[PATCH 17/17] KVM: x86/mmu: Sanity check that __kvm_faultin_pfn() doesn't create noslot pfns

[Paolo Bonzini]

From: Sean Christopherson <seanjc@xxxxxxxxxx>

WARN if __kvm_faultin_pfn() generates a "no slot" pfn, and gracefully
handle the unexpected behavior instead of continuing on with dangerous
state, e.g. tdp_mmu_map_handle_target_level() _only_ checks fault->slot,
and so could install a bogus PFN into the guest.

The existing code is functionally ok, because kvm_faultin_pfn() pre-checks
all of the cases that result in KVM_PFN_NOSLOT, but it is unnecessarily
unsafe as it relies on __gfn_to_pfn_memslot() getting the _exact_ same
memslot, i.e. not a re-retrieved pointer with KVM_MEMSLOT_INVALID set.
And checking only fault->slot would fall apart if KVM ever added a flag or
condition that forced emulation, similar to how KVM handles writes to
read-only memslots.

Cc: David Matlack <dmatlack@xxxxxxxxxx>
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Reviewed-by: Kai Huang <kai.huang@xxxxxxxxx>
Message-ID: <20240228024147.41573-17-seanjc@xxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
 arch/x86/kvm/mmu/mmu.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index d717d60c6f19..510eb1117012 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -4425,7 +4425,7 @@ static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
 	if (unlikely(is_error_pfn(fault->pfn)))
 		return kvm_handle_error_pfn(vcpu, fault);
 
-	if (WARN_ON_ONCE(!fault->slot))
+	if (WARN_ON_ONCE(!fault->slot || is_noslot_pfn(fault->pfn)))
 		return kvm_handle_noslot_fault(vcpu, fault, access);
 
 	/*
-- 
2.43.0