Re: [PATCH v9 7/8] KVM: Handle page fault for private memory

[Chao Peng]

On Wed, Nov 16, 2022 at 10:13:07PM +0000, Sean Christopherson wrote:
> On Wed, Nov 16, 2022, Ackerley Tng wrote:
> > >@@ -4173,6 +4203,22 @@ static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
> > > 			return RET_PF_EMULATE;
> > > 	}
> > >
> > >+	if (kvm_slot_can_be_private(slot) &&
> > >+	    fault->is_private != kvm_mem_is_private(vcpu->kvm, fault->gfn)) {
> > >+		vcpu->run->exit_reason = KVM_EXIT_MEMORY_FAULT;
> > >+		if (fault->is_private)
> > >+			vcpu->run->memory.flags = KVM_MEMORY_EXIT_FLAG_PRIVATE;
> > >+		else
> > >+			vcpu->run->memory.flags = 0;
> > >+		vcpu->run->memory.padding = 0;
> > >+		vcpu->run->memory.gpa = fault->gfn << PAGE_SHIFT;
> > >+		vcpu->run->memory.size = PAGE_SIZE;
> > >+		return RET_PF_USER;
> > >+	}
> > >+
> > >+	if (fault->is_private)
> > >+		return kvm_faultin_pfn_private(fault);
> > >+
> > 
> > Since each memslot may also not be backed by restricted memory, we
> > should also check if the memslot has been set up for private memory
> > with
> > 
> > 	if (fault->is_private && kvm_slot_can_be_private(slot))
> > 		return kvm_faultin_pfn_private(fault);
> > 
> > Without this check, restrictedmem_get_page will get called with NULL
> > in slot->restricted_file, which causes a NULL pointer dereference.
> 
> Hmm, silently skipping the faultin would result in KVM faulting in the shared
> portion of the memslot, and I believe would end up mapping that pfn as private,
> i.e. would map a non-UPM PFN as a private mapping.  For TDX and SNP, that would
> be double ungood as it would let the host access memory that is mapped private,
> i.e. lead to #MC or #PF(RMP) in the host.

That's correct.

> 
> I believe the correct solution is to drop the "can be private" check from the
> above check, and instead handle that in kvm_faultin_pfn_private().  That would fix
> another bug, e.g. if the fault is shared, the slot can't be private, but for
> whatever reason userspace marked the gfn as private.  Even though KVM might be
> able service the fault, the correct thing to do in that case is to exit to userspace.

It makes sense to me.

Chao
> 
> E.g.
> 
> ---
>  arch/x86/kvm/mmu/mmu.c | 36 ++++++++++++++++++++++--------------
>  1 file changed, 22 insertions(+), 14 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 10017a9f26ee..e2ac8873938e 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -4158,11 +4158,29 @@ static inline u8 order_to_level(int order)
>  	return PG_LEVEL_4K;
>  }
>  
> -static int kvm_faultin_pfn_private(struct kvm_page_fault *fault)
> +static int kvm_do_memory_fault_exit(struct kvm_vcpu *vcpu,
> +					struct kvm_page_fault *fault)
> +{
> +	vcpu->run->exit_reason = KVM_EXIT_MEMORY_FAULT;
> +	if (fault->is_private)
> +		vcpu->run->memory.flags = KVM_MEMORY_EXIT_FLAG_PRIVATE;
> +	else
> +		vcpu->run->memory.flags = 0;
> +	vcpu->run->memory.padding = 0;
> +	vcpu->run->memory.gpa = fault->gfn << PAGE_SHIFT;
> +	vcpu->run->memory.size = PAGE_SIZE;
> +	return RET_PF_USER;
> +}
> +
> +static int kvm_faultin_pfn_private(struct kvm_vcpu *vcpu,
> +				   struct kvm_page_fault *fault)
>  {
>  	int order;
>  	struct kvm_memory_slot *slot = fault->slot;
>  
> +	if (kvm_slot_can_be_private(slot))
> +		return kvm_do_memory_fault_exit(vcpu, fault);
> +
>  	if (kvm_restricted_mem_get_pfn(slot, fault->gfn, &fault->pfn, &order))
>  		return RET_PF_RETRY;
>  
> @@ -4203,21 +4221,11 @@ static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
>  			return RET_PF_EMULATE;
>  	}
>  
> -	if (kvm_slot_can_be_private(slot) &&
> -	    fault->is_private != kvm_mem_is_private(vcpu->kvm, fault->gfn)) {
> -		vcpu->run->exit_reason = KVM_EXIT_MEMORY_FAULT;
> -		if (fault->is_private)
> -			vcpu->run->memory.flags = KVM_MEMORY_EXIT_FLAG_PRIVATE;
> -		else
> -			vcpu->run->memory.flags = 0;
> -		vcpu->run->memory.padding = 0;
> -		vcpu->run->memory.gpa = fault->gfn << PAGE_SHIFT;
> -		vcpu->run->memory.size = PAGE_SIZE;
> -		return RET_PF_USER;
> -	}
> +	if (fault->is_private != kvm_mem_is_private(vcpu->kvm, fault->gfn))
> +		return kvm_do_memory_fault_exit(vcpu, fault);
>  
>  	if (fault->is_private)
> -		return kvm_faultin_pfn_private(fault);
> +		return kvm_faultin_pfn_private(vcpu, fault);
>  
>  	async = false;
>  	fault->pfn = __gfn_to_pfn_memslot(slot, fault->gfn, false, &async,
> 
> base-commit: 969d761bb7b8654605937f31ae76123dcb7f15a3
> --