linux-kernel - [PATCH v4 02/24] KVM: VMX: Drop bits 31:16 when shoving exception error code into VMCS

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Date:   Sat, 23 Jul 2022 00:51:15 +0000
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,
        Jim Mattson <jmattson@...gle.com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        Oliver Upton <oupton@...gle.com>,
        Peter Shier <pshier@...gle.com>
Subject: [PATCH v4 02/24] KVM: VMX: Drop bits 31:16 when shoving exception
 error code into VMCS

Deliberately truncate the exception error code when shoving it into the
VMCS (VM-Entry field for vmcs01 and vmcs02, VM-Exit field for vmcs12).
Intel CPUs are incapable of handling 32-bit error codes and will never
generate an error code with bits 31:16, but userspace can provide an
arbitrary error code via KVM_SET_VCPU_EVENTS.  Failure to drop the bits
on exception injection results in failed VM-Entry, as VMX disallows
setting bits 31:16.  Setting the bits on VM-Exit would at best confuse
L1, and at worse induce a nested VM-Entry failure, e.g. if L1 decided to
reinject the exception back into L2.

Cc: stable@...r.kernel.org
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
Reviewed-by: Jim Mattson <jmattson@...gle.com>
Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>
---
 arch/x86/kvm/vmx/nested.c | 11 ++++++++++-
 arch/x86/kvm/vmx/vmx.c    | 12 +++++++++++-
 2 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index a980d9cbee60..c6f9fe0b6b33 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -3827,7 +3827,16 @@ static void nested_vmx_inject_exception_vmexit(struct kvm_vcpu *vcpu,
 	u32 intr_info = nr | INTR_INFO_VALID_MASK;
 
 	if (vcpu->arch.exception.has_error_code) {
-		vmcs12->vm_exit_intr_error_code = vcpu->arch.exception.error_code;
+		/*
+		 * Intel CPUs do not generate error codes with bits 31:16 set,
+		 * and more importantly VMX disallows setting bits 31:16 in the
+		 * injected error code for VM-Entry.  Drop the bits to mimic
+		 * hardware and avoid inducing failure on nested VM-Entry if L1
+		 * chooses to inject the exception back to L2.  AMD CPUs _do_
+		 * generate "full" 32-bit error codes, so KVM allows userspace
+		 * to inject exception error codes with bits 31:16 set.
+		 */
+		vmcs12->vm_exit_intr_error_code = (u16)vcpu->arch.exception.error_code;
 		intr_info |= INTR_INFO_DELIVER_CODE_MASK;
 	}
 
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 4fd25e1d6ec9..1c72cde600d0 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -1621,7 +1621,17 @@ static void vmx_queue_exception(struct kvm_vcpu *vcpu)
 	kvm_deliver_exception_payload(vcpu);
 
 	if (has_error_code) {
-		vmcs_write32(VM_ENTRY_EXCEPTION_ERROR_CODE, error_code);
+		/*
+		 * Despite the error code being architecturally defined as 32
+		 * bits, and the VMCS field being 32 bits, Intel CPUs and thus
+		 * VMX don't actually supporting setting bits 31:16.  Hardware
+		 * will (should) never provide a bogus error code, but AMD CPUs
+		 * do generate error codes with bits 31:16 set, and so KVM's
+		 * ABI lets userspace shove in arbitrary 32-bit values.  Drop
+		 * the upper bits to avoid VM-Fail, losing information that
+		 * does't really exist is preferable to killing the VM.
+		 */
+		vmcs_write32(VM_ENTRY_EXCEPTION_ERROR_CODE, (u16)error_code);
 		intr_info |= INTR_INFO_DELIVER_CODE_MASK;
 	}
 
-- 
2.37.1.359.gd136c6c3e2-goog