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

[Sean Christopherson <seanjc@xxxxxxxxxx>]

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.

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.

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
--