KVM: x86/mmu: Guard against collision with KVM-defined PFERR_IMPLICIT_ACCESS

Add an assertion in kvm_mmu_page_fault() to ensure the error code provided
by hardware doesn't conflict with KVM's software-defined IMPLICIT_ACCESS
flag.  In the unlikely scenario that future hardware starts using bit 48
for a hardware-defined flag, preserving the bit could result in KVM
incorrectly interpreting the unknown flag as KVM's IMPLICIT_ACCESS flag.

WARN so that any such conflict can be surfaced to KVM developers and
resolved, but otherwise ignore the bit as KVM can't possibly rely on a
flag it knows nothing about.

Fixes: 4f4aa80 ("KVM: X86: Handle implicit supervisor access with SMAP")
Acked-by: Kai Huang <kai.huang@intel.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Link: https://lore.kernel.org/r/20230721223711.2334426-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/mmu/mmu.c

@@ -5742,6 +5742,17 @@ int noinline kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa, u64 err
 	int r, emulation_type = EMULTYPE_PF;
 	bool direct = vcpu->arch.mmu->root_role.direct;
 
+	/*
+	 * IMPLICIT_ACCESS is a KVM-defined flag used to correctly perform SMAP
+	 * checks when emulating instructions that triggers implicit access.
+	 * WARN if hardware generates a fault with an error code that collides
+	 * with the KVM-defined value.  Clear the flag and continue on, i.e.
+	 * don't terminate the VM, as KVM can't possibly be relying on a flag
+	 * that KVM doesn't know about.
+	 */
+	if (WARN_ON_ONCE(error_code & PFERR_IMPLICIT_ACCESS))
+		error_code &= ~PFERR_IMPLICIT_ACCESS;
+
 	if (WARN_ON(!VALID_PAGE(vcpu->arch.mmu->root.hpa)))
 		return RET_PF_RETRY;