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

Xiao Guangrong
2013-09-02T09:42:25Z

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.

> - 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?

> - kvm_setup_async_pf() also incorrect. Makes all page fault on read
> only slot to be sync.
> - kvm_vm_fault() one looks OK since function assumes write only slots,
> but it is obsolete and should be deleted anyway.