Re: [RFC PATCH v2 08/25] KVM: SEV: Stop using per-vCPU ASID for SEV VMs

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] A malicious SEV guest or an unprivileged host-userspace process can trigger a host WARN_ON_ONCE by executing VMRUN (entering nested virtualization) or via KVM_SET_NESTED_STATE.
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commit c3ac1dbd3331e2991b6bd808b207ce3395a73130
Author: Yosry Ahmed <yosry@kernel.org>

KVM: SEV: Stop using per-vCPU ASID for SEV VMs

This commit updates KVM to initialize the SEV VM ASID once in
sev_init_vmcb() instead of checking and setting it on every vCPU run. It
replaces the run-time assignment check in pre_sev_run() with a warning.

> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -3555,10 +3555,7 @@ int pre_sev_run(struct vcpu_svm *svm, int cpu)
>  	if (!cpumask_test_cpu(cpu, to_kvm_sev_info(kvm)->have_run_cpus))
>  		cpumask_set_cpu(cpu, to_kvm_sev_info(kvm)->have_run_cpus);
>  
> -	/* Assign the asid allocated with this SEV guest */
> -	svm->asid = asid;
> -
> -	if (unlikely(svm->asid != svm->vmcb->control.asid)) {
> +	if (WARN_ON_ONCE(asid != svm->vmcb->control.asid)) {

[Severity: High]
Can a malicious SEV guest or an unprivileged host-userspace process trigger
this warning by entering nested virtualization or via KVM_SET_NESTED_STATE?

KVM does not strictly prohibit userspace from exposing CPUID.SVM to an SEV
guest. If an SEV guest sets EFER.SVME and executes VMRUN, KVM intercepts it
and handles the nested transition via nested_svm_vmrun(), which switches the
active VMCB pointer to the nested vmcb02.

The nested vmcb02 does not inherit the SEV ASID assignment from
sev_init_vmcb(), and its control.asid is initialized separately. During the
subsequent svm_vcpu_run() -> pre_sev_run() call, the VM's valid SEV ASID will
no longer match the nested vmcb02's ASID, causing this warning to trigger.

Since this condition is guest-reachable and controllable by userspace, does
this create a host denial of service vector when panic_on_warn is set?

>  		svm->vmcb->control.asid = asid;
>  		vmcb_mark_dirty(svm->vmcb, VMCB_ASID);
>  	}

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Sashiko AI review · https://sashiko.dev/#/patchset/20260616004155.1435766-1-yosry@kernel.org?part=8