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

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Date:   Wed, 25 Jan 2017 10:57:28 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Paolo Bonzini <pbonzini@...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

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


-- 

David