Re: [PATCH 5/5] KVM: x86: mmu: Add guest physical address check in translate_gpa()

[Sean Christopherson]

On Thu, Feb 27, 2020 at 07:00:33PM +0100, Paolo Bonzini wrote:
> On 27/02/20 18:23, Mohammed Gamal wrote:
> > In case of running a guest with 4-level page tables on a 5-level page
> > table host, it might happen that a guest might have a physical address
> > with reserved bits set, but the host won't see that and trap it.
> > 
> > Hence, we need to check page faults' physical addresses against the guest's
> > maximum physical memory and if it's exceeded, we need to add
> > the PFERR_RSVD_MASK bits to the PF's error code.
> 
> You can just set it to PFERR_RSVD_MASK | PFERR_PRESENT_MASK (no need to
> use an "|") and return UNMAPPED_GBA.  But I would have thought that this
> is not needed and the
> 
>                 if (unlikely(FNAME(is_rsvd_bits_set)(mmu, pte, walker->level))) {
>                         errcode = PFERR_RSVD_MASK | PFERR_PRESENT_MASK;
>                         goto error;
>                 }
> 
> code would have catch the reserved bits.

That would be my assumption as well.  The only manual check should be in
the top level EPT and NPT handlers.

> > Also make sure the error code isn't overwritten by the page table walker.
> 
> Returning UNMAPPED_GVA would remove that as well.
> 
> I'm not sure this patch is enough however.  For a usermode access with
> "!pte.u pte.40" for example you should be getting:
> 
> - a #PF with PRESENT|USER error code on a machine with physical address
> width >=41; in this case you don't get an EPT violation or misconfig.
> 
> - a #PF with RSVD error code on a machine with physical address with <41.
> 
> You can enable verbose mode in access.c to see if this case is being generated,
> and if so debug it.
> 
> The solution for this would be to trap page faults and do a page table
> walk (with vcpu->arch.walk_mmu->gva_to_gpa) to find the correct error
> code.
> 
> Paolo
>