linux-kernel - [PATCH 2/2] KVM: nVMX: Do not report error code when synthesizing VM-Exit from Real Mode

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Message-ID: <20230322143300.2209476-3-seanjc@google.com>
Date:   Wed, 22 Mar 2023 07:33:00 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Maxim Levitsky <mlevitsk@...hat.com>
Subject: [PATCH 2/2] KVM: nVMX: Do not report error code when synthesizing
 VM-Exit from Real Mode

Don't report an error code to L1 when synthesizing a nested VM-Exit and
L2 is in Real Mode.  Per Intel's SDM, regarding the error code valid bit:

  This bit is always 0 if the VM exit occurred while the logical processor
  was in real-address mode (CR0.PE=0).

The bug was introduced by a recent fix for AMD's Paged Real Mode, which
moved the error code suppression from the common "queue exception" path
to the "inject exception" path, but missed VMX's "synthesize VM-Exit"
path.

Fixes: b97f07458373 ("KVM: x86: determine if an exception has an error code only when injecting it.")
Cc: stable@...r.kernel.org
Cc: Maxim Levitsky <mlevitsk@...hat.com>
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/vmx/nested.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 7c4f5ca405c7..14be92b3f34c 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -3868,7 +3868,12 @@ static void nested_vmx_inject_exception_vmexit(struct kvm_vcpu *vcpu)
 		exit_qual = 0;
 	}
 
-	if (ex->has_error_code) {
+	/*
+	 * Unlike AMD's Paged Real Mode, which reports an error code on #PF
+	 * VM-Exits even if the CPU is in Real Mode, Intel VMX never sets the
+	 * "has error code" flags on VM-Exit if the CPU is in Real Mode.
+	 */
+	if (ex->has_error_code && is_protmode(vcpu)) {
 		/*
 		 * Intel CPUs do not generate error codes with bits 31:16 set,
 		 * and more importantly VMX disallows setting bits 31:16 in the
-- 
2.40.0.rc2.332.ga46443480c-goog