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KVM: VMX: Drop bits 31:16 when shoving exception error code into VMCS

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commit eba9799b5a 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>
This commit is contained in:
Sean Christopherson 2022-08-30 23:15:49 +00:00 committed by Greg Kroah-Hartman
parent 7644786461
commit b6094c4829
2 changed files with 21 additions and 2 deletions
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11
arch/x86/kvm/vmx/nested.c
View file

@ -3427,7 +3427,16 @@ 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;
}

12
arch/x86/kvm/vmx/vmx.c
View file

@ -1676,7 +1676,17 @@ 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;
}