Add a helper to detect VMRUN failures so that KVM can guard against its
own long-standing bug, where KVM neglects to set exitcode[63:32] when
synthesizing a nested VMFAIL_INVALID VM-Exit. This will allow fixing
KVM's mess of treating exitcode as two separate 32-bit values without
breaking KVM-on-KVM when running on an older, unfixed KVM.
Cc: Jim Mattson <jmattson@google.com>
Cc: Yosry Ahmed <yosry.ahmed@linux.dev>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/svm/nested.c | 16 +++++++---------
arch/x86/kvm/svm/svm.c | 4 ++--
arch/x86/kvm/svm/svm.h | 5 +++++
3 files changed, 14 insertions(+), 11 deletions(-)
diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index ba0f11c68372..8070e20ed5a7 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -1134,7 +1134,7 @@ int nested_svm_vmexit(struct vcpu_svm *svm)
vmcb12->control.exit_info_1 = vmcb02->control.exit_info_1;
vmcb12->control.exit_info_2 = vmcb02->control.exit_info_2;
- if (vmcb12->control.exit_code != SVM_EXIT_ERR)
+ if (svm_is_vmrun_failure(vmcb12->control.exit_code))
nested_save_pending_event_to_vmcb12(svm, vmcb12);
if (guest_cpu_cap_has(vcpu, X86_FEATURE_NRIPS))
@@ -1425,6 +1425,9 @@ static int nested_svm_intercept(struct vcpu_svm *svm)
u32 exit_code = svm->vmcb->control.exit_code;
int vmexit = NESTED_EXIT_HOST;
+ if (svm_is_vmrun_failure(exit_code))
+ return NESTED_EXIT_DONE;
+
switch (exit_code) {
case SVM_EXIT_MSR:
vmexit = nested_svm_exit_handled_msr(svm);
@@ -1432,7 +1435,7 @@ static int nested_svm_intercept(struct vcpu_svm *svm)
case SVM_EXIT_IOIO:
vmexit = nested_svm_intercept_ioio(svm);
break;
- case SVM_EXIT_EXCP_BASE ... SVM_EXIT_EXCP_BASE + 0x1f: {
+ case SVM_EXIT_EXCP_BASE ... SVM_EXIT_EXCP_BASE + 0x1f:
/*
* Host-intercepted exceptions have been checked already in
* nested_svm_exit_special. There is nothing to do here,
@@ -1440,15 +1443,10 @@ static int nested_svm_intercept(struct vcpu_svm *svm)
*/
vmexit = NESTED_EXIT_DONE;
break;
- }
- case SVM_EXIT_ERR: {
- vmexit = NESTED_EXIT_DONE;
- break;
- }
- default: {
+ default:
if (vmcb12_is_intercept(&svm->nested.ctl, exit_code))
vmexit = NESTED_EXIT_DONE;
- }
+ break;
}
return vmexit;
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 7ea034ee6b6c..52b759408853 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -3530,7 +3530,7 @@ static int svm_handle_exit(struct kvm_vcpu *vcpu, fastpath_t exit_fastpath)
return 1;
}
- if (svm->vmcb->control.exit_code == SVM_EXIT_ERR) {
+ if (svm_is_vmrun_failure(svm->vmcb->control.exit_code)) {
kvm_run->exit_reason = KVM_EXIT_FAIL_ENTRY;
kvm_run->fail_entry.hardware_entry_failure_reason
= svm->vmcb->control.exit_code;
@@ -4302,7 +4302,7 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu, u64 run_flags)
/* Track VMRUNs that have made past consistency checking */
if (svm->nested.nested_run_pending &&
- svm->vmcb->control.exit_code != SVM_EXIT_ERR)
+ !svm_is_vmrun_failure(svm->vmcb->control.exit_code))
++vcpu->stat.nested_run;
svm->nested.nested_run_pending = 0;
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index 253a8dca412c..6b35925e3a33 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -426,6 +426,11 @@ static __always_inline struct vcpu_svm *to_svm(struct kvm_vcpu *vcpu)
return container_of(vcpu, struct vcpu_svm, vcpu);
}
+static inline bool svm_is_vmrun_failure(u64 exit_code)
+{
+ return (u32)exit_code == (u32)SVM_EXIT_ERR;
+}
+
/*
* Only the PDPTRs are loaded on demand into the shadow MMU. All other
* fields are synchronized on VM-Exit, because accessing the VMCB is cheap.
