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

[Gleb Natapov <gleb@redhat.com>]

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.

> >  - 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.
> 

--
			Gleb.