linux-kernel - Re: [PATCH] kvm: fix page struct leak in handle

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Date:   Wed, 25 Jan 2017 11:35:22 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        dvyukov@...gle.com
Subject: Re: [PATCH] kvm: fix page struct leak in handle_vmon



On 25/01/2017 11:11, David Hildenbrand wrote:
> Am 25.01.2017 um 10:57 schrieb David Hildenbrand:
>> Am 25.01.2017 um 10:52 schrieb Paolo Bonzini:
>>>
>>>
>>> ----- Original Message -----
>>>> From: "David Hildenbrand" <david@...hat.com>
>>>> To: "Paolo Bonzini" <pbonzini@...hat.com>, linux-kernel@...r.kernel.org, kvm@...r.kernel.org
>>>> Cc: dvyukov@...gle.com
>>>> Sent: Wednesday, January 25, 2017 10:31:13 AM
>>>> Subject: Re: [PATCH] kvm: fix page struct leak in handle_vmon
>>>>
>>>> Am 24.01.2017 um 11:56 schrieb Paolo Bonzini:
>>>>> handle_vmon gets a reference on VMXON region page,
>>>>> but does not release it. Release the reference.
>>>>>
>>>>> Found by syzkaller; based on a patch by Dmitry.
>>>>>
>>>>> Reported-by: Dmitry Vyukov <dvyukov@...gle.com>
>>>>> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
>>>>> ---
>>>>>  arch/x86/kvm/vmx.c | 9 +++++++--
>>>>>  1 file changed, 7 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>>>>> index 42cc3d6f4d20..0f7345035210 100644
>>>>> --- a/arch/x86/kvm/vmx.c
>>>>> +++ b/arch/x86/kvm/vmx.c
>>>>> @@ -7085,13 +7085,18 @@ static int nested_vmx_check_vmptr(struct kvm_vcpu
>>>>> *vcpu, int exit_reason,
>>>>>  		}
>>>>>  
>>>>>  		page = nested_get_page(vcpu, vmptr);
>>>>> -		if (page == NULL ||
>>>>> -		    *(u32 *)kmap(page) != VMCS12_REVISION) {
>>>>> +		if (page == NULL) {
>>>>>  			nested_vmx_failInvalid(vcpu);
>>>>> +			return kvm_skip_emulated_instruction(vcpu);
>>>>> +		}
>>>>> +		if (*(u32 *)kmap(page) != VMCS12_REVISION) {
>>>>
>>>> shouldn't we also check if kmap even returned a valid pointer before
>>>> dereferencing it?
>>>
>>> I don't think kmap can fail (page_address can)?
>>
>> Then I wonder why there are some checks:
>>
>> e.g. nested_vmx_merge_msr_bitmap()
>>
>> msr_bitmap_l1 = (unsigned long *)kmap(page);
>> if (!msr_bitmap_l1) {
>> 	// no unmap
>> 	...
>> 	return false;
>>
>> or vmx_complete_nested_posted_interrupt()
>>
>> vapic_page = kmap(vmx->nested.virtual_apic_page);
>> if (!vapic_page) {
>> 	// no unmap
>> 	...
>> 	return -ENOMEM;
>>
>>
>> But there is also no check in handle_vmptrld() for example.
>>
>>
>>>
>>> Paolo
>>
>>
> 
> Think you're right it can't fail.
> 
> So something like that could most likely be done
> 
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index a236dec..a9be221 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -4973,10 +4973,6 @@ static int
> vmx_complete_nested_posted_interrupt(struct kvm_vcpu *vcpu)
>                         return 0;
> 
>                 vapic_page = kmap(vmx->nested.virtual_apic_page);
> -               if (!vapic_page) {
> -                       WARN_ON(1);
> -                       return -ENOMEM;
> -               }
>                 __kvm_apic_update_irr(vmx->nested.pi_desc->pir, vapic_page);
>                 kunmap(vmx->nested.virtual_apic_page);
> 
> @@ -9730,12 +9726,6 @@ static inline bool
> nested_vmx_merge_msr_bitmap(struct kvm_vcpu *vcpu,
>                 return false;
>         }
>         msr_bitmap_l1 = (unsigned long *)kmap(page);
> -       if (!msr_bitmap_l1) {
> -               nested_release_page_clean(page);
> -               WARN_ON(1);
> -               return false;
> -       }
> -
>         memset(msr_bitmap_l0, 0xff, PAGE_SIZE);
> 
>         if (nested_cpu_has_virt_x2apic_mode(vmcs12)) {

Yes, definitely.  Want to send a patch?

Paolo