Re: [PATCH] KVM: mmu: allow page tables to be in read-only slots

[Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>]

On 09/02/2013 05:49 PM, Gleb Natapov wrote:
> On Mon, Sep 02, 2013 at 05:42:25PM +0800, Xiao Guangrong wrote:
>> On 09/01/2013 05:17 PM, Gleb Natapov wrote:
>>> On Fri, Aug 30, 2013 at 02:41:37PM +0200, Paolo Bonzini wrote:
>>>> Page tables in a read-only memory slot will currently cause a triple
>>>> fault because the page walker uses gfn_to_hva and it fails on such a slot.
>>>>
>>>> OVMF uses such a page table; however, real hardware seems to be fine with
>>>> that as long as the accessed/dirty bits are set.  Save whether the slot
>>>> is readonly, and later check it when updating the accessed and dirty bits.
>>>>
>>> The fix looks OK to me, but some comment below.
>>>
>>>> Cc: stable@vger.kernel.org
>>>> Cc: gleb@redhat.com
>>>> Cc: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
>>>> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>>>> ---
>>>> 	CCing to stable@ since the regression was introduced with
>>>> 	support for readonly memory slots.
>>>>
>>>>  arch/x86/kvm/paging_tmpl.h |  7 ++++++-
>>>>  include/linux/kvm_host.h   |  1 +
>>>>  virt/kvm/kvm_main.c        | 14 +++++++++-----
>>>>  3 files changed, 16 insertions(+), 6 deletions(-)
>>>>
>>>> diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
>>>> index 0433301..dadc5c0 100644
>>>> --- a/arch/x86/kvm/paging_tmpl.h
>>>> +++ b/arch/x86/kvm/paging_tmpl.h
>>>> @@ -99,6 +99,7 @@ struct guest_walker {
>>>>  	pt_element_t prefetch_ptes[PTE_PREFETCH_NUM];
>>>>  	gpa_t pte_gpa[PT_MAX_FULL_LEVELS];
>>>>  	pt_element_t __user *ptep_user[PT_MAX_FULL_LEVELS];
>>>> +	bool pte_writable[PT_MAX_FULL_LEVELS];
>>>>  	unsigned pt_access;
>>>>  	unsigned pte_access;
>>>>  	gfn_t gfn;
>>>> @@ -235,6 +236,9 @@ static int FNAME(update_accessed_dirty_bits)(struct kvm_vcpu *vcpu,
>>>>  		if (pte == orig_pte)
>>>>  			continue;
>>>>  
>>>> +		if (unlikely(!walker->pte_writable[level - 1]))
>>>> +			return -EACCES;
>>>> +
>>>>  		ret = FNAME(cmpxchg_gpte)(vcpu, mmu, ptep_user, index, orig_pte, pte);
>>>>  		if (ret)
>>>>  			return ret;
>>>> @@ -309,7 +313,8 @@ retry_walk:
>>>>  			goto error;
>>>>  		real_gfn = gpa_to_gfn(real_gfn);
>>>>  
>>>> -		host_addr = gfn_to_hva(vcpu->kvm, real_gfn);
>>>> +		host_addr = gfn_to_hva_read(vcpu->kvm, real_gfn,
>>>> +					    &walker->pte_writable[walker->level - 1]);
>>> The use of gfn_to_hva_read is misleading. The code can still write into
>>> gfn. Lets rename gfn_to_hva_read to gfn_to_hva_prot() and gfn_to_hva()
>>> to gfn_to_hva_write().
>>
>> Yes. I agreed.
>>
>>>
>>> This makes me think are there other places where gfn_to_hva() was
>>> used, but gfn_to_hva_prot() should have been?
>>>  - kvm_host_page_size() looks incorrect. We never use huge page to map
>>>    read only memory slots currently.
>>
>> It only checks whether gfn have been mapped, I think we can use
>> gfn_to_hva_read() instead, the real permission will be checked when we translate
>> the gfn to pfn.
>>
> Yes, all the cases I listed should be changed to use function that looks
> at both regular and RO slots.
> 
>>>  - kvm_handle_bad_page() also looks incorrect and may cause incorrect
>>>    address to be reported to userspace.
>>
>> I have no idea on this point. kvm_handle_bad_page() is called when it failed to
>> translate the target gfn to pfn, then the emulator can detect the error on target gfn
>> properly. no? Or i misunderstood your meaning?
>>
> I am talking about the following code:
> 
>         if (pfn == KVM_PFN_ERR_HWPOISON) {
>                 kvm_send_hwpoison_signal(gfn_to_hva(vcpu->kvm, gfn), current);
>                 return 0;
>         }
> 
> pfn will be KVM_PFN_ERR_HWPOISON gfn is backed by faulty memory, we need
> to report the liner address of the faulty memory to a userspace here,
> but if gfn is in a RO slot gfn_to_hva() will not return correct address
> here.

Got it, thanks for your explanation.

BTW, if you and Paolo are busy on other things, i am happy to fix these issues. :)