Re: [RFC PATCH v2 12/18] KVM: TDX: Bug the VM if extended the initial measurement fails

["Edgecombe, Rick P" <rick.p.edgecombe@intel.com>]

On Fri, 2025-08-29 at 16:18 +0800, Yan Zhao wrote:
> > +	/*
> > +	 * Note, MR.EXTEND can fail if the S-EPT mapping is somehow removed
> > +	 * between mapping the pfn and now, but slots_lock prevents memslot
> > +	 * updates, filemap_invalidate_lock() prevents guest_memfd updates,
> > +	 * mmu_notifier events can't reach S-EPT entries, and KVM's
> > internal
> > +	 * zapping flows are mutually exclusive with S-EPT mappings.
> > +	 */
> > +	for (i = 0; i < PAGE_SIZE; i += TDX_EXTENDMR_CHUNKSIZE) {
> > +		err = tdh_mr_extend(&kvm_tdx->td, gpa + i, &entry,
> > &level_state);
> > +		if (KVM_BUG_ON(err, kvm)) {
> I suspect tdh_mr_extend() running on one vCPU may contend with
> tdh_vp_create()/tdh_vp_addcx()/tdh_vp_init*()/tdh_vp_rd()/tdh_vp_wr()/
> tdh_mng_rd()/tdh_vp_flush() on other vCPUs, if userspace invokes ioctl
> KVM_TDX_INIT_MEM_REGION on one vCPU while initializing other vCPUs.
> 
> It's similar to the analysis of contention of tdh_mem_page_add() [1], as
> both tdh_mr_extend() and tdh_mem_page_add() acquire exclusive lock on
> resource TDR.
> 
> I'll try to write a test to verify it and come back to you.

I'm seeing the same thing in the TDX module. It could fail because of contention
controllable from userspace. So the KVM_BUG_ON() is not appropriate.

Today though if tdh_mr_extend() fails because of contention then the TD is
essentially dead anyway. Trying to redo KVM_TDX_INIT_MEM_REGION will fail. The
M-EPT fault could be spurious but the second tdh_mem_page_add() would return an
error and never get back to the tdh_mr_extend().

The version in this patch can't recover for a different reason. That is 
kvm_tdp_mmu_map_private_pfn() doesn't handle spurious faults, so I'd say just
drop the KVM_BUG_ON(), and try to handle the contention in a separate effort.

I guess the two approaches could be to make KVM_TDX_INIT_MEM_REGION more robust,
or prevent the contention. For the latter case:
tdh_vp_create()/tdh_vp_addcx()/tdh_vp_init*()/tdh_vp_rd()/tdh_vp_wr()
...I think we could just take slots_lock during KVM_TDX_INIT_VCPU and
KVM_TDX_GET_CPUID.

For tdh_vp_flush() the vcpu_load() in kvm_arch_vcpu_ioctl() could be hard to
handle.

So I'd think maybe to look towards making KVM_TDX_INIT_MEM_REGION more robust,
which would mean the eventual solution wouldn't have ABI concerns by later
blocking things that used to be allowed.

Maybe having kvm_tdp_mmu_map_private_pfn() return success for spurious faults is
enough. But this is all for a case that userspace isn't expected to actually
hit, so seems like something that could be kicked down the road easily.