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

Gleb Natapov
2013-09-02T10:11:15Z

On Mon, Sep 02, 2013 at 06:05:10PM +0800, Xiao Guangrong wrote:
> 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. :)
I am busy with reviews mostly :). If you are not to busy with lockless
write protection then fine with me. Lest wait for Paolo's input on
proposed API though.

--
Gleb.