Re: [PATCH 08/16] KVM: x86/mmu: WARN and skip MMIO cache on private, reserved page faults

[Huang, Kai]

On 1/03/2024 12:06 pm, Sean Christopherson wrote:
> On Fri, Mar 01, 2024, Kai Huang wrote:
> > On 28/02/2024 3:41 pm, Sean Christopherson wrote:
> > > WARN and skip the emulated MMIO fastpath if a private, reserved page fault
> > > is encountered, as private+reserved should be an impossible combination
> > > (KVM should never create an MMIO SPTE for a private access).
> > >
> > > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> > > ---
> > >    arch/x86/kvm/mmu/mmu.c | 3 ++-
> > >    1 file changed, 2 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > > index bd342ebd0809..9206cfa58feb 100644
> > > --- a/arch/x86/kvm/mmu/mmu.c
> > > +++ b/arch/x86/kvm/mmu/mmu.c
> > > @@ -5866,7 +5866,8 @@ int noinline kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa, u64 err
> > >    		error_code |= PFERR_PRIVATE_ACCESS;
> > >    	r = RET_PF_INVALID;
> > > -	if (unlikely(error_code & PFERR_RSVD_MASK)) {
> > > +	if (unlikely((error_code & PFERR_RSVD_MASK) &&
> > > +		     !WARN_ON_ONCE(error_code & PFERR_PRIVATE_ACCESS))) {
> > >    		r = handle_mmio_page_fault(vcpu, cr2_or_gpa, direct);
> > >    		if (r == RET_PF_EMULATE)
> > >    			goto emulate;
> >
> > It seems this will make KVM continue to call kvm_mmu_do_page_fault() when
> > such private+reserve error code actually happens (e.g., due to bug), because
> > @r is still RET_PF_INVALID in such case.
>
> Yep.
>
> > Is it better to just return error, e.g., -EINVAL, and give up?
>
> As long as there is no obvious/immediate danger to the host, no obvious way for
> the "bad" behavior to cause data corruption for the guest, and continuing on has
> a plausible chance of working, then KVM should generally try to continue on and
> not terminate the VM.

Agreed.  But I think sometimes it is hard to tell whether there's any 
dangerous things waiting to happen, because that means we have to sanity 
check a lot of code, and when new patches arrive we need to keep that in 
mind too, which could be a nightmare in terms of maintenance.

> E.g. in this case, KVM will just skip various fast paths because of the RSVD flag,
> and treat the fault like a PRIVATE fault.  Hmm, but page_fault_handle_page_track()
> would skip write tracking, which could theoretically cause data corruption, so I
> guess arguably it would be safer to bail?
>
> Anyone else have an opinion?  This type of bug should never escape development,
> so I'm a-ok effectively killing the VM.  Unless someone has a good argument for
> continuing on, I'll go with Kai's suggestion and squash this:
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index cedacb1b89c5..d796a162b2da 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -5892,8 +5892,10 @@ int noinline kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa, u64 err
>                  error_code |= PFERR_PRIVATE_ACCESS;
>   
>          r = RET_PF_INVALID;
> -       if (unlikely((error_code & PFERR_RSVD_MASK) &&
> -                    !WARN_ON_ONCE(error_code & PFERR_PRIVATE_ACCESS))) {
> +       if (unlikely(error_code & PFERR_RSVD_MASK)) {
> +               if (WARN_ON_ONCE(error_code & PFERR_PRIVATE_ACCESS))
> +                       return -EFAULT;

-EFAULT is part of guest_memfd() memory fault ABI.  I didn't think over 
this thoroughly but do you want to return -EFAULT here?