[PATCH 03/21] KVM: VMX: Drop bits 31:16 when shoving exception error code into VMCS

Sean Christopherson
2022-03-11T03:28:29Z

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@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/vmx/nested.c | 9 ++++++++-
arch/x86/kvm/vmx/vmx.c | 11 ++++++++++-
2 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index f09c6eff7af9..7bdda9ef2828 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -3808,7 +3808,14 @@ 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 will never generate an error code 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.
+ */
+ 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 e8963f5af618..a8ebe91fe9a5 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -1613,7 +1613,16 @@ 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 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;
}

--
2.35.1.723.g4982287a31-goog