Re: [PATCH v10 049/108] KVM: x86/tdp_mmu: Support TDX private mapping for TDP MMU

Isaku Yamahata
2022-11-17T19:59:57Z

On Tue, Nov 08, 2022 at 09:41:44PM +0800,
Binbin Wu <binbin.wu@linux.intel.com> wrote:

> > diff --git a/arch/x86/include/asm/kvm-x86-ops.h b/arch/x86/include/asm/kvm-x86-ops.h
> > index f28c9fd72ac4..1b01dc2098b0 100644
> > --- a/arch/x86/include/asm/kvm-x86-ops.h
> > +++ b/arch/x86/include/asm/kvm-x86-ops.h
> > @@ -94,6 +94,11 @@ KVM_X86_OP_OPTIONAL_RET0(set_tss_addr)
> > KVM_X86_OP_OPTIONAL_RET0(set_identity_map_addr)
> > KVM_X86_OP_OPTIONAL_RET0(get_mt_mask)
> > KVM_X86_OP(load_mmu_pgd)
> > +KVM_X86_OP_OPTIONAL(link_private_spt)
> > +KVM_X86_OP_OPTIONAL(free_private_spt)
> > +KVM_X86_OP_OPTIONAL(set_private_spte)
> > +KVM_X86_OP_OPTIONAL(remove_private_spte)
> > +KVM_X86_OP_OPTIONAL(zap_private_spte)
> > KVM_X86_OP(has_wbinvd_exit)
> > KVM_X86_OP(get_l2_tsc_offset)
> > KVM_X86_OP(get_l2_tsc_multiplier)
> >
> > @@ -509,9 +524,81 @@ static void handle_removed_pt(struct kvm *kvm, tdp_ptep_t pt, bool shared)
> > WARN_ON_ONCE(ret);
> > }
> > + if (is_private_sp(sp) &&
> > + WARN_ON(static_call(kvm_x86_free_private_spt)(kvm, sp->gfn, sp->role.level,
> > + kvm_mmu_private_spt(sp)))) {
> > + /*
> > + * Failed to unlink Secure EPT page and there is nothing to do
> > + * further. Intentionally leak the page to prevent the kernel
> > + * from accessing the encrypted page.
> > + */
> > + kvm_mmu_init_private_spt(sp, NULL);
>
> Do you think is it better to add some statistics for the intentinal leakage?

No because this error case happens only when race condition bug(TDX_OPERAND_BUSY)
or TDX module was shutdown due to bug or attack. They are (should be) rare.

The race condition bug should be fixed. It's covered by WARN_ON(). In the case
of TDX module shutdown, the current TDX KVM implementation checks unexpected
error and mark it bugged, KVM_BUG_ON(), in vmx/tdx.c. So adding statistic
counter doesn't help. It's another story to harden the code against TDX module
shutdown.
--
Isaku Yamahata <isaku.yamahata@gmail.com>