Re: [PATCH] KVM: SVM: Call SEV Guest Decommission if ASID binding fails

[Marc Orr]

On Thu, Jun 10, 2021 at 10:46 AM Alper Gun <alpergun@xxxxxxxxxx> wrote:
>
> Send SEV_CMD_DECOMMISSION command to PSP firmware if ASID binding
> fails. If a failure happens after  a successful LAUNCH_START command,
> a decommission command should be executed. Otherwise, guest context
> will be unfreed inside the AMD SP. After the firmware will not have
> memory to allocate more SEV guest context, LAUNCH_START command will
> begin to fail with SEV_RET_RESOURCE_LIMIT error.
>
> The existing code calls decommission inside sev_unbind_asid, but it is
> not called if a failure happens before guest activation succeeds. If
> sev_bind_asid fails, decommission is never called. PSP firmware has a
> limit for the number of guests. If sev_asid_binding fails many times,
> PSP firmware will not have resources to create another guest context.
>
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: 59414c989220 ("KVM: SVM: Add support for KVM_SEV_LAUNCH_START command")
> Reported-by: Peter Gonda <pgonda@xxxxxxxxxx>
> Signed-off-by: Alper Gun <alpergun@xxxxxxxxxx>
> ---
>  arch/x86/kvm/svm/sev.c | 20 +++++++++++++++-----
>  1 file changed, 15 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index e0ce5da97fc2..8d36f0c73071 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -199,9 +199,19 @@ static void sev_asid_free(struct kvm_sev_info *sev)
>         sev->misc_cg = NULL;
>  }
>
> -static void sev_unbind_asid(struct kvm *kvm, unsigned int handle)
> +static void sev_decommission(unsigned int handle)
>  {
>         struct sev_data_decommission decommission;
> +
> +       if (!handle)
> +               return;
> +
> +       decommission.handle = handle;
> +       sev_guest_decommission(&decommission, NULL);
> +}
> +
> +static void sev_unbind_asid(struct kvm *kvm, unsigned int handle)
> +{
>         struct sev_data_deactivate deactivate;
>
>         if (!handle)
> @@ -214,9 +224,7 @@ static void sev_unbind_asid(struct kvm *kvm, unsigned int handle)
>         sev_guest_deactivate(&deactivate, NULL);
>         up_read(&sev_deactivate_lock);
>
> -       /* decommission handle */
> -       decommission.handle = handle;
> -       sev_guest_decommission(&decommission, NULL);
> +       sev_decommission(handle);
>  }
>
>  static int sev_guest_init(struct kvm *kvm, struct kvm_sev_cmd *argp)
> @@ -341,8 +349,10 @@ static int sev_launch_start(struct kvm *kvm, struct kvm_sev_cmd *argp)
>
>         /* Bind ASID to this guest */
>         ret = sev_bind_asid(kvm, start.handle, error);
> -       if (ret)
> +       if (ret) {
> +               sev_decommission(start.handle);
>                 goto e_free_session;
> +       }
>
>         /* return handle to userspace */
>         params.handle = start.handle;
> --
> 2.32.0.272.g935e593368-goog
>

Reviewed-by: Marc Orr <marcorr@xxxxxxxxxx>