[6.0,178/862] KVM: VMX: Drop bits 31:16 when shoving exception error code into VMCS
Commit Message
Greg KH
Oct. 19, 2022, 8:24 a.m. UTC
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 @@ -3832,7 +3832,16 @@ 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 @@ -1695,7 +1695,17 @@ 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; }