Re: [PATCH] KVM: nVMX: Clear nested.smm.guest_mode on a failed RSM-to-L2

[Sean Christopherson]

On Sat, Oct 17, 2026, Amirmohammad Eftekhar wrote:
> vmx_leave_smm() consumes nested.smm.vmxon into nested.vmxon up front, but
> clears nested.smm.guest_mode only on the success path.  On either failure
> arm (nested_vmx_check_restored_vmcs12() < 0, or
> nested_vmx_enter_non_root_mode() != NVMX_VMENTRY_SUCCESS) it returns early
> with nested.smm.guest_mode still true while nested.smm.vmxon is already
> false.
> 
> em_rsm() turns that non-zero return into a triple fault, but the vCPU is
> left with the internal pair (smm.guest_mode=true, smm.vmxon=false).  That
> pair is not representable in migration state: vmx_get_nested_state()
> serializes KVM_STATE_NESTED_SMM_GUEST_MODE without
> KVM_STATE_NESTED_SMM_VMXON, and vmx_set_nested_state() rejects exactly
> SMM_GUEST_MODE && !SMM_VMXON with -EINVAL.  KVM thus serializes a state it
> refuses to restore -- a save/restore (live-migration) round-trip break.
> 
> The leak is sticky: free_nested() and vCPU reset do not clear
> nested.smm.guest_mode (kvm_vcpu_reset() gates the nested unwind on
> is_guest_mode(), false here), so a later VMXON re-exposes the bad
> KVM_GET_NESTED_STATE with no SMI/RSM involved, for the life of the vCPU.
> 
> Clear nested.smm.guest_mode before the consistency check and re-entry, so
> it is cleared on every exit from the block.  nested.vmxon is left true,
> which is correct (L1 does have VMX on) and round-trips cleanly on its own.
> Not a memory-safety issue; fidelity / migration only.
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Amirmohammad Eftekhar <amirmohammad.eftekhar@xxxxxxxx>
> ---
>  arch/x86/kvm/vmx/vmx.c | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 1f802cdbe..56332a85c 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -8381,6 +8381,18 @@ int vmx_leave_smm(struct kvm_vcpu *vcpu, const union kvm_smram *smram)
>  	}
>  
>  	if (vmx->nested.smm.guest_mode) {
> +		/*
> +		 * Clear smm.guest_mode up front so the failure arms below, which
> +		 * triple-fault the vCPU, cannot leave the internal pair
> +		 * (smm.guest_mode=true, smm.vmxon=false): smm.vmxon was already
> +		 * consumed into nested.vmxon above, and KVM_GET_NESTED_STATE
> +		 * would then serialize SMM_GUEST_MODE without SMM_VMXON, which
> +		 * KVM_SET_NESTED_STATE rejects (-EINVAL), breaking the
> +		 * save/restore round-trip.  The flag has served its only purpose
> +		 * (bringing the vCPU into this re-entry) by now.
> +		 */

The gorty details of exactly how this shows up in migration are great for the
changelog, but the comment should focus on why this is "architecturally" correct.
Unless someone objects, I'll fixup to this when applying:

		/*
		 * Note, even if RSM ultimately fails (hits SHUTDOWN), the vCPU
		 * is no longer in SMM with the intent to return to L2.
		 */
		vmx->nested.smm.guest_mode = false;

> +		vmx->nested.smm.guest_mode = false;
> +
>  		/* Triple fault if the state is invalid.  */
>  		if (nested_vmx_check_restored_vmcs12(vcpu) < 0)
>  			return 1;
> @@ -8390,7 +8402,6 @@ int vmx_leave_smm(struct kvm_vcpu *vcpu, const union kvm_smram *smram)
>  			return 1;
>  
>  		vcpu->arch.nested_run_pending = KVM_NESTED_RUN_PENDING;
> -		vmx->nested.smm.guest_mode = false;
>  	}
>  	return 0;
>  }
> -- 
> 2.54.0 (Apple Git-157)
>