Re: [PATCH v19 038/130] KVM: TDX: create/destroy VM structure

Xiaoyao Li
2024-04-15T08:17:42Z

On 2/26/2024 4:25 PM, isaku.yamahata@intel.com wrote:

...

> +
> + kvm_tdx->tdcs_pa = tdcs_pa;
> + for (i = 0; i < tdx_info->nr_tdcs_pages; i++) {
> + err = tdh_mng_addcx(kvm_tdx->tdr_pa, tdcs_pa[i]);
> + if (err == TDX_RND_NO_ENTROPY) {
> + /* Here it's hard to allow userspace to retry. */
> + ret = -EBUSY;

So userspace is expected to stop creating TD and quit on this?

If so, it exposes an DOS attack surface that malicious users in another
can drain the entropy with busy-loop on RDSEED.

Can you clarify why it's hard to allow userspace to retry? To me, it's
OK to retry that "teardown" cleans everything up, and userspace and
issue the KVM_TDX_INIT_VM again.

> + goto teardown;
> + }
> + if (WARN_ON_ONCE(err)) {
> + pr_tdx_error(TDH_MNG_ADDCX, err, NULL);
> + ret = -EIO;
> + goto teardown;
> + }
> + }
> +
> + /*
> + * Note, TDH_MNG_INIT cannot be invoked here. TDH_MNG_INIT requires a dedicated
> + * ioctl() to define the configure CPUID values for the TD.
> + */
> + return 0;
> +
> + /*
> + * The sequence for freeing resources from a partially initialized TD
> + * varies based on where in the initialization flow failure occurred.
> + * Simply use the full teardown and destroy, which naturally play nice
> + * with partial initialization.
> + */
> +teardown:
> + for (; i < tdx_info->nr_tdcs_pages; i++) {
> + if (tdcs_pa[i]) {
> + free_page((unsigned long)__va(tdcs_pa[i]));
> + tdcs_pa[i] = 0;
> + }
> + }
> + if (!kvm_tdx->tdcs_pa)
> + kfree(tdcs_pa);
> + tdx_mmu_release_hkid(kvm);
> + tdx_vm_free(kvm);
> + return ret;
> +
> +free_packages:
> + cpus_read_unlock();
> + free_cpumask_var(packages);
> +free_tdcs:
> + for (i = 0; i < tdx_info->nr_tdcs_pages; i++) {
> + if (tdcs_pa[i])
> + free_page((unsigned long)__va(tdcs_pa[i]));
> + }
> + kfree(tdcs_pa);
> + kvm_tdx->tdcs_pa = NULL;
> +
> +free_tdr:
> + if (tdr_pa)
> + free_page((unsigned long)__va(tdr_pa));
> + kvm_tdx->tdr_pa = 0;
> +free_hkid:
> + if (is_hkid_assigned(kvm_tdx))
> + tdx_hkid_free(kvm_tdx);
> + return ret;
> +}
> +