--
2.52.0.rc1.455.g30608eb744-googOn Thu, Nov 13, 2025 at 02:56:15PM -0800, Sean Christopherson wrote:
> Add a helper to detect VMRUN failures so that KVM can guard against its
> own long-standing bug, where KVM neglects to set exitcode[63:32] when
> synthesizing a nested VMFAIL_INVALID VM-Exit. This will allow fixing
> KVM's mess of treating exitcode as two separate 32-bit values without
> breaking KVM-on-KVM when running on an older, unfixed KVM.
>
> Cc: Jim Mattson <jmattson@google.com>
> Cc: Yosry Ahmed <yosry.ahmed@linux.dev>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
> arch/x86/kvm/svm/nested.c | 16 +++++++---------
> arch/x86/kvm/svm/svm.c | 4 ++--
> arch/x86/kvm/svm/svm.h | 5 +++++
> 3 files changed, 14 insertions(+), 11 deletions(-)
>
> diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> index ba0f11c68372..8070e20ed5a7 100644
> --- a/arch/x86/kvm/svm/nested.c
> +++ b/arch/x86/kvm/svm/nested.c
> @@ -1134,7 +1134,7 @@ int nested_svm_vmexit(struct vcpu_svm *svm)
> vmcb12->control.exit_info_1 = vmcb02->control.exit_info_1;
> vmcb12->control.exit_info_2 = vmcb02->control.exit_info_2;
>
> - if (vmcb12->control.exit_code != SVM_EXIT_ERR)
> + if (svm_is_vmrun_failure(vmcb12->control.exit_code))
This was flipped, wasn't it?
> nested_save_pending_event_to_vmcb12(svm, vmcb12);
>
> if (guest_cpu_cap_has(vcpu, X86_FEATURE_NRIPS))
> @@ -1425,6 +1425,9 @@ static int nested_svm_intercept(struct vcpu_svm *svm)
> u32 exit_code = svm->vmcb->control.exit_code;
> int vmexit = NESTED_EXIT_HOST;
>
> + if (svm_is_vmrun_failure(exit_code))
> + return NESTED_EXIT_DONE;
> +
> switch (exit_code) {
> case SVM_EXIT_MSR:
> vmexit = nested_svm_exit_handled_msr(svm);
> @@ -1432,7 +1435,7 @@ static int nested_svm_intercept(struct vcpu_svm *svm)
> case SVM_EXIT_IOIO:
> vmexit = nested_svm_intercept_ioio(svm);
> break;
> - case SVM_EXIT_EXCP_BASE ... SVM_EXIT_EXCP_BASE + 0x1f: {
> + case SVM_EXIT_EXCP_BASE ... SVM_EXIT_EXCP_BASE + 0x1f:
> /*
> * Host-intercepted exceptions have been checked already in
> * nested_svm_exit_special. There is nothing to do here,
> @@ -1440,15 +1443,10 @@ static int nested_svm_intercept(struct vcpu_svm *svm)
> */
> vmexit = NESTED_EXIT_DONE;
> break;
> - }
> - case SVM_EXIT_ERR: {
> - vmexit = NESTED_EXIT_DONE;
> - break;
> - }
> - default: {
> + default:
> if (vmcb12_is_intercept(&svm->nested.ctl, exit_code))
> vmexit = NESTED_EXIT_DONE;
> - }
> + break;
> }
>
> return vmexit;
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 7ea034ee6b6c..52b759408853 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -3530,7 +3530,7 @@ static int svm_handle_exit(struct kvm_vcpu *vcpu, fastpath_t exit_fastpath)
> return 1;
> }
>
> - if (svm->vmcb->control.exit_code == SVM_EXIT_ERR) {
> + if (svm_is_vmrun_failure(svm->vmcb->control.exit_code)) {
