Re: [PATCH 01/16] KVM: x86/mmu: Exit to userspace with -EFAULT if private fault hits emulation

[Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx>]

On 3/7/2024 8:52 PM, Gupta, Pankaj wrote:
> > Exit to userspace with -EFAULT / KVM_EXIT_MEMORY_FAULT if a private 
> > fault
> > triggers emulation of any kind, as KVM doesn't currently support 
> > emulating
> > access to guest private memory.  Practically speaking, private faults 
> > and
> > emulation are already mutually exclusive, but there are edge cases upon
> > edge cases where KVM can return RET_PF_EMULATE, and adding one last 
> > check
>
> edge cases upon edge cases?
>
> Just curious about the details of the edge cases scenarios?

Same question out of curiosity.

> > to harden against weird, unexpected combinations is inexpensive.
> >
> > Suggested-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
> > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> > ---
> >   arch/x86/kvm/mmu/mmu.c          |  8 --------
> >   arch/x86/kvm/mmu/mmu_internal.h | 13 +++++++++++++
> >   2 files changed, 13 insertions(+), 8 deletions(-)
> >
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index e4cc7f764980..e2fd74e06ff8 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -4309,14 +4309,6 @@ static inline u8 kvm_max_level_for_order(int 
> > order)
> >       return PG_LEVEL_4K;
> >   }
> >   -static void kvm_mmu_prepare_memory_fault_exit(struct kvm_vcpu *vcpu,
> > -                          struct kvm_page_fault *fault)
> > -{
> > -    kvm_prepare_memory_fault_exit(vcpu, fault->gfn << PAGE_SHIFT,
> > -                      PAGE_SIZE, fault->write, fault->exec,
> > -                      fault->is_private);
> > -}
> > -
> >   static int kvm_faultin_pfn_private(struct kvm_vcpu *vcpu,
> >                      struct kvm_page_fault *fault)
> >   {
> > diff --git a/arch/x86/kvm/mmu/mmu_internal.h 
> > b/arch/x86/kvm/mmu/mmu_internal.h
> > index 0669a8a668ca..0eea6c5a824d 100644
> > --- a/arch/x86/kvm/mmu/mmu_internal.h
> > +++ b/arch/x86/kvm/mmu/mmu_internal.h
> > @@ -279,6 +279,14 @@ enum {
> >       RET_PF_SPURIOUS,
> >   };
> >   +static inline void kvm_mmu_prepare_memory_fault_exit(struct 
> > kvm_vcpu *vcpu,
> > +                             struct kvm_page_fault *fault)
> > +{
> > +    kvm_prepare_memory_fault_exit(vcpu, fault->gfn << PAGE_SHIFT,
> > +                      PAGE_SIZE, fault->write, fault->exec,
> > +                      fault->is_private);
> > +}
> > +
> >   static inline int kvm_mmu_do_page_fault(struct kvm_vcpu *vcpu, 
> > gpa_t cr2_or_gpa,
> >                       u32 err, bool prefetch, int *emulation_type)
> >   {
> > @@ -320,6 +328,11 @@ static inline int kvm_mmu_do_page_fault(struct 
> > kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
> >       else
> >           r = vcpu->arch.mmu->page_fault(vcpu, &fault);
> >   +    if (r == RET_PF_EMULATE && fault.is_private) {
> > +        kvm_mmu_prepare_memory_fault_exit(vcpu, &fault);
> > +        return -EFAULT;
> > +    }
> > +
> >       if (fault.write_fault_to_shadow_pgtable && emulation_type)
> >           *emulation_type |= EMULTYPE_WRITE_PF_TO_SP;