[PATCH] KVM: apic: avoid calculating pending eoi from an uninitialized val

[PATCH] KVM: apic: avoid calculating pending eoi from an uninitialized val

[linmiaohe]

From: Miaohe Lin <linmiaohe@huawei.com>

When get user eoi value failed, var val would be uninitialized and result
in calculating pending eoi from an uninitialized val. Initialize var val
to 0 to fix this case.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
---
 arch/x86/kvm/lapic.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 4f14ec7525f6..7e77e94f3176 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -626,7 +626,7 @@ static inline bool pv_eoi_enabled(struct kvm_vcpu *vcpu)
 
 static bool pv_eoi_get_pending(struct kvm_vcpu *vcpu)
 {
-	u8 val;
+	u8 val = 0;
 	if (pv_eoi_get_user(vcpu, &val) < 0)
 		printk(KERN_WARNING "Can't read EOI MSR value: 0x%llx\n",
 			   (unsigned long long)vcpu->arch.pv_eoi.msr_val);
-- 
2.19.1

Re: [PATCH] KVM: apic: avoid calculating pending eoi from an uninitialized val

[Vitaly Kuznetsov]

linmiaohe <linmiaohe@huawei.com> writes:

> From: Miaohe Lin <linmiaohe@huawei.com>
>
> When get user eoi value failed, var val would be uninitialized and result
> in calculating pending eoi from an uninitialized val. Initialize var val
> to 0 to fix this case.

Let me try to suggest an alternative wording,

"When pv_eoi_get_user() fails, 'val' may remain uninitialized and the
return value of pv_eoi_get_pending() becomes random. Fix the issue by
initializing the variable."

>
> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
> ---
>  arch/x86/kvm/lapic.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 4f14ec7525f6..7e77e94f3176 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -626,7 +626,7 @@ static inline bool pv_eoi_enabled(struct kvm_vcpu *vcpu)
>  
>  static bool pv_eoi_get_pending(struct kvm_vcpu *vcpu)
>  {
> -	u8 val;
> +	u8 val = 0;
>  	if (pv_eoi_get_user(vcpu, &val) < 0)
>  		printk(KERN_WARNING "Can't read EOI MSR value: 0x%llx\n",
>  			   (unsigned long long)vcpu->arch.pv_eoi.msr_val);

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>

But why compilers don't complain?

-- 
Vitaly

Re: [PATCH] KVM: apic: avoid calculating pending eoi from an uninitialized val

[Sean Christopherson]

On Thu, Feb 20, 2020 at 05:33:17PM +0100, Vitaly Kuznetsov wrote:
> linmiaohe <linmiaohe@huawei.com> writes:
> 
> > From: Miaohe Lin <linmiaohe@huawei.com>
> >
> > When get user eoi value failed, var val would be uninitialized and result
> > in calculating pending eoi from an uninitialized val. Initialize var val
> > to 0 to fix this case.
> 
> Let me try to suggest an alternative wording,
> 
> "When pv_eoi_get_user() fails, 'val' may remain uninitialized and the
> return value of pv_eoi_get_pending() becomes random. Fix the issue by
> initializing the variable."
> 
> >
> > Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
> > ---
> >  arch/x86/kvm/lapic.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> > index 4f14ec7525f6..7e77e94f3176 100644
> > --- a/arch/x86/kvm/lapic.c
> > +++ b/arch/x86/kvm/lapic.c
> > @@ -626,7 +626,7 @@ static inline bool pv_eoi_enabled(struct kvm_vcpu *vcpu)
> >  
> >  static bool pv_eoi_get_pending(struct kvm_vcpu *vcpu)
> >  {
> > -	u8 val;
> > +	u8 val = 0;

Rather than initialize @val, I'd prefer to explicitly handle the error,
similar to pv_eoi_clr_pending() and pv_eoi_set_pending(), e.g.

	u8 val;

	if (pv_eoi_get_user(vcpu, &val) < 0) {
		printk(KERN_WARNING "Can't read EOI MSR value: 0x%llx\n",
			   (unsigned long long)vcpu->arch.pv_eoi.msr_val);
		return false;
	}
	return val & 0x1;

> >  	if (pv_eoi_get_user(vcpu, &val) < 0)
> >  		printk(KERN_WARNING "Can't read EOI MSR value: 0x%llx\n",
> >  			   (unsigned long long)vcpu->arch.pv_eoi.msr_val);
> 
> Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> 
> But why compilers don't complain?

Clang might?

Re: [PATCH] KVM: apic: avoid calculating pending eoi from an uninitialized val

[linmiaohe]

Vitaly Kuznetsov <vkuznets@redhat.com> writes:
>linmiaohe <linmiaohe@huawei.com> writes:
>> When get user eoi value failed, var val would be uninitialized and 
>> result in calculating pending eoi from an uninitialized val. 
>> Initialize var val to 0 to fix this case.
>
>Let me try to suggest an alternative wording,
>
>"When pv_eoi_get_user() fails, 'val' may remain uninitialized and the return value of pv_eoi_get_pending() becomes random. Fix the issue by initializing the variable."

Sounds much better. You're really good at it. :) Thanks.

>>
>>
>Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
>
>But why compilers don't complain?

Maybe it's because @val only remain uninitialized when pv_eoi_get_user() failed?

Re: [PATCH] KVM: apic: avoid calculating pending eoi from an uninitialized val

[linmiaohe]

Sean Christopherson <sean.j.christopherson@intel.com> wrote:
>On Thu, Feb 20, 2020 at 05:33:17PM +0100, Vitaly Kuznetsov wrote:
>> linmiaohe <linmiaohe@huawei.com> writes:
>> 
>
>Rather than initialize @val, I'd prefer to explicitly handle the error, similar to pv_eoi_clr_pending() and pv_eoi_set_pending(), e.g.
>
>	u8 val;
>
>	if (pv_eoi_get_user(vcpu, &val) < 0) {
>		printk(KERN_WARNING "Can't read EOI MSR value: 0x%llx\n",
>			   (unsigned long long)vcpu->arch.pv_eoi.msr_val);
>		return false;
>	}
>	return val & 0x1;

Looks good. Handle the error explicitly can help remind us @val is unusable.
Will do. Thanks.

>> >  	if (pv_eoi_get_user(vcpu, &val) < 0)
>> >  		printk(KERN_WARNING "Can't read EOI MSR value: 0x%llx\n",
>> >  			   (unsigned long long)vcpu->arch.pv_eoi.msr_val);
>> 
>> Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
>> 
>> But why compilers don't complain?
>
>Clang might?
>