On 2/18/25 19:26, Sean Christopherson wrote:
> Terminate the VM if userspace attempts to run an SEV-SNP (or -ES) vCPU
> that has an invalid VMSA. With SNP's AP Create/Destroy hypercalls, it's
> possible for an SNP vCPU to end up with an invalid VMSA, e.g. through a
> deliberate Destroy or a failed Create event. KVM marks the vCPU HALTED so
> that *KVM* doesn't run the vCPU, but nothing prevents a misbehaving VMM
> from manually making the vCPU RUNNABLE via KVM_SET_MP_STATE.
>
> Fixes: e366f92ea99e ("KVM: SEV: Support SEV-SNP AP Creation NAE event")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
> arch/x86/kvm/svm/sev.c | 18 +++++++++++++++---
> arch/x86/kvm/svm/svm.c | 7 +++++--
> arch/x86/kvm/svm/svm.h | 2 +-
> 3 files changed, 21 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 6c6d45e13858..e14a37dbc6ea 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -3452,10 +3452,21 @@ void sev_es_unmap_ghcb(struct vcpu_svm *svm)
> svm->sev_es.ghcb = NULL;
> }
>
> -void pre_sev_run(struct vcpu_svm *svm, int cpu)
> +int pre_sev_run(struct vcpu_svm *svm, int cpu)
> {
> struct svm_cpu_data *sd = per_cpu_ptr(&svm_data, cpu);
> - unsigned int asid = sev_get_asid(svm->vcpu.kvm);
> + struct kvm *kvm = svm->vcpu.kvm;
> + unsigned int asid = sev_get_asid(kvm);
> +
> + /*
> + * Terminate the VM if userspace attempts to run the vCPU with an
> + * invalid VMSA, e.g. if userspace forces the vCPU to be RUNNABLE after
> + * an SNP AP Destroy event.
> + */
> + if (sev_es_guest(kvm) && !VALID_PAGE(svm->vmcb->control.vmsa_pa)) {
> + kvm_vm_dead(kvm);
> + return -EIO;
> + }
If a VMRUN is performed with the vmsa_pa value set to INVALID_PAGE, the
VMRUN will fail and KVM will dump the VMCB and exit back to userspace
with KVM_EXIT_INTERNAL_ERROR.
Is doing this preferrable to that? If so, should a vcpu_unimpl() message
be issued, too, to better identify the reason for marking the VM dead?
>
> /* Assign the asid allocated with this SEV guest */
> svm->asid = asid;
> @@ -3468,11 +3479,12 @@ void pre_sev_run(struct vcpu_svm *svm, int cpu)
> */
> if (sd->sev_vmcbs[asid] == svm->vmcb &&
> svm->vcpu.arch.last_vmentry_cpu == cpu)
> - return;
> + return 0;
>
> sd->sev_vmcbs[asid] = svm->vmcb;
> svm->vmcb->control.tlb_ctl = TLB_CONTROL_FLUSH_ASID;
> vmcb_mark_dirty(svm->vmcb, VMCB_ASID);
> + return 0;
> }
>
> #define GHCB_SCRATCH_AREA_LIMIT (16ULL * PAGE_SIZE)
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index b8aa0f36850f..46e0b65a9fec 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -3587,7 +3587,7 @@ static int svm_handle_exit(struct kvm_vcpu *vcpu, fastpath_t exit_fastpath)
> return svm_invoke_exit_handler(vcpu, exit_code);
> }
>
> -static void pre_svm_run(struct kvm_vcpu *vcpu)
> +static int pre_svm_run(struct kvm_vcpu *vcpu)
> {
> struct svm_cpu_data *sd = per_cpu_ptr(&svm_data, vcpu->cpu);
> struct vcpu_svm *svm = to_svm(vcpu);
> @@ -3609,6 +3609,8 @@ static void pre_svm_run(struct kvm_vcpu *vcpu)
> /* FIXME: handle wraparound of asid_generation */
> if (svm->current_vmcb->asid_generation != sd->asid_generation)
> new_asid(svm, sd);
> +
> + return 0;
> }
>
> static void svm_inject_nmi(struct kvm_vcpu *vcpu)
> @@ -4231,7 +4233,8 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu,
> if (force_immediate_exit)
> smp_send_reschedule(vcpu->cpu);
>
> - pre_svm_run(vcpu);
> + if (pre_svm_run(vcpu))
> + return EXIT_FASTPATH_EXIT_USERSPACE;
Since the return code from pre_svm_run() is never used, should it just
be a bool function, then?
Thanks,
Tom
>
> sync_lapic_to_cr8(vcpu);
>
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index 5b159f017055..e51852977b70 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -713,7 +713,7 @@ void avic_refresh_virtual_apic_mode(struct kvm_vcpu *vcpu);
>
> /* sev.c */
>
> -void pre_sev_run(struct vcpu_svm *svm, int cpu);
> +int pre_sev_run(struct vcpu_svm *svm, int cpu);
> void sev_init_vmcb(struct vcpu_svm *svm);
> void sev_vcpu_after_set_cpuid(struct vcpu_svm *svm);
> int sev_es_string_io(struct vcpu_svm *svm, int size, unsigned int port, int in);
