KVM: X86: avoid uninitialized 'fault.async_page_fault' from fixed-up #PF

kvm_fixup_and_inject_pf_error() was introduced to fixup the error code(
e.g., to add RSVD flag) and inject the #PF to the guest, when guest
MAXPHYADDR is smaller than the host one.

When it comes to nested, L0 is expected to intercept and fix up the #PF
and then inject to L2 directly if
- L2.MAXPHYADDR < L0.MAXPHYADDR and
- L1 has no intention to intercept L2's #PF (e.g., L2 and L1 have the
  same MAXPHYADDR value && L1 is using EPT for L2),
instead of constructing a #PF VM Exit to L1. Currently, with PFEC_MASK
and PFEC_MATCH both set to 0 in vmcs02, the interception and injection
may happen on all L2 #PFs.

However, failing to initialize 'fault' in kvm_fixup_and_inject_pf_error()
may cause the fault.async_page_fault being NOT zeroed, and later the #PF
being treated as a nested async page fault, and then being injected to L1.
Instead of zeroing 'fault' at the beginning of this function, we mannually
set the value of 'fault.async_page_fault', because false is the value we
really expect.

Fixes: 89786147 ("KVM: x86: Add helper functions for illegal GPA checking and page fault injection")
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=216178Reported-by: Yang Lixiao <lixiao.yang@intel.com>
Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>
Reviewed-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220718074756.53788-1-yu.c.zhang@linux.intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -13022,6 +13022,7 @@ void kvm_fixup_and_inject_pf_error(struct kvm_vcpu *vcpu, gva_t gva, u16 error_c
 		fault.error_code = error_code;
 		fault.nested_page_fault = false;
 		fault.address = gva;
+		fault.async_page_fault = false;
 	}
 	vcpu->arch.walk_mmu->inject_page_fault(vcpu, &fault);
 }