> kvm_run->exit_reason = KVM_EXIT_FAIL_ENTRY;
> kvm_run->fail_entry.hardware_entry_failure_reason
> = svm->vmcb->control.exit_code;
> @@ -4302,7 +4302,7 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu, u64 run_flags)
>
> /* Track VMRUNs that have made past consistency checking */
> if (svm->nested.nested_run_pending &&
> - svm->vmcb->control.exit_code != SVM_EXIT_ERR)
> + !svm_is_vmrun_failure(svm->vmcb->control.exit_code))
> ++vcpu->stat.nested_run;
>
> svm->nested.nested_run_pending = 0;
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index 253a8dca412c..6b35925e3a33 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -426,6 +426,11 @@ static __always_inline struct vcpu_svm *to_svm(struct kvm_vcpu *vcpu)
> return container_of(vcpu, struct vcpu_svm, vcpu);
> }
>
> +static inline bool svm_is_vmrun_failure(u64 exit_code)
> +{
> + return (u32)exit_code == (u32)SVM_EXIT_ERR;
> +}
> +
> /*
> * Only the PDPTRs are loaded on demand into the shadow MMU. All other
> * fields are synchronized on VM-Exit, because accessing the VMCB is cheap.
> --
> 2.52.0.rc1.455.g30608eb744-goog
>
On Thu, Nov 13, 2025, Yosry Ahmed wrote: > On Thu, Nov 13, 2025 at 02:56:15PM -0800, Sean Christopherson wrote: > > Add a helper to detect VMRUN failures so that KVM can guard against its > > own long-standing bug, where KVM neglects to set exitcode[63:32] when > > synthesizing a nested VMFAIL_INVALID VM-Exit. This will allow fixing > > KVM's mess of treating exitcode as two separate 32-bit values without > > breaking KVM-on-KVM when running on an older, unfixed KVM. > > > > Cc: Jim Mattson <jmattson@google.com> > > Cc: Yosry Ahmed <yosry.ahmed@linux.dev> > > Signed-off-by: Sean Christopherson <seanjc@google.com> > > --- > > arch/x86/kvm/svm/nested.c | 16 +++++++--------- > > arch/x86/kvm/svm/svm.c | 4 ++-- > > arch/x86/kvm/svm/svm.h | 5 +++++ > > 3 files changed, 14 insertions(+), 11 deletions(-) > > > > diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c > > index ba0f11c68372..8070e20ed5a7 100644 > > --- a/arch/x86/kvm/svm/nested.c > > +++ b/arch/x86/kvm/svm/nested.c > > @@ -1134,7 +1134,7 @@ int nested_svm_vmexit(struct vcpu_svm *svm) > > vmcb12->control.exit_info_1 = vmcb02->control.exit_info_1; > > vmcb12->control.exit_info_2 = vmcb02->control.exit_info_2; > > > > - if (vmcb12->control.exit_code != SVM_EXIT_ERR) > > + if (svm_is_vmrun_failure(vmcb12->control.exit_code)) > > This was flipped, wasn't it? Ugh, yes. Hrm, I'm surprised this wasn't caught by svm_nested_soft_inject_test.c. Oof. We should probably also extend svm_is_vmrun_failure() (in the future) to detect any failure, e.g. VMEXIT_INVALID_PMC might be relevant soon? > > nested_save_pending_event_to_vmcb12(svm, vmcb12);
© 2016 - 2026 Red Hat, Inc.