0530  Jue Wang <juew@google.com> wrote
> Fix an access to vcpu->arch.apic when KVM_X86_SETUP_MCE is called
> without KVM_CREATE_IRQCHIP called or KVM_CAP_SPLIT_IRQCHIP is
> enabled.
> 
> Fixes: 4b903561ec49 ("KVM: x86: Add Corrected Machine Check Interrupt (CMCI) emulation to lapic.")
> Signed-off-by: Jue Wang <juew@google.com>
> ---
>  arch/x86/kvm/x86.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 4322a1365f74..d81020dd0fea 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4820,8 +4820,9 @@ static int kvm_vcpu_ioctl_x86_setup_mce(struct kvm_vcpu *vcpu,
>          if (mcg_cap & MCG_CMCI_P)
>              vcpu->arch.mci_ctl2_banks[bank] = 0;
>      }
> -    vcpu->arch.apic->nr_lvt_entries =
> -        KVM_APIC_MAX_NR_LVT_ENTRIES - !(mcg_cap & MCG_CMCI_P);
> +    if (vcpu->arch.apic)
> +        vcpu->arch.apic->nr_lvt_entries =
> +            KVM_APIC_MAX_NR_LVT_ENTRIES - !(mcg_cap & MCG_CMCI_P);
>  
>      static_call(kvm_x86_setup_mce)(vcpu);
>  out:
> -- 
> 2.37.0.rc0.161.g10f37bed90-goog
> 
>

Hello Jue,

There is a syzkaller bug regarding null ptr dereference which is caused by
vcpu->arch.apic being NULL, first reported on 27th June. You might want to
add it's reported-by line so that it can be marked as fixed.

Link: https://syzkaller.appspot.com/bug?id=10b9b238e087a6c9bef2cc48bee2375f58fabbfc

I was looking at this bug too and fixed it (i.e. reproducer won't crash)
using lapic_in_kernel(vcpu) as a condition instead of null ptr check on
vcpu->arch.apic, as it makes more sense to the code reader (the lapic is
not there since during kvm_arch_vcpu_create(), it isn't created due to
irqchip_in_kernel() check being false).

May I suggest that lapic_in_kernel(vcpu) be used instead of the null ptr
check?

Thanks,
Siddh

Re: [PATCH 2/2] KVM: x86: Fix access to vcpu->arch.apic when the irqchip is not in kernel

[Jue Wang]

Hi Siddh,

Thanks for the note.

I've sent out an updated v2 patch:
https://lore.kernel.org/kvm/20220706145957.32156-2-juew@google.com/T/#u

Thanks,
-Jue


On Sun, Jul 3, 2022 at 7:44 AM Siddh Raman Pant <code@siddh.me> wrote:
>
> On Fri, 01 Jul 2022 22:20:45 +0530  Jue Wang <juew@google.com> wrote
> > Fix an access to vcpu->arch.apic when KVM_X86_SETUP_MCE is called
> > without KVM_CREATE_IRQCHIP called or KVM_CAP_SPLIT_IRQCHIP is
> > enabled.
> >
> > Fixes: 4b903561ec49 ("KVM: x86: Add Corrected Machine Check Interrupt (CMCI) emulation to lapic.")
> > Signed-off-by: Jue Wang <juew@google.com>
> > ---
> >  arch/x86/kvm/x86.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 4322a1365f74..d81020dd0fea 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -4820,8 +4820,9 @@ static int kvm_vcpu_ioctl_x86_setup_mce(struct kvm_vcpu *vcpu,
> >          if (mcg_cap & MCG_CMCI_P)
> >              vcpu->arch.mci_ctl2_banks[bank] = 0;
> >      }
> > -    vcpu->arch.apic->nr_lvt_entries =
> > -        KVM_APIC_MAX_NR_LVT_ENTRIES - !(mcg_cap & MCG_CMCI_P);
> > +    if (vcpu->arch.apic)
> > +        vcpu->arch.apic->nr_lvt_entries =
> > +            KVM_APIC_MAX_NR_LVT_ENTRIES - !(mcg_cap & MCG_CMCI_P);
> >
> >      static_call(kvm_x86_setup_mce)(vcpu);
> >  out:
> > --
> > 2.37.0.rc0.161.g10f37bed90-goog
> >
> >
>
> Hello Jue,
>
> There is a syzkaller bug regarding null ptr dereference which is caused by
> vcpu->arch.apic being NULL, first reported on 27th June. You might want to
> add it's reported-by line so that it can be marked as fixed.
>
> Link: https://syzkaller.appspot.com/bug?id=10b9b238e087a6c9bef2cc48bee2375f58fabbfc
>
> I was looking at this bug too and fixed it (i.e. reproducer won't crash)
> using lapic_in_kernel(vcpu) as a condition instead of null ptr check on
> vcpu->arch.apic, as it makes more sense to the code reader (the lapic is
> not there since during kvm_arch_vcpu_create(), it isn't created due to
> irqchip_in_kernel() check being false).
>
> May I suggest that lapic_in_kernel(vcpu) be used instead of the null ptr
> check?
>
> Thanks,
> Siddh