[PATCH 5.19 163/717] KVM: VMX: Drop bits 31:16 when shoving exception error code into VMCS

Greg Kroah-Hartman
2022-10-22T07:50:29Z

From: Sean Christopherson <seanjc@google.com>

commit eba9799b5a6efe2993cf92529608e4aa8163d73b upstream.

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>
Reviewed-by: Jim Mattson <jmattson@google.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Link: https://lore.kernel.org/r/20220830231614.3580124-3-seanjc@google.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/x86/kvm/vmx/nested.c | 11 ++++++++++-
arch/x86/kvm/vmx/vmx.c | 12 +++++++++++-
2 files changed, 21 insertions(+), 2 deletions(-)

--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -3839,7 +3839,16 @@ static void nested_vmx_inject_exception_
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;
}

--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -1687,7 +1687,17 @@ static void vmx_queue_exception(struct k
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;
}